Cost-Effectiveness of Lane and Shoulder Widening of Rural, Two-Lane Roads in Kentucky

Transcription

1 Transportation Kentucky Transportation Center Research Report University of Kentucky Year 17 Cost-Effectivene of Lane and Shoulder Widening of Rural To-Lane Roads in Kentucky Charles V. Zegeer Jee G. Mayes Kentucky Department of Transportation Kentucky Department of Transportation This paper is posted at UKnoledge. researchreports/13

2 FRANK R. METTS SECRETARY COMMONWEALTH OF KENTUCKY DEPARTMENT OF TRANORTATION Division of Research 33 South Limestone Lexington KY 48 JOHN Y. BROWN Jr. GOVERNOR July 17 H 3-4 MEMO TO: G. F. Kemper State Highay Engineer Chairman Research Committee SUBJECT: Research Report 4 Cost-Effectivene of Lane and Shoulder Widening of Rural To-Lane Roads in Kentucky KYP-74 HPR-PL-1(1) Part III-B. belo: The report submitted hereith comcludes a series hich began in 17. The foregone are listed 1. Report 341 K. R. Agent Accident and Economic Analyses of Acce Control on Several Bypaes September 17.. Report 37 K. R. Agent Evaluation of the High-Accident Location Spot-Improvement Program in Kentucky February Report 387 K. R. Agent Relationship beteen Roaday Geometries and Accidents {An Analysis of Kentucky Records) Apri Report 43 C. V. Zegeer Pedestrian Accidents in Kentucky: March 17.. Report 47 K. R. Agent Accidents Aociated ith Highay Bridges May Report 4 C. V. Zegeer The Effectivene of School Signs ith Flashing Beacons in Reducing Vehicle Speeds July Report 433 K. R. Agent Transverse Pavement Markings for Speed Control and Accident Reduction September Report 44 K. R. Agent Guardrail Performance: An Analysis of Accident Records March 176. On April 1 Gerald Love Aociate Administrator for R & D in the FHWA relayed a request through R. E. Johnson for a draft copy of the report. The request orginated ith Davey L. Warren hos aignment concerned development of FHWA guidelines for R-R-R ork. He had no other resource of information pertaining to cost effectivene of idening in the category covered by our study. A copy as forarded. Further studies in this vein ill be advanced in KYP To reports have been iued from that designation already they are. No. 471 D. R. HerdK. R. Agent and R. L. Rizenbergs Traffic Accidents: Day versus Night May 177. gd Attachment No. 13 J. G. Pigman R. L. Rizenbergs and D. R. Herd Analysis of Weekday Weekend and Holiday Accident Frequencies November ()PC:?& 178. ccs: Research Committee Havens Director of Research

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4 Technical Report Documentation Page 1. Report No.. Government Acceion No. 3. Recipients Catalog No. 4. Title and Subtitle. Report Date Cost-Effectivene of Lane and Shoulder Widening of July 17 Rural To-Lane Roads in Kentucky 6. Performing Organization Code 7. Author(s) Zegeer C. V. and Mayes J. G. B. Performing Organization Report No. 4. Performing Organization Nome and Addre 1. Work Unit No. (TRAIS) Division of Research Kentucky Bureau of Highays 33 S. limestone St. KYP-7-4 Lexington KY Sponsoring Agency Name and Addre 11. Contract or Grant No. 13. Type of Report anel Period Covered Final 14. Sponsoring Agency Code 1. Supplementary Notes Study Title: Effects of Geometries and Operations on Accidents 16. Abstract The purpose of the study as to determine the cost-effectivene of idening lanes and shoulders on rural to-lane roads. Infonnation concerning geometries accidents and traffic volumes as obtained for over 1 miles ( krn) of roads. Reductions in accident rates occurred as lane and shoulder idths increased. Run-off-road and oppositedirection accidents ere the primary accident types aociated ith narro lanes and shoulders. Reductions in accidents ranged from 1 to 3 percent for lane idening and 6 to 1 percent for shoulder idening. Priority listings ere prepared for 31 projects based on critical rate factor. Priority listings ere also prepared for tbe top 1 laneidening projects and the top 36 shoulder-idening projects based on benefit-cost ratios. 17. Key Words Benefit/cost ratio Accident Rate Lane Widening AADT Shoulder Widening Critical Rate Factor Rate-Quality Control Method 1. Security Claif. (of this report). 18. Distribution Statement Security Claif. (of this page) 1. No of Pages. Price Unclaified Unclaified Form DOT F 17.7 IB-71 Reproduction of completed page authorized

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6 Research Report 4 COST-EFFECTIVENESS OF LANE AND SHOULDER WIDENING OF RURAL TWO-LANE ROADS IN KENTUCKY KYP-7-4 HPR-PL-1(1) Part III-B by Charles V. Zegeer Formerly Research Engineer Principal and Jee G. Mayes Research Engineer Chief Division of Research Bureau of Highays DEPARTMENT OF TRANORTATION Commonealth of Kentucky The contents of this report reflect the vies of the authors ho are responsible for the facts and the accuracy of the data presented herein The contents do not necearily reflect the official vies or policies of the Bureau of Highays This report does not constitute a standard specification or regulation July 17

