HEADWAY AND SAFETY ANALYSIS OF SPEED LAW ENFORCEMENT TECHNIQUES IN HIGHWAY WORK ZONES

HEADWAY AND SAFETY ANALYSIS OF SPEED LAW ENFORCEMENT TECHNIQUES IN HIGHWAY WORK ZONES Ming-Heng Wang*, Ph.D. Post Doctoral Researcher Transportation Research Institute The University of Kansas 2160 Learned Hall, 1530 W. 15th Street, Lawrence, Kansas 66045 Phone : (785)-766-0339 Fax: (785) 864-3199 E-mail : mwang4@ku.edu Rahim (Ray) F. Benekohal, Ph.D. Professor Department of Civil and Environmental Engineering The University of Illinois at Urbana-Champaign R1213 NCEL205 N. Mathews Ave. Urbana, IL 61801 USA Phone: (217)-244-6288 Fax: (217)-333-1924 E-mail : rbenekoh@illinois.edu Hani Ramezani Graduate Research Assistant Department of Civil and Environmental Engineering B106 Newmark Civil Engineering Laboratory University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA E-mail: Hrameza2@illinois.edu Phone: (217) 333-5967 *: Corresponding Author Word Count: 4522 Figures and Tables: 8 Submitted for Presentation at the 89 th TRB Annual Meeting First Submitted: August 1 st, 2009 Revision submitted: Nov 15 th, 2009

1 ABSTRACT HEADWAY AND SAFETY ANALYSIS OF SPEED LAW ENFORCEMENT TECHNIQUES IN HIGHWAY WORK ZONES This paper investigated the headway distribution of platooning vehicles, presence of very short headways (<0.7secnond), and frequency of applying of brakes and changing lane in work zones with and without law enforcement activities. Law enforcement activities include the police patrol car presence (PCP) and automated speed photo-radar enforcement (SPE). Data from two work zones were collected and analyzed. Mean headway of vehicles in work zones increased when SPE or police patrol car presence was utilized in work zones. This is a beneficial effect and indicates that drivers had a longer time to react to the lead vehicles. The law enforcement presence in work zones, either the SPE or police patrol car presence, in general decreased the number of vehicles traveling with a very short headway (<0.7 seconds). The difference in braking behavior was not significant when the SPE case was compared to police car presence. However, lane changing behavior was significantly different for cars traveling in the median lane; also for trucks traveling in the shoulder lane. It is recommended to continue using police car presence or the SPE system to reduce work zone speed and improve traffic flow condition. The use of SPE is recommended particularly in work zones where the lateral space does not allow parking a car on the shoulder (police or violator) or where the traditional law enforcement has limited effectiveness. Future research for modeling the headway distribution of other speed reduction treatments in work zone is recommended.

2 INTRODUCTION The severity and frequency of highway traffic crashes have always been a major concern, particularly in work zones. Excessive speeding is a major contributing factor in the severity of crashes. The presence of police patrol in work zones consistently produced the largest speed reductions (1, 2). However, its temporal and special effects are contained. With a limited lateral space to park the patrol car and restricted sight distance in some work zones, it becomes difficult for law enforcement officers to detect, pursue, and apprehend violators within the confines of the work zone. To enhance the effectiveness of law enforcement, automated Speed Photo-radar Enforcement (SPE) systems were used in Illinois work zones for the first time in 2006. An evaluation study (3) showed that SPE significantly decreased the average speed of traffic and the percentage of vehicles speeding. However speed reduction alone should not be the sole criteria for assessing the safety improvement created by law enforcement presence in work zones. For instance, headway reduction in conjunction with speed reduction is also a desirable outcome. However, headway reduction without speed reduction ends up decreasing spacing between vehicles which is not a desirable outcome. Hence headway analysis is required to explore possible disadvantages of using SPE. The necessity of this kind of study becomes clearer when one investigates the short headways. If the headway of a vehicle is less than the driver s reaction time, a sudden braking by the lead vehicle will increase the likelihood of rear-end collision for the following vehicle. The platooning and time headway characteristics in highway work zones was investigated by Benekohal and Sadeghhosseini (4) and Sadeghhosseini and Benekohal (5).They examined the effects of traffic volume on distribution of time headways and on the percentage of platooning vehicles. Other researchers also investigated and modeled car following characteristics, such as headways or gaps (6, 7, and 8). Although headway characteristics have been used widely in these analyses, none of them have addressed the issue when any enforcement activity was present. Understanding of the effects of law enforcement methods on headway distribution, safe car following behavior in the platoon, and the platoon size can help in better management of traffic in construction zones. Another issue about the implementation of the SPE system in work zones is whether the presence of the SPE van would change the drivers behavior leading to a reduction in safety compared to using the traditional method of a police car presence in work zones. Work zone safety deteriorates when vehicles suddenly change lanes, rapidly reduce their speeds, or maintain a short following distance as they approach the law enforcement vehicle. The objective of this paper is to investigate the headway distribution of platooning vehicles in work zones with and without law enforcement activities. Law enforcement includes police car presence and automated speed enforcement by the SPE van. In addition, platooning vehicles with a very short headways (<0.7 seconds) are examined. And finally, the behavior of vehicles near the law enforcement vehicles were recorded and analyzed in terms of braking and lane changing behavior. RESEARCH SCOPE AND METHODOLOGY Research Objectives The purpose of this study is to investigate the effect of different law enforcement methods on car following and driver behavior in interstate work zones. Two law enforcement activities, Police Car Presence (PCP) and Speed Photo-radar enforcement (SPE) were implemented and evaluated

3 in this study. For comparison, the base data (BASE) without any enforcement activity on site were collected. Field data were analyzed to address the following issues: Effects of law enforcement activities on car following behavior and platoonnng characteristics. Effects of the law enforcement activities on the headway distribution of platooning vehicles. Effects of the law enforcement activities on the number of vehicles traveling with very short headway. Effects of the enforcement activity on the frequency of braking and drivers lane change behavior. Methodology Vehicles are grouped into either free-flowing or platooning, and only vehicles traveling in platoon are analyzed in this paper. To distinguish free flowing vehicles from in-platoon vehicles, a minimum criterion of three-second headway was used based on the Highway Capacity Manual (9). This means that if the headway between vehicles was three seconds or greater, the following vehicle was considered as a free flowing vehicle. Two statistical test methods were conducted to compare the headway distribution. The student t-test was used to test the difference in the mean headways. The Kolmogorov-Smirnov (K-S) test was used to test the fitness of the cumulative distribution of headways among treatments for the number of very short headway; chi-square test was used to test the difference in the portion between different treatments. Automatic Speed Photo-Radar Enforcement The SPE system, implemented in Illinois work zones, is a self contained van with equipment that can measure speed and take pictures of speeders. A conventional radar with a range of approximate 0.5 mile monitors the speed of the approaching vehicles and displays it on an LED board mounted on top of the van. A speeding driver still has a chance to reduce their speeds and comply with the speed limit. An advanced radar (second radar) also measures the speed of vehicles when they are about 150 ft upstream of the van. If the speed is greater than a specified value, the second radar activates cameras that take pictures of the front and rear of the speeding vehicle. From the pictures the face of the driver and the license plate of the vehicle are obtained. A police officer sits in the van because Illinois law requires it, but he/she is not needed for system to operate. The officer in the van can only issue a citation for speeding vehicles, if the speed exceeds the speed limit by a predetermined threshold. Finally, after a quality control process, the SPE vendor processes the approved citations and mails them to the registered owners of the vehicle within 14 days. Additional details on the implementation of the speed photo enforcement in Illinois can be found in an earlier work by the authors (10). DATA COLLECTION AND REDUCTION Two work zones with different traffic flow characteristics were selected for data collection. The work zone on I-64 had some speeders and the traffic volume may be characterized as low-tomoderate. The work zone on I-55 had a lot more speeders and the traffic volume was moderateto-high for a work zone. Further descriptions of the sites are given below. I-64 Eastbound in southern Illinois near St Louis, MO:

4 This dataset was collected during off-peak hours in the morning on a typical weekday. The construction zone at this site was for adding a third lane in the median. Concrete barriers separated the work area from the traveled lanes. The work zone extent was around 7 miles long. The starting milepost was 9 and treatments were placed around milepost 14. I-55 Northbound near Chicago, IL: This dataset was collected during off-peak hours in the afternoon on a typical weekday. In this construction zone, lane addition and bridge deck repair was taking place at the time of data collection. The work zone extent was about 7 miles long. The starting milepost was 255 and treatments were placed around milepost 258. At this site, there was a sign asking trucks to use the left lane (the median lane). In both the work zones, the posted speed limit was 55 mph. Two lanes were open to through traffic and a normal-width-right-hand-side shoulder was available at the data collection locations. Data was collected at a location several hundred feet downstream of where the speed control treatment was located. This distance provided sufficient room for the drivers who may have delayed reaction to the speed control treatments. The speed control treatments were visible at least 0.5 miles or more before the drivers reached them. Two types of data were collected using camcorders: Headway data: the traffic flow and headway data were collected using two markers that were about 200 feet apart. All the data collection equipment was placed outside of the shoulder with no interference to the traffic stream. The camcorder was placed far from the traveled lane such that drivers could not easily see it in order to reduce any potential effect on the speeds of vehicles. Vehicle braking and lane change behavior: these data were only collected in the I-55 work zone site. A camcorder was placed on a nearby overpass in the work zone and aimed at the rear of vehicle approaching the treatment location. The overpass is about 0.6 miles in advance of the site where the SPE or police car was placed. For data reduction, all recorded tapes were time stamped with frame numbers allowing the accuracy in time measurements to be 0.033 second (one frame). The time at which a vehicle passed the second marker, the vehicle type (cars or trucks) and traveling lane (shoulder lane or median lane) were reduced for headway calculation. RESULTS AND ANALYSIS Data Overview The data reduction time, total number of vehicles observed, overall mean headway and the percentage of vehicles in platoon are listed in Table 1. Generally, the proportion of vehicles in platoon increased when the traffic volume increased, and the overall mean headways decreased when the traffic volume increased. However, the mean headways of vehicles in the platoon were independent of the traffic volume. This supports the previous research (11) that headway distribution on non work zone highways was independent of the traffic volume. In a work zone, the effect of law enforcement on the headway of platooning vehicles is more important than their effects on all headways, thus the focus in this paper was on the platoonning vehicles only.

5 TABLE 1 Data Collection Components and Headway of Vehicle in Platoon Cars Trucks Site Treatment Lane I-64 I-55 BASE Police Car Presence SPE Van BASE Police Car Presence SPE Van Shoulder Shoulder Shoulder Shoulder Shoulder Shoulder Data period (mins) 40 90 60 60 60 60 Volume (veh/hr) 414 770 311 672 489 811 808 1094 765 995 774 1028 Total sample size 276 513 466 1008 489 811 808 1094 765 995 774 1028 Overall mean headway (sec) 7.69 3.66 9.08 3.99 5.85 3.36 2.43 2.80 2.61 3.26 2.44 2.95 No. of veh. 134 303 201 583 277 517 653 783 608 671 626 740 Vehicles in platoon % of veh. 49% 59% 67% 43% 57% 64% 81% 72% 79% 67% 81% 72% Mean headway (sec) 1.48 1.77 1.68 1.77 1.61 1.84 1.41 1.54 1.43 1.54 1.49 1.57 Volume (veh/hr) 102 