Right-TurnTreatments at Signalized Intersections

Transcription

1 IGuidelines for Right-TurnTreatments at Signalized Intersections BY RICHARD A PEREZ The City of Salem, in conjunction with other impacted agencies, has begun a long-range study of a major arterial corridor greatly impacted by strip commercial development. The data-gathering efforts consisted of extensive traffic counts and travel-time and delay studies. This enabled us to see firsthand the great congestion that was occurring on the street. At the four most-congested intersections, unequal queuing between the two through traffic lanes was observed. In all cases, the right lane s queue appeared at least 20 percent, and up to 50 percent, longer than the left lane s queue during the most congested hours. By comparison, the default lane utilization factor in the Highway Capacity Manuall procedures is 1.05, thus assuming the more saturated lane will carry about 5 percent more volume. Since each approach to these intersections had peak-hour right-turn volumes of 200 or more, it appeared that a right-turn lane at these intersections would measurably improve levels of service. At the same time, the Oregon Traffic Control Devices Committee was presented with a request by the City of Salem to consider the use of protected/permitted right-turn overlap phasing. A protected/permitted rightturn overlap phase is a signal indication in which the right-turn movement is displayed a green ball during a permitted phase, and a green right arrow during a protected phase. This operation can occur only if the cross-street of the subject approach has a protected leftturn phase and no U-turn is permitted. For example, an eastbound approach would be displayed circular green indications during the eastbound throughtraffic phase. During the following northbound left-turn phase, the eastbound through lanes would be displayed circular red indications. But the eastbound right-turn lane could be displayed a green right-arrow because this movement would have no conflicting movement, as long as U-turns were not permitted from the northbound leftturn lane. Currently, the Oregon Traffic Control Devices Committee has approved a right-turn overlap phase that must be protected only by displaying right-arrow signal indications, and prohibition of conflicting pedestrian crossings or permitted only with a circular green indication on a programmed-visibility signal head to limit the visibility of the right-turn signal indications to the right-turn lane. The committee referred to the city the task of studying guidelines for protected/permitted right-turn overlap phasing as well as warrants for installing right-turn lanes at signalized intersections. Conversion Factor 1 foot = 0.3 meter 1 square foot = square meter 1 mile per hour= 1.6kilometers per hour 1 acre = hectare Data Search Historically, a great deal of attention has been paid to the provision of left-turn lanes and phasing. The subject of right-turn lanes does not garner the same attention since right-turn movements involve fewer conflicting movements. The American Association of State Highway and Transportation Officials (AA SHTO) A Policy on Geometric Design of Highways and Streets2 makes mention of considerations for turn lanes, but restricts the discussion of application to left turns. The Oregon Department of Transportation Highway Design Manuals makes no mention of rightturn lanes at signalized intersections, but directs readers to use AASHTO for guidance on items not discussed its own manual. The Washington State Department of Transportation s Design Manua[4 devotes one section of a chapter on right-turn lanes and provides warrants for unsignalized intersections; but it refers to the Highway Capacity Manual (HCM) procedures to determine the need at signalized intersections. HCM only suggests a right-turn lane should be considered if both the right-turn volume and the through volume from the adjacent lane each exceed 300 vehicles per hour (vph). The Federal Highway Administration s Design of Urban Streetss has a limited discussion of right-turn lanes but gives no particular guidelines. California Department of Transportation s Tr@ic Marzua/fi suggests right-turn overlap ITE JOURNAL* FEBRUARY