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8 INTRODUCTION One question facing highay engineers is hether to iden lanes and shoulders on existing rural roads to meet current design standards. In most states a percentage of highay funds is allocated annually for design and operational improvements on rural roads. Limited funds of course compel implementation of those improvements hich are most cost effective. Improvements in rideability capacity and safety are primary objectives. Narro lanes are aociated ith head-on collisions and lo highay capacity. Narro unstabilized shoulders may reduce capacity and increase the chance of run-off-road accidents. Some researchers claim hoever that ide paved shoulders encourage leisure stops by motorists hich cause rear-end and side-sipe accidents particularly at night. The available funds should be spent on projects hich are cost-effective. Therefore before lane and shoulder improvements are decided upon the relationship beteen idths of lanes (and shoulders} and accident experience on different types of roads should be ascertained. Highay conditions such as traffic volume acce control and highay type should be considered to determine ho these relate to safety of various lane and shoulder idths. BACKGROUND Lane and Shoulder Width Standards Bureau The design standards used by the Kentucky of Highays for pavement (driving lanes) and shoulder idths are given in Table 1 for the six claes of highays. Minimum standards for pavement idth range from 16 feet (4. m} in Oa 6 (AADT belo 1) to 4 feet (7.3 m) for Cla I highays (AADT above ). Minimum idth of pavements are dependent on highay design speeds in Claes 3 and 4. Standards for shoulder idth range from feet (. 6 m} for Cla 6 to 1 feet (.6 m} for Cla. Cla 1 roads call for special design of shoulders ( 1). The recommended design standards for pavement and shoulder idths as given by AASHTO are presented in Tables and 3 (). Pavement idths exceed Kentucky standards for lo traffic volume roads. Minimun AASHTO idths range from to 4 feet (6.1 to 7.3 m} depending on traffic volume and design speed. While Kentucky standards allo shoulders of and 3 feet (.6 and. m} on lo volume roads AASHT() and Kentucky standards are about the same (1). Standards for pavement idth based on economic considerations ere developed in a 176 study by Shannon and Stanley for the Idaho Department of Transportation. An analysis of 671 highay sections covering a total of 336 miles (468 krn) as made. Construction found maintenance and accident costs ere for various idths. Based on the economic analysis standards for the paved surface (pavement plus shoulders} ere set for to-lane roads and traffic volumes as follos (3): Current AADT Minimum Paved Width ft (m) -4 () -3 (6.1) (7.3} 7-8 (8.) (l.4) - 4 (1.} Lane Width and Safety On 18-foot (.-m) pavements (-foot (.7-m) lanes) cars pa oncoming trucks at clearances averaging only.6 feet (.8 m}. Cars usually do not reduce speeds or move to the right to increase this clearance. On -foot (6.1-m} pavements average clearances are 3. feet (1.1 m}. Clearances beteen vehicles are I foot (.3 m} or le for one of every eight meetings on 18-foot (.-m) pavements and one of every meetings on -foot (6.1-m) pavements. When a truck meets an oncoming truck all of the clearance distances are le. Trucks overtaking other trucks remain center ed in their lanes only hen lanes are 1 feet (3.7 m) ide or greater. Clearances for cars overtaking other cars are only.3 feet (.7 m) on 18-foot (.-m) pavements aud 4.8 feet (1. m) on 4-foot (7.3-m) pavement (4). Results from several studies suggest benefits from idening roads in Illinois the idening of an: 18-foot (.-m} pavement to feet (6.7 m} on 4 miles (361 km} caused a reduction from 3 to 14 accidents per 1 million vehicle-miles (16 million vehicle-kilometers) (3-percent reduction) ( 6). In Louisiana it as concluded that narro lanes contribute significantly to injury and fatal accidents and et-eather accidents. There accident rates on rural roads decreased from.4 accidents per million vehicle-miles (1.6 million vehicle-kilometers) on -foot (.7-m) lanes to 1.8 on 1-foot (3.1-m} lanes and 1. on 11- and 1-foot (3.4- and 3.7-m} lanes (6 7!

9 TABLE 1. KENTUCKY DESIGN STANDARDS FOR LANE AND SHOULDER WIDTHS (1) HIGHWAY CLASS TRAFFIC VOLUME 6 CURRENT AADT -1 CURRENT AADT CURRENT AADT CURRENT AADT UNDER mph m/o mph m/o DHV -1 (AADT 1 to 7) mph m/o 1 DHV OVER 6 {AADT and up) mph m/s I Design Speed Design Speed ill be Controlled by. the Horizontal and Vertical -ALignments Pavement Width Feet (m} 3 mph (13.4 m/s) 4 mph (17. m/s) mph (.4 m/s) 6 mph (6.8 m/s} 7 mph (31.3 m/sl 16 (4.) 18(.) (6.1) (6.1) (6.7) (6.1) (6.7) (6. 7) (6.7) 4 (7. 3) 4(7.3) 4(7.3) 4 (7.3) 4 (7.3) 4 Pavement Initial -Lanes ith 4-Lanes Ultimate or 4 or more Lanes Initial Depending on DHV Minimum Shoulder Width Feet (m) (.6) 3 {.) 4(1. ) 6 (1.8) 1 (7. 3) Special Design Minimum Roadbed Width Feet {m) 3 mph (13.4 m/s) 4 mph {17. m/s) mph (.4 m/s) 6 mph {6.8 m/s) 7 mph (31.3 m/s} (6.1) 4 {7. 3) 8(8.) (8.) 34(1.4) 8(8.) 34[1.4] 3(.1) 34(1.4) 36(11.) 48(14.6) 48(14.6) 48(14.6) 48(14.6) Special Design * Date of Revision 3/1/7

10 TABLE. AASHTO MINIMUM WIDTHS OF PAVEMENT FOR TWO-LANE HIGHWAYS () MINIMUM WIDTHS OF PAvEMENT FOR DESIGN VOLUMES OF DESIGN EED CURRENT AADT CURRENT AADT CURRENr AADT MPH M/S DHV DHV DHV AND OVER FT M FT M FT M FT M FT M Note: For design speeds of 3 4 and mph ( and.4 m/s idths that are feet (Oa6 m) narroer may be used on minor roads ith fe trucks TABLE 3. WIDTHS OF SHOULDERS FOR TWO-LANE RURAL HIGHWAYS () DESIGN VOLUME USABLE SHOULDER WIDTH CURRENT AADT DHV MINIMUM DESIRABLE FT M FT M and over