255 67 209 91 220 341 113 382 72 379 90 Total sample size 68 170 100 314 91 220 341 113 382 72 379 90 Overall Mean headway (sec) 6.36 4.06 11.54 4.39 7.85 4.00 4.47 3.89 4.08 4.52 4.17 4.37 No. of veh 33 92 25 132 40 105 170 65 224 25 211 44 Vehicles in platoon % of veh 49% 54% 58% 25% 44% 48% 50% 58% 59% 35% 56% 49% Mean headway (sec) HEADWAY DISTRIBUTION OF PLATOONING VEHICLES Headway Distribution in travel lane The cumulative distribution of headways by vehicle type and travel lane is shown in Figure 1. Generally, cars have a smooth cumulative distribution curve while trucks have a jumpy curve due to the fewer number of observations. The curves for cars are on the left side of the curves for trucks. Similarly, the curves for vehicles traveling in median lane are located on the left side of the curves for the shoulder lane. This indicates most cars have shorter time headway than trucks in a platoon. Similarly, vehicles traveling in the median lane have shorter time headway than vehicles traveling in the shoulder lane. However, when the law enforcement was implemented, the headway distribution curves for cars in median and shoulder lanes became closer together, indicating the effect of the treatment.. 1.65 2.01 1.82 2.03 1.82 1.97 1.99 1.68 1.86 1.76 1.94 1.93

6 100% 100% 80% 80% Percentage (%) Percentage (%) Percentage (%) 20% 20% 0% 0% (a) Base condition in I-64 (d) Base condition in I-55 100% 100% 80% 80% 20% 20% 0% 0% (b) Police Car Presence in I-64 (e) Police Car Presence in I-55 100% 100% 80% 80% Percentage (%) Percentage (%) Percentage (%) 20% 20% 0% 0% (c) SPE Van presence in I-64 (f) SPE Van presence in I-55 FIGURE 1 Headway cumulative distribution in platoon (headway < 3sec). Table 2 lists the results of comparing headways for vehicles traveling in median and shoulder lanes. For the base condition on I-64, the mean headways of vehicles in median and shoulder lanes were significantly different for both cars (1.48 sec to 1.77sec) and trucks (1.65 sec to 2.01 sec). When a police car was present, the mean headways of cars and trucks in the median lane were 1.68 sec and 1.82 sec, respectively, which were not significantly different from the mean headways of cars (1.77 sec) and trucks (2.03 sec) in shoulder lane. Similarly, the mean

7 headway of trucks in the median lane increased to 1.82 seconds when the SPE van was implemented. The mean headways for cars traveling in the median lane (1.61 sec) and shoulder lane (1.84 sec) were still significantly different since the mean headways of cars in both lanes increased. TABLE 2 Comparison of Platoonning Headway Distribution in Different Traveling Lane Cars Trucks Location Treatment Lane Sample Mean p-value Sample Mean p-value Size Headway Pr> t Pr>K-S Size Headway Pr> t Pr>K-S BASE 134 1.48 33 1.65 <0.001 <0.001 Shoulder 303 1.77 92 2.01 <0.01 <0.01 I-64 PCP 201 1.68 25 1.82 0.05 0.05 Shoulder 583 1.77 132 2.03 0.11 0.25 SPEV 277 1.61 40 1.82 <0.001 <0.001 Shoulder 517 1.84 105 1.97 0.16 0.28 BASE 653 1.41 170 1.99 <0.001 <0.001 Shoulder 783 1.54 65 1.68 <0.01 <0.01 I-55 PCP 608 1.43 224 1.86 <0.001 0.02 Shoulder 671 1.54 25 1.76 0.41 0.31 SPEV 626 1.49 211 1.94 0.01 0.04 Shoulder 740 1.57 44 1.93 0.92 0.97 In the higher speed site (I-55), the difference in mean headways for cars in median and shoulder lanes was significant whether law enforcement activity was present or not. The mean headway for trucks in median lane in BASE condition was significantly higher than the mean headway for truck in shoulder lane. However, the mean headways of trucks between median and shoulder lanes were not significantly different when either one of the enforcement methods was implemented. Effect of Law Enforcement on Headway distribution The headway cumulative distribution curves for cars under different enforcement treatments are shown in Figure 2. For the median lane on I-64, the distribution curve for the base condition is on the left side of the curves for law enforcement conditions It indicates the effect of law enforcement on the headway of vehicles in the median lane. The curves for PCP and SPE are close when headways are less than 1.7 seconds. This means that the proportions of headways less than 1.7 seconds are similar when the enforcement activities were present. In the shoulder lane, PCP curve is close to the BASE curve after 1.3 seconds indicating the portion of headways less than 1.2 seconds under the BASE condition is larger than the portion of headways when the police car was present.