2 phasing for volumes greater than 200 vph, but does not mention criteria for right-turn only lanes. The Intersection Channelizatiorr Design GaideT provides the most extensive discussion of considerations found for right-turn lanes, but provides no quantifiable guidelines. Texas Transportation Institute s (TTI) City Street Design documents the increase in capacity capable by the provision of right-turn lanes with the following summary As a general rule, the provision of right-turn lanes at an arterial intersection will increase the intersection capacity by at least one percent for each one percent of the intersection volume that consists of right turns. In as much as capacity is (or will be) an issue at major intersections, right-turn bays should always be provided as part of original design for capacity as well as safety considerations. On existing arterials, the addition of right-turn bays should be a high priority for the same reasons. x The Colorado State Highway Access Codey provides warrants for right-turn lanes for access drives to state highways, which could presumably be applied to signalized intersection design. As a minimum, it requires right-turn lanes for any right-turn movement from a state highway greater than 25 vph. The application of Colorado s warrants, as well as those from TTI, would require right-turn lanes at virtually every signalized intersection, which would be a radical departure from existing practice for many agencies. Methodology Twenty signalized intersections were studied during the morning, noon and evening peak hours. Each approach of each intersection was studied for impacts of right-turn-only lanes and right-turn over- Are your breakawa~$ ~, p ~~, The SPEED?.I Telespar@ slipbase,., system meets tough AASHTO 1800 lb. change-of-, velocity vehicle standard$,., giving you multi-diretiional breakaway performance., - A low-cost system that,, installs in minutes, the Telespar slip-base delivers q extra safety with long- : a lasting performance..,,,j Send for free literature.,,, / d. ~~ ALL STANDARD FASTENERS 1:. 1: **,.-, K,,. 4 t 7 square-and many other d.,,,.,,., ~ anchors.,-., ~ TELESPAR%W:IRWtion Wayne, Ml UNISTRUT Sign Support System (600) Fax (313) lap phasing. In order to quantify the impacts of freeing up additional green time, the timing of each intersection was optimized using the signal timing optimization software package TRANSYT-7F, and analyzed using HCM procedures. Right turns on red were estimated at 10percent without a right-turn lane, 30 percent with a rightturn lane, and 10 percent with a right-turn lane with overlap phasing. These are slightly more conservative than published values, I~ but were confirmed by limited sampling at intersections along the subject arterial. Default values from Highway Capacity Software for the percent of right-turns on a protected phase were accepted. For each scenario analyzed, the following data were recorded. Note that lane group is used as defined by HCM procedures. Through volume, right-turn volume and intersection volume (in vph) Approach volume/capacity ratio (v/c) without right-turn lane Approach delay without right-turn lane (in seconds per vehicle [see/veh]) Intersection v/c without right-turn lane Through lane group v/c with right-turn lane Through lane group delay with right-turn lane (see/veh) Right-turn lane v/c Right-turn lane delay (see/veh) Total intersection v/c with right-turn lane Total intersection delay with right-turn lane (see/veh) Through lane group v/c with right-turn phasing Through lane group delay with right-turn phasing (seclveh) Right-turn lane v/c with right-turn phasing Right-turn delay with right-turn phasing (see/veh) Total intersection v/c with right-turn phasing Total intersection delay with right-turn phasing (see/veh) Right-Turn Lanes The total reduction in delay in vehicle-seconds per hour was calculated for each scenario. The average and standard deviation of these scenarios was calculated and distributions plotted to determine logical break points. Due to changes in signal timing recommended by TRANSYT-7F and the assumed right-turn-on-red rates, delays were increased by the addition of a right-turn lane in some cases. The resulting distributions are shown in Figures 1 and 2 and are correlated well to a break-even analysis of the cost of delay vs. the cost of construction of a typical right-turn lane, as follows: Assume 12 feet (ft) of additional right-of-way for 200 ft: 12ft x 20Qft= 2,400ftZ= 0.06 acre Assume right-of-way cost at $100,000/acre: $100,000/acre x 0.06 acre= $6,003 Assume paving cost at $10/square ft (ft ): 2400ft2 X $1O/ft = $24,030 Total cost: $24,00U+ $6,000 = $30,000 Assume cost of delay at $15/hour (hr): $151hr+ 3,600 seclhr = $0.0Q4167/sec Assume right-turn lane should pay for itself in one year: $30,000 + $ /sec = 7,200,000 sec of delay Assume 10 percent of average daily volume occurs in the peak hour: 24 ITE JOURNAL* FEBRUARY 1995