11 Shoulder Width and Safety Several previous studies involving rural to-lane roads have included correlations of shoulder idth ith accident occurrences. Considerable variation in findings have been cited. One study in Oregon concluded that total accidents increase ith increasing shoulder idth except for roads ith AADT s of36 to (8). Shoulders over 8 feet (.4 m) ere found to cause significantly more accidents than 3- to 4-foot (.- to 1.--m) shoulders in another study (). Connecticut found a decrease in all accident types ith increased shoulder idth for AADT s beteen 6 and 4. Reverse correlation existed belo an AADT of 6 ( 1 ). Only a slight correlation as noted beteen shoulder idth and accidents in Louisiana (7 ). Others have found a definite benefit from ide shoulders. In California about tice as many injury accidents occurred on roads ith 1- to 3-foot (.3- to.-m) shoulders than for shoulders over 6 feet (1.8 m) (for most AADT ranges) (11). In one Ne York study reductions in accidents ere observed as shoulder idth increased especially in the - 6 AADT range no correlation as found for AADT s belo (1). In another study in Ne York it as concluded that 4- to -foot (1.- to 1.-m) shoulders ere adequate on roads of good alignment but shoulders over 8 feet (.4 m) ide ere preferred on roads ith poor geometries (13). A number of studies on shoulder idths indicate a lack of correlation ith accidents on to-lane roads here AADT s are belo. Wide shoulders appear to be most beneficial here AADT s are beteen 3 and. Shoulders 4 to 7 feet (1. to.1 m) ide ere preferred to ider ones. Others suggested that shoulders as ide as 1 to 1 feet (3.1 to 3.7 m) ere the safest. Hoever the economic justification for idening shol)lders on different types of road and traffic volumes has not been determined. Several geometric variables ere found to be significant in accident occurrences in some of the studies. Lane idth acce control conflict points per mile cro slope of shoulder traffic volumes and sight distance ere all mentioned as variables. Shoulder Stability To derive full beneflts it is very important for the shoulders to be stable. Shoulders should support vehicle loads in all kinds of eather. The poibility of a vehicle skidding out of control or turning over is increased hen the shoulder is covered ith loose gravel sand or mud or is soft. One of the greatest problems ith unstabilized shoulders is that they are sometimes one-half to several inches loer than the pavement. Th driver therefore may be faced ith a hazard hen his outer heels drop onto the shoulder and he tries to quickly steer back onto the road (). A stable shoulder should have a compacted subgrade preferably of a granular material. Base courses can be topped by a more stable surface course. Untreated gravel or crushed stone shoulders may be adequate on lo volume roads but may become dusty and erode. Bituminous shoulders offer several advantages including protection of the pavement from structural deterioration. Ride quality of paved shoulders is also better than unpaved shoulders and they allo easy reentry of a vehicle onto the pavement (). In a study of cost effectivene of paved shoulders in North Carolina it as found that a significantly loer accident experience and severity index ere aociated ith paved shoulders on to-lane roads hen compared ith unpaved shoulders on similar highay sections. Shoulders 3 to 4 feet (. to 1. rn) ide ere predominant in that study. Paving of shoulders as cost effective ithin I to years and varied according to traffic volume. The oniy benefits used in the analysis ere the expected reductions in accidents. Hoever a similar analysis of paved shoulders on four-lane roads did not pay off after years. A sample of 34 miles (41 km) of rural primary roads as used in that analysis the interest rates ere 6 and 1 percent and traffic groth rates ere and 8 percent (14). Ohio concluded that shoulder stabilization on to-lane roads resulted in a reduction of 38 percent of all accidents and 46 percent of injury and fatality accidents. The shoulders ere stabilized because of the abnormally high percentages of run-off-road and headon accidents. The criteria used in Ohio for stabilizing shoulders as a minimum of 4 percent of the accidents being (1). run-off-the-road and head-on collisions 4

12 Capacity Considerations Figure I (from the Highay Capacity Manual) relates lane idth lateral clearance and capacity. A tolane road ith foot (.7-m) lanes and no shoulders has a capacity of under 11 vehicles per hour compared ith a capacity of ith 1-foot (3.7-m) lanes and 6-foot (1.8-m) shoulders (on level roads ith no trucks and under ideal conditons). A capacity of about 13 is found for 1-foot (3.1-m) lanes ith -foot (.6-m) shoulders (16). The expected gain from idening lanes (Figure ) and shoulders (Figure 3) can be determined. Increasing lane idth from to 1 feet (.7 to 3.7 m) ill increase capacity by about 48 vehicles per hour (under ideal conditions). Increasing lane idth from 1 to 1 feet (3.1 to 3.7 m) should result in a capacity increase of about 38 vehicles. The result of shoulder idening from (no lateral clearance) to 6 feet ( to 1.8 m) is a capacity increase of over vehicles per hour. Widening shoulders from to 4 feet ( to 1. m) ill increase capacity by 4 vehicles per hour ( 16 ) LATERAL CLEARANCE FEET (METERS) 3.4 :J: 1- Cl : 1-3: 3. z <f..j II L--- L L_L A _LL J CAPACITY ( VPH) Figure I. Relationship beteen Lane Width Lateral Clearance and Capacity.

13 .. Figure. Expected Gain in Capacity due to Lane Widening.. Ui 1. :I: z 4 1. z >- <( > : ob L CAPACITY INCREASE IVPHI. 6 LU en 1. l: u <t z 4 u 1. z <t U> : UJ <t UJ t- UJ u -- <t : LU.... <t CAPACITY INCREASE (VPH) 6 Figure 3. Expected Gain in Capacity due to Shoulder Widening.

14 PROCEDURE To compare accident occurrences for various lane and shoulder idths to different >rocedures may be folloed. The first ould involve conducting an analysis of before-and-after accidents for sections hich ere idened. The second procedure compares accident experiences for existing highay sections ith various lane and shoulder idths. Several problems ere found ith the before-and-after accident analysis. First a very limited sample size ould be available. Second such improvements often include improved d elineation skid resistance realignment and shoul:ier leveling. These improvements of course affect the accident experience. Third additional traffie may be generated by such improvements and therefore affect accidents. The other procedure involves selecting a large sample of highay sections here geometric and accident data are knon. Sections of similar geometries could then be grouped. Accidents (numbers and rates) and severities for highays of different lane and shoulder idths could then be compared. This procedure -as selected. The accident records consisted of 176 accidents investigated by state county and city police agencies and stored on computer tape. Highay geometries ere obtained from the Division of Maintenance and ere also stored on computer tape. Data from both sources ere coded by county number route number and milepost. Accident summaries ere merged ith geometric data on a third computer tape. The state-maintained highay system in Kentucky consists of approximately miles (4 km) of urban and rural roads. Claifications of these roads include interstate parkay state primary state secondary rural secondary special and unclaified roads. Interstates and parkays are generally constructed to meet maximum standards and include 1-foot (3.7-m) lanes and 8- to 1-foot (.4- to 3.7-m) shoulders. Special and unclaified roads include entrances to state or national parks and very lo-volume facilities hich do not arrant pavement and shoulder idening. Only rural highays claified as state primary state secondazy or rural secoridary routes ere selected. Also only to-lane roads ere considered since most four-lane highays are adequately designed and there are only about 4 miles (644 km) of such roads in Kentucky: Geometric information as coded by mileposts. Wherever geometric changes occur along the system a break as made at that point and data ere given separately for each fraction of a mile. Sometimes geometric information as given separately for several highay sections ithin the same mile. Such breaks may be major junctions urban limits railroad croings and changes in surface idth shoulder idth number of lanes median idth acce control and surface types. Accident generators such as major junctions and abrupt changes in roaday idth or acce control ere considered undesirable since they ould bias the data. Therefore all sections of road ith such locations ere omitted. Also all sections in urban areas ere omitted as ere all highays in urbanized Jefferson and Fayette Counties. Using the above criteria for selection of a test sample a total of 144 miles (67 km) of roads ere included in the analysis. A total of eight claifications based on AADT (Table 4) as used. Rural secondary roads had the largest mileage in the test sample ith 783 miles (1 km) folloed by 871 miles (46 km) on state secondary routes and 43 miles (3611 km) on state primary roads. Nearly half of the sections had an AADT le than about 4 percent of the data as beteen an AADT of and. There ere about 1! miles (37 km) of roads ith an AADT over I but only 46 miles (74 km) had an AADT over. 7