8 100% 100% 80% 80% Percentage (%) Percentage (%) 20% 20% 0% 0% (a) Cars at median lane in I-64 (c) Cars at median lane in I-55 100% 100% 80% 80% Percentage (%) Percentage (%) 20% 20% 0% 0% (b) Cars at shoulder lane in I-64 (d) Cars at shoulder lane in I-55 FIGURE 2 Platooning headway distributions with law enforcement presence in work zones. For the I-55 work zone, all headway distribution curves are similar, indicating that the effects of law enforcement on platooning vehicles were limited at this site. Table 3 shows the difference in mean headways under different treatments. For the I-64 work zone, with 95% confidence, the mean headway of cars in median lane significantly increased (from 1.48 seconds to 1.61 seconds for SPE and to 1.68 seconds for PCP) when the SPE van or police car was present.. The difference in headways for PCP and SPE was not significant for the median lane. The mean headway of cars in the shoulder lane increased when the SPE van was present from 1.77 seconds to 1.84 seconds, but the difference was not statistical significant. The cars headway in the shoulder lane when the SPE van was present (1.84) was significantly different from that when the police car was present (1.77 seconds). The mean headway of trucks in median lane increased from 1.65 seconds to 1.82 seconds when either the police car or SPE van was present, but this increase was not statistically significant. Similarly, the mean headway difference for trucks in shoulder lane was not significant for the entire comparison.

9 TABLE 3 Comparison of Platoonning Headway When Law Enforcements Present Cars Trucks Vehicle Mean Headway p-vale Mean Headway p-vale Location Comparison Type Differenc Differenc vs Pr> t Pr>K-S vs Pr> t Pr>K-S e e BASE vs SPEV 1.48-1.61-0.13 0.038 * 0.069 + 1.65-1.82-0.17 0.244 0.436 BASE vs PCP 1.48-1.68-0.20 0.005 * 0.03 * 1.65-1.82-0.18 0.304 0.597 I-64 SPEV vs PCP 1.61-1.68-0.06 0.265 0.613 1.82-1.82 0.00 0.961 0.970 BASE vs SPEV 1.77-1.84-0.07 0.104 0.263 2.01-1.97 0.04 0.610 0.960 Shoulder BASE vs PCP 1.77-1.77 0.00 0.998 0.480 2.01-2.03-0.01 0.873 0.970 SPEV vs PCP 1.84-1.77 0.07 0.044 * 0.031* 1.97-2.03-0.05 0.467 0.636 BASE vs SPEV 1.41-1.49-0.08 0.013 * 0.016 * 1.99-1.94 0.04 0.453 0.435 BASE vs PCP 1.41-1.43-0.03 0.404 0.084 + 1.99-1.86 0.13 0.035 * 0.03 * I-55 SPEV vs PCP 1.49-1.43 0.05 0.096 + 0.057 + 1.94-1.86 0.08 0.132 0.055 + BASE vs SPEV 1.54-1.57-0.03 0.326 0.775 1.68-1.93-0.25 0.048 * 0.077 + Shoulder BASE vs PCP 1.54-1.54-0.01 0.777 0.989 1.68-1.76-0.08 0.620 0.924 SPEV vs PCP 1.57-1.54 0.02 0.512 0.541 1.93-1.76 0.17 0.281 0.434 Note: *: significantly different with 95% confidence level +: significantly different with 90% confidence level In the I-55 work zone, the mean headway of cars in the median and shoulder lanes increased when the law enforcement was present, however, the increase (from 1.41 seconds to 1.49 seconds) was statistically significant only for cars on median lane when SPE van was present. For trucks, the mean headway increase was significant only in the shoulder lane when the SPE was present (from 1.68 seconds to 1.93 seconds). The mean headways when the police car was present were not significantly different from the mean headways when the SPE was implemented. Knowing that the average headway due to SPE slightly increased, and considering the speed reduction due to SPE as reported by Benekohal et al. (10), it is concluded that SPE effects on headway was positive and it did not increase the overall chance of rear-end crashes in the work zones. To investigate the SPE effect on spacing of two successive vehicles, both speed and headway changes should be considered simultaneously, but such data were not available Very Short Headway Analysis Vehicles with very short