3 5-11. i F l&/ Change,. Delay Frequency Distribution. u ~ W-... -,.,, ~,,.< >, - I ~ , WD,V , Break Figure 1. Change in delay with right-turn lane, one-lane lane group. 7,200,000secx 0.10 = 720,000secof delay Assume peak hour volume occurs every day: 720,000 sec dayslyear = sec of delay/hr = 2,000 sec of delay/hr Using these assumptions, a right-turn lane would pay for itself in one year if it results in a 2,000-second reduction in vehicular delay during a peak hour. Right-Turn Overlap Phasing The addition of right-turn overlap phasing for a right-turn lane would consist of replacing one three-section signal head with at least one, but preferably two, five-section heads, at a cost of at least $200, and possibly up to $1,000. Using the same criteria for the establishment of a right-turn lane, the rightturn overlap phase would pay for itself within one year if its installation resulting in a reduction of 66 seconds of vehicular delay in a peak hour over that reduction achieved by the addition of a right-turn lane. This analysis assumes that the complementary left-turn movement has a protected phase. Providing right-turn phasing in the absence of a complementary protected left-turn phase was not examined. The Manual on Uniform Traffic Control Devices (MUTCD)tl recognizes only two lens arrangements for protect- I ed/permitted right-turns, one horizontal and one vertical (see figures from MUTCD). One head should be provided directly ahead of the right-turn lane as Can your trafilc.- t practicable. This could be either on a signal pole or a mast arm or span wire. To ensure visibility of the signals controlling the right-turn movement around large vehicles, a second signal head is recommended. This second head should be located on a pole on the far right side of the intersection if it is at least 8 ft from the first right-turn signal head, or on the mast arm or span wire between the pole-mount and through-traffic signal heads. If this requirement cannot be met, the second signal head should be located on the near right side of the intersection. The latter location may aid in focusing the attention of the driver to potentially conflicting pedestrian movements during the permitted portion of the right-turn movement. Data Analysis Linear regression analysis showed that the variables most closely related to the reduction in vehicular delay were, not surprisingly, the lane group delay before adding the right-turn lane, the through movement delay after the right-turn lane was added, the intersection delay before counter... Give you a listing of every speeding vehicle, by type, and time of day? Ours can! ITE JOURNALO FEBRUARY

4 Frequency Distribution 9M II J = , T 11 } %79 -E % X S1dDe , ,10 +0,5 00 Break Figure 2. Change in delay with right-turn lane, two-lane lane group. 0: 1 1 i 1 I, Zm m Em 7k [ Lane Group Volume (DHV) Figure 3. Right-turn treatments at signalized intersections for l-lane lane groups. the right-turn lane was added, and the total intersection delay after the rightturn lane was added. Unfortunately, in order to quantify these variables, a levelof-service analysis must be conducted both with and without the right-turn lane. Then the difference in total intersection delay multiplied by the totaf intersection volume will yield the total delay reduction. Therefore, the use of these formulae do not result in any data that could not be calculated more easily or more directly. However, some similarities were --1 observed in the raw data that were used to develop guidelines. These could be used to quickly determine if a right-turn was not warranted, and borderline cases could be studied more thoroughly, perhaps incorporating costs particular to the site to determine an appropriate breakeven point in a cost/benefit anafysis. After the break-even points were chosen, the parameters for each scenario above the break-even point were analyzed to derive the minimum values of each for meeting guidelines. These were then tested to ( determine the validity of these guidelines against the sample data set. The false positive rate (the percentage of the sample data set wherein the guidelines falsely predicted the requisite delay reduction would be achieved) was less than 2 percent. The false negative rate (wherein the guidelines falsely predicted the requisite delay reduction would not be achieved) was less than 6 percent. The resulting guidelines are shown in Figures 3 and 4 and summarized in Table 1. These use the design hourly volumes (DHV) of the right-turn movement and the lane group (as defined by HCM) from which the right-turn would be made, as well as the v/c ratio of the intersection. It should also be noted that in order to attain a reduction in delay, a minimum volume of traffic not turning right from the same lane group as the right-turn movement is required. As this volume increases, so does the probability of righttuming traftlc being prevented from making a right-turn-on-red. Although not documented here, it appeam that a