15 TABLE 4. DISTRIBUTION OF ROAD MILES (km) BY TRAFFIC VOLUME AND BY ROUTE TYPE STATE STATE RURAL AADT PRIMARY SECONDARY SECONDARY TOTAL MILES KM MILES KM MILES KM MILES KM to to to to to to to to Totals Information input included the location (county route and milepost) lane idth shoulder idth AADT road claification pavement type (bituminous or concrete) shoulder type (bituminous dense-graded aggregate or other) number of lanes acce control (full partial or permit) and number of public approaches (acce points) A computer program as then ritten hich matched accident records ith each!-mile (1.6-km) section. The accident information for each section as summarized and included the folloing: I. total number of accidents. number of run-off-road accidents 3. number of fatal accidents 4. number of injury accidents. number of property-damage-only accidents 6. number of accidents on level roads 7. number of accidents on grade II number of accidents on hill crest number of accidents on straight roads number of accidents on curve number of dry-pavement accidents number of et-pavement accidents number of ice-sno accidents number of daylight accidents number of nighttime accidents number of defective-shoulder accidents number of fatalities number of A-injuries number of B-injuries number of C-injuries number of accidents -- going opposite direction both moving. number of accidents -- left roaday at intersection 3. number of accidents -- left roaday not at intersection 4. number of accidents -- left roaday on straight road not at intersection. number of rear end accidents 6. number of overtaking accidents 7. number of left-turn accidents 8. number of intersection accidents. number of right-turn accidents -- aay from approaching vehicle 3. number of right-turn accidents -- into approaching vehicle 31. number of head-on accidents 3. number of sidesipe accidents and 33. number of other accidents. Certain information hich ould have been useful as not available. These included skid number location of drainage ditches shoulder slope speed limit and number and degree of vertical and horizontal curves. Because of the large data sample (about 16 miles (6 krn)) much of their influence in causing accidents as expected to be minimized. Also the claifications of accidents by type (rear-end run-off. road opposite-direction driveay-related etc.) alloed for the exclusion of accidents hich ere unrelated to lane and shoulder idths. After accident data ere summarized the relationships beteen accident and various geometric characteristics ere determined. Several hundred summary tables ere generated hich gave cumula tive accident numbers for each lane idth shoulder idth and AADT.

16 LANE WIDTH AND ACCIDENTS For this analysis lane idths ere rounded to the nearest foot (.3 m). Accident and traffic volume statistics for lane idths of 7 to 13 feet (.1 to 4. m) are cited in Table. There ere 1676 accidents on 146 miles (468 krn) of road. Traffic volumes ere higher for roads ith ider lanes and increased from on 7-foot (.1-m) lanes to 4483 on 13-foot (4.-m) lanes. Accidents per mile (1.6 krn) increased from.31 to 3. because of tbe higher volumes. Accident rate decreased from 4.16 to.6 accidents per million vehicle-miles (1.6 million vehicle-kilometers) as lane idth increased to 11 feet (3.4 m) as shon in Figure 4. The rate then leveled off at around. beyond 11-fJt (3.4-m) lanes. Accidents considered ere run-off-road opposite-direction (head-n or sidesipe collision beteen opposing vehicles) rear-ends paing situations driveay and intersection and collisions ith pedestrians bicycles animals and trains. The summary of those accidents is given in Table 6. The most common accidents for all lane idtbs ere run-off-road (73) opposite-direction (64) and rear-end (I34). Corresponding accident rates ere computed for each type of accident as presented in Table 7. Rates ere the highest for run-off-road and opposite-direction accidents for narro lanes and decreased steadily as lane idth increased. Accident rates for other accidents generally increased as lane idtbs increased. This is shon more clearly in Figure and it indicates that tbe only accidents hich ould be expected to decrease ith lane idening ere tbe run-off-road and. opposite-direction accidents. A plot of percentage of accidents for each lane idth are shon in Figure 6. Injury and fatality rates for each lane idtb are given in Table 8. Rates of property damage and injury accidents decreased as lane idth increased. This re lationship merely corresponds to the overall accident rate for various lane idths. No signfiicant changes in fatality rate occurred as lane idtb changed. Also tbe percentage of injury and fatal accidents increased slightly and tben decreased as lane idtb increased. Based on this infomration no definite relationship as found beteen lane idth and accident severity. TABLE. LANE WIDTH AND ACCIDENTS SAMPLE NUMBER ACCIDENlS LANE WIDTH SIZE OF PER FEET METERS MILES KM ACCIDENTS MILE (1.6 ACCIDENT RATE TOTII.L VEHICLE- ACCIDENTS PER 1ILBS (1.6 HILLION VEHILES-MILES KM) MDT VEHICLE-KM) (1.6 MILLION KM)) B TOTALS

17 Figure 4. Relationship beteen Lane Width and Accident Rate on Rural To-Lane Roads. E 4.. > ::E VI - :il 1-.J3 <( :: u : z > z u - u.j <( ::E a. 1- z u <( 7 8 FEET 1 II 1.. METERS 3 LANE WIDTH 4. TABLE 6. NUMBER OF ACCIDENTS FOR VARIOUS LANE WIDTHS BY ACCIDENT TYPE PEDESTRIAN OTHER RUN DRIVEWAY BICYCLE OR LANE WIDTH OFF OPPOSITE REAR VEHICLE AND ANIMAL OR NOT FEET M ROAD DIRECTION END PASSING INTERSECTION TRAIN STATED TOTALS () * () (13) (1) (14) (13) Totals (11) (3) 447 * Number in parenthesis indicates percentage of Other or Not Stated lo