headways have very little time to react when the corresponding lead vehicles slow down rapidly, thus they have a higher chance of being involved in a rear-end collision in work zones. In this study, a very short headway is defined as the headway less than 0.7 second. This threshold is selected based on the median brake reaction time of 0.666 seconds for alerted drivers (12). Table 4 lists the percent of cars traveling with very short headway and the results of Chisquare tests. Using a 95 percent confidence level, when χ 2 value is higher than 3.84 (χ 2, 0.05, 1), the difference of the portion of vehicles with very short is significant. On I-64, the proportion of cars traveling with a very short headway in the median lane significantly decreased from 9 percent to 1.5 percent when the SPE van was implemented; and it decreased to 2.5 percent when the police car was present. In the shoulder lane, very short headway was not observed when the SPE was implemented. In the BASE condition, 18 cars in the median lane had a very short headway. Assuming a car with a very short headway represents a potential rear-end crash, then 18 potential crashes per hour existed in the base condition. The law enforcement activity, either SPE or police car presence reduced, this number significantly for the I-64 work zone.

10 TABLE 4 Proportions of Cars Traveling with Very Short Headways and Comparison Location Lane I-64 Shoulder Both Lanes I-55 Shoulder Both Lanes Total Number Treatment of Vehicles in Platoon Number of Vehicle Headway <0.7 sec Vehicle per hr Proportion Comparison χ 2 Result BASE PCP 134 201 12 3 18.0 2.0 9.0% 1.5% BASE vs PCP BASE vs SPEV 10.47 8.46 diff diff SPEV 277 7 7.0 2.5% PCP vs SPEV 0.61 same BASE 303 2 3.0 0.7% BASE vs PCP 0.58 same PCP 583 7 4.7 1.2% BASE vs SPEV 3.42 same SPEV 517 0 0.0 0.0% PCP vs SPEV 6.25 diff BASE 437 14 21.0 3.2% BASE vs PCP 5.41 diff PCP 784 10 6.7 1.3% BASE vs SPEV 9.06 diff SPEV 794 7 7 0.9% PCP vs SPEV 0.57 same BASE 653 44 44 6.7% BASE vs PCP 1.51 same PCP 608 31 31 5.1% BASE vs SPEV 3.08 same SPEV 626 28 28 4.5% PCP vs SPEV 0.27 same BASE 783 35 35 4.5% BASE vs PCP 0.51 same PCP 671 25 25 3.7% BASE vs SPEV 2.86 same SPEV 740 21 21 2.8% PCP vs SPEV 0.88 same BASE PCP SPEV 1436 1279 1366 79 56 49 79 56 49 5.5% 4.4% 3.6% BASE vs PCP BASE vs SPEV PCP vs SPEV In the I-55 work zone, the portion of cars traveling with very short headway also decreased when the SPE van or police car was present, but the decreases were not statistically significant. The number of very short headways decreased from 79 to 56 or 49 per hour when the police car or the SPE van was present, respectively. The Effect on Drivers Lane Changing and Braking Behavior In addition to the travel speed and car following headway, drivers lane changing and braking behavior were analyzed to address the concern that SPE van may increase the probability of vehicle braking or lane changing as they approach the van compared to the traditional law enforcement method. Thus, vehicle braking and lane changing lane data were collected. This type of data was collected in the I-55 work zone, and but was not feasible in the I-64 work zone. Figure 3 shows the percentage of vehicles applying brakes and changing lanes when the SPE van or police car was placed on the site. A higher percentage of cars applying brakes were found when the SPE was implemented compared to when the police car was present in either the median or shoulder lane. The percent of cars changing lane was also greater when the SPE van was placed except for the cars in the shoulder lane. 1.81 5.88 1.08 same diff same