right-turn lane usually is warranted where such a lane would be a critical movement, as defined by HCM procedures. Another caveat is that, due to the minimum v/c requirement of these guidelines, an intersection analyzed using these guidelines should be analyzed incrementally. The addition of one right-turn lane may be adequate to reduce the intersection v/c such that right-turn treatments on other approaches of the same intersection would no longer have a significant impact on the reduction of total intersection delay. However, for a planning-level analysis, this caveat could be ignored, based on an assumption of high VICratios in the design year. This analysis does not explore the potential for safety benefits derived from right-turn treatments, but ample studies indicate the contribution of high-speed differentials to accident involvement rates.lz As a substitute, it is recommended that 70 percent of the values of these guidelines be used when the operating speed of the subject approach is greater than 40 miles per hour (mph) in order to provide a refuge area for decelerating traffic. This is consistent with the MUTCD S warrants for signalization in high-speed locations. However, no analysis was performed to justify this. It is recognized that other factors may exist which may require or preclude the use of right-turn treatments as discussed 26 ITE JOURNAL* FEBRUARY 1995

5 I I I I I I I I I! I Lane Group Volume (DHV) Figure 4. Right-turn treatments at signalized intersections for 2-lane lane groups. here. As usual, the use of these guidelines is not a substitute for good engineering judgment. Conclusion As transportation system management measures become more prevalent, guidelines for providing operational improvements will become increasingly necessary. The provision of right-turn treatments, when used appropriately, can reduce delays and improve air quality. Although the guidelines here can provide a good preliminary indication of the need for such treatments, they should not be used as a substitute for a detailed feasibility analysis. However, Table 1. Resulting Guidelines the guidelines suggest that right-turn overlap phasing would result in reduced delay in locations where right-turn volumes meet these guidelines for a rightturn lane and the complementary left turn has a protected phase. It is hoped that this article can provoke thoughtful discussion and research into the subject of operational improvements for rightturning traffic. References 1.Transportation Research Board, Highway Capacity Manual, Special Report 209 (Washington, D. C.: National Research Council, 1985) Chapter 9, Appendix I. I-LaneApproach 2-LaneApproach Parameter Right-lurn Right-Turn Right-Turn Overlap Right-Turn Overlap Lane Phase Lane Phase Lane Group Minimum Volume (vph) Percent of Lane Group Turning Right 10.0 % 15.0 % 15.0% 15.0% Minimum Lane Group Volume Not Turning Right (vph) Intersection Volume/Capacity Ratio Without Right-Turn Lane False Positive Rate 1.8% 1.2% 1.9% 0.0 % False Negative Rate 1.8% 3.6 % 5.7 % 0.0 % Sample Size I 2. American Association of State Highway and Transportation Officials, A Policy on Geometric Design of Highways and Streets (Washington, D. C.: AASHTO, 1990). 3. Oregon Department of Transportation, Highway Division, Highway Design Manual (Salem, Ore.: ODOT, 1985). 4. Washington State Department of Transportation, Design Manual (Olympia, Wash.: WSDOT, 1989). 5. Federal Highway Administration, Design of Urban Streets (Washington, D. C.: FHWA, January 1980). 6. California Department of Transportation, Traffic Manual (Sacramento, Calif CalTrans, 1977). 7. Timothy R. Neuman, Transportation Research Board, Intersection Channelization Design Guide, National Cooperative Highway Research Program Report 279 (Washington, D. C.: National Research Council, November 1985) pp Texas Transportation Institute, Texas A&M University, City Street Design, Short Course Notes, August 1988, Revised September The State Highway Access Code, State of Colorado Department of Highways, August 15, ITE Technical Council Committee 4M-20, Driver Behavior at Right-Turnon-Red Locations, ITE Journal 62(4), pp U.S. Department of Transportation, Manual on Uniform Traffic Control Devices (Washington, D. C.: Federal Highway Administration, 1988), Figure Stover, Virgil G., and Frank J. Koepke, Transportation and Land Development (Washington, D. C.: Institute of Transportation Engineers, 1987). I Richard A. Perez, P. E., is the Assistant Traffic Engineer for the City of Salem, Ore., and is a member of the Technical Committee of the Oregon Section of ITE. He earned a B.S. in civil engineering technology from Oregon Institute of Technology, and is an Associate Member of ITE. ITE JOURNAL* FEBRUARY