18 TABLE 7. ACCIDENT RATES OF VARIOUS LANE WIDTHS BY ACCIDENT TYPE* PEDESTRIAN OTHER LANE WIDTH RUN DRIVEWAY BICYCLE OR OFF OPPOSITE REAR VEHICLE AND ANIMAL1 NOT FEET M ROAD DIRECTION END PASSING I:iTERSECTION TRAIN STATED TOTALS ** ? * Based on 1676 accidents on 146 sections of 1 mile (1. 6 km) each ** Accident Rates in terms of accidents per million vehicle miles (1.6 million vehicle-kilometers} E > :I <D - (f)....j 1. ::! ti...j :! 1- z l: > z u...j...j :E : a. e 1. (f).... z c u = RUN OFF-ROAD ACCIDENTS :.. OPPOSITE DIRECTION ACCIDENTS =ALL OTHER ACCIDENTS FEET II LANE METERS WIDTH Figure. Accident Rate versus Lane Width for Various Accident Types. ll

19 Figure 6. Percentage of Various Accident Types for Different Lane Widths. 7r-----r = RAN-OFF-ROAD ACCIDENTS 6= OPPOSITE DIRECTION ACCIDENTS El =ALL OTHER ACCIDENTS 4 1- z (.) 3 a METERS LANE WIDTH TABLE 8. INJURY AND FATALITY RATES AND PERCENT FOR VARIOUS LANE WIDTHS LANE FEET WIDTH M NUMBER OF ACCIDENTS ACCIDENT RATE* NUMBER OF PERCENT 1-MILE (1.6-km) PROPERTY PROPERTY INJURY AND SECTIONS DAMAGE INJURY FATAL DAMAGE INJURY FATAL FATAL TOTALS * Accidents per million vehicle-miles {1.6 million vehicle kilometers) 1

20 SHOULDER WIDTH AND ACCIDENTS The number of miles (Juri) number of accidents accidents per mile (1.6 km) traffic volumes vehiclemiles (vehicle-kilometers) and accident rates ere found for each shoulder idth form to 1 feet ( to 3.7 m). Of the 1788 miles (41 km) of roads miles (1788 km) had no shoulders (most of the rural to-lane roads have gra or soil adjacent to the pavement). Only paved or DGA (dense-graded aggregate) shoulders are counted as shoulders since gra and soil are not suitable driving surfaces and therefore these surfaces do not function as shoulders. idth and all accidents as expected since other factors such as lane idth and volume ere thought to have a greater influence on accidents. The small sample of locations for shoulder idths greater than 3 feet (. m) may also be a factor. A summary of accidents for various shoulder idths is given in Table 1 and by accident rates in Table 11. Aa ith lane idth the run-off-road and opposite-direction rates decreased as shoulder idth increased to feet (.7 m). There as a slight increase in rate for 1- to 1-foot (3.- to 3.7-m) shoulders. Accident rates for categories other than run-off-road TABLE. EFFECT OF SHOULDER WIDTH ON ACCIDENT RATE SHOULDER WIDTH SAMPLE SIZE NUMBER ACCIDENTS ACCIDENT RATE OF PER MILE TOTAL VEHICLE- (ACCIDENTS PER FEET M MILES KM ACCIDENTS (1. 6 km) AADT MILES (1.6 km) MVM) (1.6 MVkm) one 1 to 3 4 to 6 None.3 to 1. to to 1 to 1.1 to.7 3 to TOTALS Because of the small sample sizes for some shoulder idths considerable differences ere found in the accident rates. Shoulder idths ere then categorized as no shoulder 1 to 3 feet (.3 to. m) 4 to 6 feet (1. to 1.8 m) 7 to feet (.1 to.7 m) and 1 to 1 feet (3. to 3.7 m) -- as shon in Table. Accident rates remained nearly the same for shoulders up to 6 feet (1.8 m) but beyond that idth the rates decreased. The poor relationship beteen shoulder and opposite direction tended to remain fairly constant or increased slightly. Rates for property-damage injury and fatal accidents ere calculated as shon in Table 1. Aa before rates for each type generally decreased as shoulders idened but the percentage of injury and fatal accidents did not sho any trends. No reduction in accident severity therefore may be expected from shoul der idening. TABLE 1. NUMBERS OF ACCIDENTS FOR VARIOUS SHOULDER WIDTHS BY ACCIDENT TYPE PEDESTRIAN SHOULDER WIDTH RUN DRIVEWAY BICYCLE OFF OPPOSITE REAR VEHICLE AND ANIMAL NOT FEET M ROAD DIRECTION END PASSING INTERSECTION TRAIN STATED TOTALS None None to to (1)* (13) to 6 1. to () 37 7 to.1 to.7 1 to 1 3. to TOTALS (13) (1) (11) 16 * Number in p8renthesis indicates the percentage of Other or Not Stated 13

21 TABLE 11. ACCIDENT RATES OF VARIOUS SHOULDER WIDTHS BY ACCIDENT TYPE PEDESTRIAN OTHER SHOUWER WIDTH RUN DRIVEWAY BICYCL& OR OFF OPPOSITE REAR VEHICLE AND ANIMAL NOT FEET M ROAD DIRECTION END PASSING INTERSECTION TRAIN STATED TOTALS None None to 3.3 to to 6 1. to to.1 to to 1 3. to TABLE 1. INJURY AND FATALITY RATES AND PERCENT FOR VARIOUS SHOULDER WIDTHS SHOULDER WIDTH NUMBER OF NUMBER OF ACCIDENTS ACCIDENT RATE* PERCENT 1-MILE (1. 6-kml PROPERTY PROPERTY INJURY AND FEET M SECTIONS DAMAGE INJURY FATAL DAMAGE INJURY FATAL FATAL None None to 3.3 to to 6 1. to to.1 to to I 3. to TOTAL * Accidents per million vehicle iles (1.6 million vehicle-kilometers) LANE AND SHOULDER WIDTH COMBINATIONS An analysis as made of accident rates for various combinations of lane and shoulder idths. The first analysis included rates for all accident types as is shon in Table 13. For roads ith no shoulders. accident rates decreased from.1 to about 1. as lane idths increased form 7 to II feet (.1 to 3.4 m). No improvement in accident rate as found beteen 11 and 1 feet (3.4 to 3.7 m). For other shoulder idths accident rates generally decreased ith increasing lane idth although the relationships ere not as pronounced. For the same lane idths accident rates also tended to decrease as shoulder idth increased. The decrease in accident rates for foot (.7 m) lanes ent from 3.. to 1.8 as shoulders increased to feet (.7 m). Hoever for 8 foot (.4 m) lanes the rate increased slightly (from 3.6 to 4.1) as shoulder idth increased to 3 feet (. m). Overall the decrease in accident rate as greater for increases in lane idths than for equivalent increases in shoulder idths. Another analysis as made of accident rates for various lane and shoulder idths using only run off. road and opposite direction accidents (Table 14). In most cases more uniform decreases in accident rates ere found than hen all accidents ere included. Again increases in lane idths resulted in a greater reduction in accident rates than for the same idening of shoulder. The previous analysis (Tables 13 and 14) indicated that a greater accident savings can be realized by Jane idening than by shoulder idening. While little reduction in accidents may be gained by increasing a foot (6.8 m) road to a 4 foot (7.4 m) pavement the added idth ould provide slightly better service to the motorist in terms of capacity and safe dliving speed. 14