11 35% 35% 30% 30% Percentage (%) 25% 20% 15% 10% 5% 0% 20.8% 18.8% Brake(BR) 19.5% 15.8% 21.9% 21.0% 8.5% 6.3% 4.9% 4.8% 4.9% 4.6% Lane Change (LC) Brake(BR) Lane Change (LC) Brake(BR) Lane Change (LC) Percentage (%) 25% 20% 15% 10% 5% 0% 14.0% 10.9% Brake(BR) 3.6% 1.9% Lane Change (LC) 12.3% 10.1% Brake(BR) 1.6% 0.2% Lane Change (LC) 22.1% Brake(BR) 14.9% 3.2% 21.6% Lane Change (LC) All Shoulder All Shoulder (a) Passenger Cars (b) Trucks FIGURE 3 Driver lane change and braking behavior in I-55. Similar trends in lane changing were found for trucks except for the truck in the shoulder lane. The percentage of trucks changing lanes from the shoulder to the median lane was very high because the trucks on I-55 work zone were required to travel in the median lane. When truck drivers noticed the police presence, most trucks changed lanes from shoulder to median to comply with the local requirement. Drivers lane changing and braking behaviors when SPE was present were compared to when police car was present using Chi-square test with a 95% confidence level, see Table 5. The difference in braking behavior was not significant. However, lane changing behavior was significantly different for cars traveling in the median lane; also for trucks traveling in the shoulder lane. The portion of cars changing lanes from the median lane increased from 5 percent to 8 percent when the SPE van was on site compared to when the police car was present. It should be noted that only several trucks went from median lane to shoulder lane when the enforcement activities were present because the trucks were supposed to stay in the median lane. However, a significant number of trucks changed lanes from shoulder to median when the police car was present.

12 TABLE 5 Comparison Test Results of Drivers Lane Change and Braking Behaviors Total Making Brakes Changing Lane Lane Veh Type Treatment Sample Size N % χ 2 N % χ 2 SPE 1837 383 Cars Both PCP 1764 332 Lanes SPE 527 74 Trucks PCP 478 52 SPE 814 159 Cars PCP 747 118 SPE 432 53 Trucks PCP 404 41 SPE 1023 224 Cars PCP 1017 214 Shoulder SPE 95 21 Trucks PCP 74 11 Note: *: significantly different with 95% confidence level 21% 19% 14% 11% 20% 16% 12% 10% 22% 21% 22% 15% 2.33 2.29 3.73 0.94 0.22 1.42 116 86 10 17 69 36 7 1 47 50 3 16 6% 5% 2% 4% 8% 5% 2% 0% 5% 5% 3% 22% 3.52 2.64 8.30 * 4.15 * 0.12 14.21 * CONCLUSION AND RECOMMENDATION To provide more traffic operational characteristics and help in better management of traffic and safety in construction zones, this paper investigated the headway distribution of platooning vehicles in work zones with and without law enforcement activities. Law enforcement includes police car presence and automated speed enforcement (SPE). Data came from two work zones. The results indicate that the mean headways of cars in the median lane significantly increased in both work zones when the SPE van was present compared to the base condition. When the police car was present, the significant increase was found only in the work zone with a lower average speed. For trucks, the mean headway increase was significant only in shoulder lane when the SPE was present. The mean headways when the police car was present were not significantly different than those with SPE van. The increase in mean headway is considered as beneficial effects of the traditional law enforcement and SPE system. Furthermore, the law enforcement presence in work zones, either the SPE or police patrol vehicle presence, decreased the number of vehicles traveling with a very short headway (<0.7 seconds). The proportion of cars traveling with a very short headway in the median lane significantly decreased when either the police car or SPE van was present in one work zone (I- 64). In the shoulder lane, very short headway was not observed when the SPE was implemented. In the other work zone (I-55), the portion of cars traveling with very short headways