22 TABLE 13. ACCIDENT RATES FOR VARIOUS COMBINATIONS OF LANE AND SHOULDER WIDTHS ON TWO-LANE RURAL ROADS (ALL ACCIDENTS) ACCIDENT RATES AND NUMBER OF 1-MILE {1.6pkm) -SECTIONS INCLUDED LANE FEET WIDTH M NO SHOULDER SHOULDER WIDTH 1 to 3 FEET (.3 to. m) SHOULDER WIDTH SHOULDER WIDTH SHOULDER WIDTH 4 to 6 FEET 7 to FEET 1 to 1 FEET (1. to 1.8 rn) (.1 to. 7 rn) (3. to 3. 7 rn) (86) 1.71{11) --- () ---() ---() {46).(63) {1384) 1.3(38) D. 77 (168) 3.4 (344) 1. (18) 1. 6 (lobo) 1.{7) 1.8 (87) ---(1) ---(1) ---() 1.34() 1.(6) ---(4) 1.1(3) 1.3(8) 1.3(1).81(31).1(1).84(38).8 {7).7(34).(61) TABLE 14. ACCIDENT RATES FOR RUN-OFF-ROAD AND OPPOSITE-DIRECTION ACCIDENTS FOR VARIOUS LANE AND SHOULDER WIDTHS ACCIDENT RATES AND NUMBER OF 1-MILE (1. 6-krn) SECTIONS INCLUDED LANE WIDTH NO SHOULDER SHOULDER WIDTH SHOULDER WIDTH SHOULDER WIDTH SHOULDER WIDTH 1 to 3 FEET 4 to 6 FEET 7 to FEET 1 to 1 FEET FEET M (.3 to. m) (1. to 1.8 rn) {.1 to.7 m) (3. to 3. 7 m) 7.1.(86) * J.. 4 (11) ----** (46) 4. 6 (344) {63).86(18). () 1.83(6) (1384).73(18) 3.11(3). 6 (8).4(1) (38).1(7). 1 (31).8(1).1(38) (168).43(87).6(7) 1.8(34) 1.86(6) * Number of 1-mile (1.6-km) sections used to calculate the accident rate ** A blank as used hen le than five sections ere available in test sample OTHER HIGHWAY FEATURES The previous summaries of accidents by lane and shoulder idths ere influenced by other geometric characteristics. Their effect on the accident data needed to be determined and adjustments made in the accident summaries by lane and shoulder idths. The effect of traffic volume highay type and acce control on accidents as examined and summarized. Traffic Volume The number of accidents per mile (1.6 km) in creased considerably ith AADT as shon in Figure 7. Each data point represents a volume group as discued previously. For example there as an average of.3 accidents per mile on roads ith AADTs from to. The number of accidents per mile increases to about for AADTs of around 6. TI1ese numbers ould be higher if intersection data ere included in the sample of highay sections. The relationship beteen traffic volume and ac ident rate is shon in Figure 8 for all sections (over 1 miles (4 km)) of rural to-lane roads. The accident rate remained at 3.3 as AADT increased from to I. The rate then decreased to.3 at an AADT of and as belo.7 for AADTs above. 1

23 Figure 7. Relationship beteen Accidents and Traffic Volume All Accidents....J 4 a z g u :o-:co----o:-o:-----:4:o-:co--g:::o:-:o:----.l o:-:o-:o:----:l o-:o-=o----..:-oo=-o:--:6: o-=o-=o---t=o ooo AADT E > - <n...j :E f-...j «u I >- z> z U:i U...J a. <n... z g u u.. o o --L-- s-o-=o...j_ -...J p o o :o-:o-:o- -- -L o-:o-:ol_...j L_L...J o_ o_o_o --.1o- oo-o AADT 16 Figure 8. Relationship beteen Accident Rate and Traffic Volume All Accidents.

24 It appears (Figure 8) that loer accident.. rates are aoicated ith higher volumes. Hoever the relationship is not that simple. Higher volumes ere also aociated ith higher cla of roads hich normally have ider lanes and shoulders and le and more gradual curvature than loer-volume facilities. To determine ho accident rates ere affected by volume alone summaries ere made of rates versus volumes for specific highay types and lane idths (Table 1). Accident rates ere calculated for each AADT group and lane idth for rural secondary state secondary and state primary routes. To exclude other geometric variables only routes ith no shoulder and ith le than four public approaches (acce points) per mile (1.6 km) ere included. By comparing rates in each vertical column for rural secondary state secondaryand state primary routes no clear relationship as found. Rates for each claification and lane idth remained roughly the same or fluctuated slightly as AADT increased. This may be expected since all accident types ere included in the calculation of accident rate. Previous research has shon that single-vehicle accidents ere affected differently than multivehicle accidents as AADT increas<ed. This as verified in Figure. The accident rate of single-vehicle (run-offroad) accidents as over 1.8 for lo AADTs ( to ). As AADT increases the rate drops sharply to about.6 for a 6 AADT. Rates of multi-vehlcle accidents remained virll!allv cnstant. If the test sections had included all intersections the rate for multi vehicle accidents (particularly rear-end accidents) ould have increased as AADT increased. The probable reason that the rate of run-off-road accidents decreases as AADT increases is that vehicles tend to be driven sloer since paing may not be poible. On lovolume roads vehicles are not able to caravan (follo each other in groups) and unfamiliar motorists may take curves at exceive speeds particularly at night or in the rain. At night motorists sometimes follo tail lights ahead of them hich help arn of sharp curves.. H N rl rl OO:r Lfl \. ::: :: ::: N <:l Lfl ixl O M I- M M M M rl O <:r 1- Lfl i.o N N N N Mrlr::- oooo 88 :!8 M Lfl M rl <:r Lfl W Lfl r- co..... N rl rl rl rl..... M N N N N M r- rl 1 1- CO M N O\ rl i.o \ rl <:r N M M M m \D O N OI M «l O N Lfl rl rl i<r- r- i<co Lf1 r- *8 ::: I<Lfl co +:N N 1... M _. M O Lfl O Lfl - O N Lfl r- rl O rl O Q O O -ij.j..).j..) -ij -ij rl rl rl rl o o o o -ij rl O Lfl O Lfl O Lfl rl N Lfl f- 17

25 . r----- E.>< > 1.8 ::E SINGLE-VEHICLE ACCIDENTS en 1.6 :iii!jj W 1- - : U :: :: 1.4 IJJ 1- > Z z IJJ O - c: j 1. :: a. (/) z <3 u.8 <( MULTI- VEHICLE ACCIDENTS o.sl l--l-l--l-----l----l---l AADT Figure. Effect of Traffic Volume on Single Vehicle and Multi-Vehicle Accident Rates. Since the rate of run-off-road accidents decreases as lane idth and AADT increase it as unclear hat the effet of lane idth as on the rate of run-off road accidents. To determine the individual effect of AADT and lane idth the rate of run-off-road acci dents as plotted versus AADT for different lane idths (Figure 1). The slope of the lines indicate the effect of AADT on rates and the vertical distance be teen lines indicate the effect of lane idth on rates. For to 4-foot (6.7- to 7.3-m) pavements the rate decreased from about 1. to.4 as AADT increased from 17 to 8. The rate for 18 to -foot (.- to 6.1-m) pavements dropped from about 1. to. as the AADT increased from to 6. No relation ship as found for pavements belo feet (6.1 m) due to the limited range of traffic volumes for those sections. These data are in Table 16. The difference in rates beteen the to curves in Figure 1 is about.4 hich is a decrease of. 1 accidents per million vehicle-miles (1.6 million vehicle kilometers) per foot (.3 m) of total idening or a. decrease for each foot (.3 m) of lane idening. The total decrease in run-off-road rates as from 1.6 to. 1 for various lane idths (Table 7). This is a decrease in rate of 1.4 of hich only 8 percent (.4/1.4) as due to ider lanes. Most of the de Grease (71 percent) as due to volume increases. 18

26 TABLE 16. ACCIDENT RATES OF RUN-OFF-ROAD ACCIDENTS FOR VARIOUS LANE WIDTHS AADT 7 to 8 FEET (.1 to.4 to m} (. 7 1 FEET to 3. m} 11 to lz FEET ( 3. 4 to 3. 7 m} to to to to 1 to 7 71 to * All rates are for 4 or more 1-mile (1.6-krn) sections 1.8 E > 1.6 (/)...J :: <t O a::: _ z > 1. z Q CJ...J = :::1. a: a. 18 T FT ( TO 6.1 m) PAVEMENTS.8 z (.).6 to 4 FT (6.7 to7.3m) PAVEMENTS Q4L-----L L-L----L L--L -L...J AADT Figure I. Rates of Run-off. Road Accidents by Lane Width Various Traffic Volumes. 1

27 The effect of traffic volume on opposite-direction accid e nts as also determined ith respect to various pavement idths as shon in Figure II. The rate for 14- to 16-foot (4.3- to 4.-m) pavements decreased slightly from about 1.3 to 1. for AADTs of to 1. The rate for 18- to -foot (.- to 6.1-m) pavements decreased slightly from.6 to.4 for AADTs of 3 to. Rates for - to 4-foot (6.7- to 7.3-m) pavements decreased slightly from. to.1 for AADTs of 17 to. The total decrease in rate of opposite-direction accidents as 1.6 as shon in Table 7. The increase in volume as responsible for only about 4 percent of this decrease as determined from data in Figure 11 and Table 17. Therefore about 76 percent of the reduction in opposite-direction accidents as due to ider lanes. As can be seen in Figure II the greatest reduction in accident rate per foot of idening can be achieved by idening the narro-idth pavements (14- to 16-foot) (4.3- to 4.-m) to medium-idth (18- to -foot) (.- to 6.1-m) pavements. Because of the limited sample of highay sections ith shoulder idths above 3 feet (. m) an analysis of rate versus traffic volume as not poible for various shoulder idths. The effect of volume on rates of run-off-road and opposite-direction accidents as found using the lane-idth analysis. The effect of volume on rates can be accounted for in a similar manner in a shoulder-idth analysis E > 1. :: <D o-----_::o - 14 TO 16FT (4.3 TO 4.rn) PAVEMENTS :IE <l u o a :: s:. _ z > :z!:! (.)...J.6 U -- cti :..4 (/) 1- z. () <l: o--- a TO FT (.T 6.1m)PAVEMENTS x TO 4 FT(6.7 TO 7.3m)PAV EMENTS ol l-- L- --L- -L AADT Figure II. Rates of Opposite-Direction Accidents by Lane Width Various Traffic Volumes.