also decreased when the SPE van or police car was present, but the decreases were not statistically significant. In addition, the difference in braking behavior was not significant when the SPE case was compared to police car presence. However, lane changing behavior was significantly different for cars traveling in the median lane; also for trucks traveling in the shoulder lane. The proportion of cars changing lanes from the median lane to the shoulder was 8 percent when the SPE van was on site compared to 5 percent when the police car was present. Only several trucks went from median lane to shoulder lane when the enforcement activities were present because

13 they were supposed to stay in the median lane. A significant number of trucks changed lanes from shoulder to median when the police car was present. It is recommended to continue using police car presence or the SPE system to reduce work zone speed and improve traffic flow condition. The use of SPE is recommended particularly in work zones where the lateral space does not allow parking a car on the shoulder (police or violator) or where the traditional law enforcement has limited effectiveness. Future research for modeling the headway distribution of other speed reduction treatments in work zone is recommended. REFERENCE 1. Richards, S. H., and C. L. Dudek. Implementation of Work-Zone Speed Control Measures. In Transportation Research Record 1086, TRB, National Research Council, Washington, D.C., 1986, pp. 36~42. 2. Richards, S.H., R.C. Wunderlich, and C.L Dudek. Field Evaluaiton of Work Zone Speed Control Techniques, In Transportation Research Record 1035, TRB, National Research Council, Washington, D.C., 1985, pp. 66~78. 3. Benekohal, R. F., M. Chitturi, A. Hajbabaie, M.-H. Wang and J.C. Medina. Automated Speed Photo Enforcement Effects on Speeds in Work Zones. In Transportation Research Record: Journal of the Transportation Research Board, No. 2055, Transportation Research Board of the National Academies, Washington, D.C., 2008, pp. 11-20. 4. Benekohal, R. F., and S. Sadeghhosseini, Platooning Characteristics of Vehicles in Highway Construction Zones, Modeling and Simulation, 22, 1991, pp.16-23. 5. Sadeghhosseini, S. and R.F. Benekohal. 1995. Space Headway and Safety of Platooning Highway Traffic, Traffic Congestion and Traffic Safety in the 21st Century, Chicago, Illinois, pp. 472-478. 6. Sun, D. and R. F. Benekohal, Analysis of Work Zone Gaps and Rear-End Collision Probability, Journal of Transportation and Statistics volume 8 No. 2, 2005, pp. 71-86. 7. Pesti, G. and M. A. Brewer, Variable Time Headway Criteria to Improve Reliability of Speed Control System Evaluations, In Transportation Research Record: Journal of the Transportation Research Board, No. 1973, Transportation Research Board of the National Academies, Washington, D.C., 2006, pp. 89 94. 8. Zwahlen, H. T., F. Oner and K. R. Suravaram, Approximated Headway Distributions of Free-Flowing Traffic on Ohio Freeways for Work Zone Traffic Simulations, In Transportation Research Record: Journal of the Transportation Research Board, No. 1999, Transportation Research Board of the National Academies, Washington, D.C., 2007, pp. 131 140. 9. Highway Capacity Manual, Special Report No. 209, TRB, National Research Council, Washington, D.C., 2000. 10. Benekohal R. F., A. Hajbabaie, J. C. Medina, M. Wang, and M. V. Chitturi. (2008). Speed Photo-Radar Enforcement Evaluation in Illinois Work Zones. Report to IDOT. University of Illinois, Urbana, IL, 2008 11. Wasielewski, P. 1979. Car-Following Headways on Freeways Interpreted by the Semi- Poisson Headway Distribution Model. Transportation Science 13(1):36 55. 12. Traffic Flow Theory a Monograph, TRB, National Research Council, Washington, D.C., 1975.