28 TABLE 17. ACCIDENT RATES OF OPPOSITE-DIRECTION ACCIDENTS FOR VARIOUS LANE WIDTHS AADT 7 to 8 FEET to 1 FEET 11 to 1 FEET (.1 to.4 m) (. 7 to 3. m) (3.4 to 3. 7 m) to to to to to to 1.16 Acce Points Another geometric feature thought to have some influence on accident rates as the effect of acce points per mile (1.6 km). This is the number of public approaches or minor entrances onto the highay hich could adversely affect accident rates. An analysis as made of accident rates versus lane idths for zero to four and five or more acce points per mile (1.6 km) (Table 18). For every lane-idth category the accident rate as higher for the five Or more acce group than for sections ith zero to four acce points per mile (1.6 km). Differences in accident rates ranged fr om abont (for 1-foot (3.-m) lanes) to.7 accidents per miilion vehiclemiles (1.6 million vehicle-kilometers) for -foot (.7-m) lanes. A plot of these data is in Figure 1. While more acce points per mile (1.6 km) did increase accident rate only about six percent (1 miles (16 km)) of the sample had five or more acce points per mile (1.6 km). Also those sections ere distributed evenly throughout the test sections. Therefore the effect of this geometric feature did not significantly influence the accident rates for various lane and shoulder idths. TABLE 18. EFFECT OF ACCESS POINTS ON ACCIDENT RATE FOR VARIOUS LANE WIDTHS LANE ACCESS POINTS/MILE (1.6 km) WIDTH FT M to 4 OR MORE (3) (71) 4.1 (7).7.7 (78) 3.7(3) (84).86(3) (663).17 (1) (48).36 (11) (4). (14) Note : Only data ith 8 or more 1-mile (1. 6-km) sections ere used 1

29 OR MORE ACCESS POINTS PER MI LE E 3.4 :< > ::: 1-3. <( ::: :: > 1- z :: Cl a...8 (.) (.) (/) 1- <( z.6 Cl ::: 3. (.) (.).4 <( TO 4 ACCESS POl NTS PER MILE FEET II METERS LANE WIDTH Figure 1. Effect of Acce Points per Mile (kilometer) on Accident Rates Various Lane Widths. Highay Claification Another variable hich as studied included the effect of highay claification on accident rate these results are given in Table 1. To limit the effect of other variables only sections ith no shoulder and ith le than five acce points per mile (1.6 km) ere used. Rates ere compared for each lane idth for rural secondary state secondary and state primary routes. Rates for 8-foot (.4-m) lanes ere slightly higher for state secondary than for rural secondary routes. For -foot (.7-m) lanes rates ere generally higher for rural secondary routes and loer for state primary routes. For 1-foot (3.-m) groups ith lo AADT1s a similar trend as found. Hoever as AADT increased rates became highest for state primary routes. This could indicate that 1-foot (3.-m) lanes are not acceptable for state primary roads ith high volumes. For!!-foot {3.4-m) lanes no obvious differences ere found beteen state secondary and state primary routes.

30 ACCIDENT SAVINGS Savings due to accident reductions ere the only benefits included in the economic analysis. The expect ed reductions in accident rates ere computed as a function of lane and shoulder idths. The cost per accident as used along ith expected traffic volume and accident reduction to compute accident savings. As discued previously lane and shoulder idths ere shon to have an effect on only run-off-road and opposlte direction accidents. Other accident types ere not found to decrease as a function of ider lanes and shoulders. Thus average accident costs ere com puled for these to categories of accidents based on information in Table 1. The number of accidents and injuries for various severities as found for th follom ing accident types in order of increasing severity: 1. Property-Damage-Only Accident (PDQ) no injuries ere sustained Using this information the severity index of the accident types ere computed. The severity index formula developed in a 173 study ( 17) follos: in hich SI SI K A B c = = PDQ = N = [.(K + A) + 3.(B + C) +PDQ] /N severity index number of fatal accidents number of A type injury acci dents number of B-type injury acci dents number of C-type injury acci dents number of property damage only accidents and total number of accidents. TABLE 1. NUMBER OF ACCIDENTS BY rype AND SEVERITY ooa AND RORb ACCIDENTS OTHER ACCIDENTS ACCIDENT PERCENT ACCIDENT NUMBER OF NUMBER _OF NUMBER OF NUMBER OF ROR AND OTHER TYPE ACCIDENTS INJURIES ACCIDENTS INJURIES OD ACCIDENTS ACCIDENTS Property Damage c - Injury B - Injury Injury F - Injury A - Total a b Opposite -direction accident Ran-off-road accident. C-type Injury Accident.. involving no visible injuries but complaints of pain 3. B type Injury Accident - bruises abrasions selling or limping 4. A-type Injury Accident - bleeding ound distorted member or person carried from scene and. Fatal Accident -- one or more deaths. Of all run-off-road and opposite-direcdon accidents 4.3 percent involved injuries or fatalities compared ith only 1.6 percent for the other types of a accidents. The percentage of fatal and A-injury acci dents as nearly three times as high for run off-road and opposite-direction accidents than for all other types. The sample included about 4 accidents (Table 1). The maximum severity index computed by tills formula is. and ould occur if all accidents ere fatal and A-type injury accidents. The mmunum severity index is 1. and applies hen all accidents are property damage only. The combined severity index of the run-off-road and opposite-direction accidents as.74 compared to 1.74 for the other accidents. T11e average cost per accident as then computed for use in the calculation of expected accident savings. The folloing accident costs as reported by the National Safety Council for 176 (18) ere used: Death $1 Nonfatal disabling injury $47 and Property-damage accident.. $67. The average cost of a run-off-road or opposite-direction accident as $6 compared to $1 for other accident types on rural to-lane roads. 3

31 Lane Width The expected reduction in accident rate as computed and plotted for various degrees of lane idening as shon in Figure 13. The values represent reductions in the combined rate of run-off-road and opposite-direction accidents {Table 7) adjusted for traffic volume. For example by idening a 7-foot {.1-m) lane to II feet {3.4 m) an accident rate reduction of 1.8 (accidents per million vehicle-miles {1.6 million vehicle-kilometers)) may be realized. Widening a -foot {.7-m) lane to 1 feet {3. m) ill result in only a.16 reduction in accident rate. Very little additional benefit ill be realized by idening a road beyond II feet {3.4 m). Based on information in Table 11 the relationship for percentage reduction in run-off-road and opposite-direction accidents for various degrees of pavement idening as determined. The effect of traffic volume on accident rate as also taken into account. By idening lanes from 7 to 11 feet {.1 to 3.4 m) a 3percent reduction in run-off-road and opposite-direction accidents should result (Table ). An 8-foot {.4-m) lane idened to 1 feet {3. m) ill produce a 3-percent reduction. 1.6 e > :: 1.4 _ Li.l {j) :=! 1. a: :: >- Z...J W <.> 1. o - - :X: <.> W <.> > <t z z.8 - ::: Z - :::! t- a:.6 <.> => a.. fil(l) ::.4 u <.> <t. l.!! 7 (.1 ) 8 (. 4) (.7) 1 (3.) FEET 1.. METERS LANE WIDTH AFTER WI DENING Figure 13. Reduction in Accident Rate due to Lane Widening. 4