Work zone speed limits and management of work zone speeds continue to be critical areas of concern for transportation agencies . This field study sought to evaluate select strategies for improving compliance with work zone speed limits , which included a spe ed feedback trailer (SFT) and the presence of law enforcement. A SFT was tested at the start and end of the taper within a freeway work zone single -lane closure to determine which position provided the most favorable speed reduction effects. In general, the magnitude of the speed reduction effects was the greatest in the general proximity of the SFT. Accordingly, positioning the SFT near the end of the taper led to lower speeds for a more sustained distance into the work zone compared to when the SFT was positioned near the start of the taper . When SFT was positioned near end of the taper , the average speed was 1.5 mph and 0.8 mph lower at the SFT location and at the end of the measurement area (2,150 -ft beyond the start of the taper) compared to no SFT conditions. The second evaluation assessed the effectiveness of a specialized work zone enforcement strategy that included a covert speed measurement vehicle positioned near the end of the work zone along with four police cars positione d just beyond the end of the work zone to stop speeding drivers. The visible presence of law enforcement at this location reduced work zone speed by approximately 5 mph, which increased to 7 mph shortly beyond the end of the work zone as motorists passed by the police cars positioned on the shoulder. These speed reduction effects were only observed when at least one law enforcement vehicle was visibly present at the site. Keywords: work zones , enforcement presence, speed feedback trailers , speed limit increase , freeways 2 Mahmud et al. INTRODUCTION AND BACKGROUND Speed management continues to be a high priority nationally, both in regards to setting appropriate speed limits and the degree to which drivers comply with the work zone speed limits. One area that r emains a particular challenge for speed management is construction work zones, particularly as maximum speed limits continue to increase nationwide. In 20 20, there were an estimated 102,000 work zone crashes, resulting in 4 4,000 injuries and 857 fatalitie s, including 1 56 workers (1). Many of these crashes can be attributed to excessive speed or speed vari ance, as speeding has been identified as a contributory factor in approximately 25 percent of all work zone fatal crashes (2). Consequently, setting appropriate work zone speed limits and ensuring compliance to it is an important component towards improving work zone safety. The Manual on Uniform Traffic Control Devices (MUTCD) recommends that reduced speed limits should be used only where conditions or restrictive features are present (3). The MUTCD notes that frequent changes in th e speed limit should be avoided and that reductions should not exceed 10 mph. Further, where a speed reduction of more than 10 mph is required, additional driver notification should be provided. Recently, with the increase in speed limits, many work zones now require to drop speed above 10 mph, particularly, at the locations near workers ' presence. The magnitude of these speed reductions relies on several factors. For example in Michigan , the work zone speed limit policy considers various factors such as the existing speed limit, type of work activity, presence of construction workers, and the presence of channelizing devices or concrete barriers (4). One important concern in establishing work zone speed limits is the degree to which drivers comply with these limit s. Several studies have concluded that although certain measures can reduce speeds, motorists generally tend to regulate their speeds as they feel necessary (5, 6). Work zone speeds have also been shown to vary based on free -flow speeds under normal conditions, as well as under various levels of traffic volume, and at different times of day (7). The physical characteristic of the work zone and the associated temporary traffic control plan also play an important role. For example, reduced lane widths have been sh own to be effective in reducing average speeds , though it decrease s the capacity (8). The presence of workers and the level of work activity that is ongoing are also important concerns as research has shown that speeds tend to be lower during periods of construction activity. These are also the periods during which the risks to workers are the highest, leading to states such as Michigan introducing lower work zone speed limits where workers are present. However, speeds often remain above these limits r egardless of whether the activity is ongoing (8). From an agency perspective, additional research is warranted to assess the degree to which drivers comply with work zone speed limits under various conditions. National Cooperative Highway Research Pr ogram (NCHRP ) Synthesis 482 (9) focused on speed management strategies for work zones on high -speed roads. This review focused on various speed management techniques, including speed management devices, changes in the physical driving environment, and enforcement. Several other studies have also shown enforcement to reduce speeds (6, 10–12) and these reductions tend to be greatest when enforcement activity is highest (12). However , these effects dissipate almost immediately after enforcement activities cease (11, 12). The efficacy of enforcement also tends to be influenced by the normal operating speeds of the roadway, as well as details of the temporary traffic control plan (12). NCHRP Report 746 details pertinent information about the administration of work zone speed enforcement, along with related issues such as determining how much enforcement is required and where to position police vehicles (13). 3 Mahmud et al. Given practical difficulties that arise with speed enforcement in work zones, there remains a clear need to examine how other strategies can help to maintain work zone speed limit compliance. Different speed display signs , including speed feedback trailer (SFT) were found to be an effective speed reduction strategy and are being utilized across different states (14–20). However, speeding over the work zone speed limits, particularly , at the further reduced speed areas near the workers continue to be a stat ewide issue. To this end, a study was designed to find empirical evidence in support of the most effective means of maintaining acceptable levels of compliance with work zone speed limits. This include s consider ation of how temporary traffic control devices such as SFT and the presence of law enforcement impact driver speed s while approaching, entering, and exiting the work zone . The objective of this study is to ut ilize vehicle speed profiles while approaching and traversing through different critical locations in the work zone and this will help to fully understand how and when driver behavior changes in response to different speed reduction strategies. This will ultimat ely help to p rovide recommendations for specific traffic control devices and other speeding -related countermeasures of interest. METHODOLOGY A series of field evaluation s were performed at t wo freeway work zones to determine the effectiveness of SFT and the presence of law enforcement in reducing drivers’ speeds . The following subsections detail various aspects of this evaluation, including study design, study sites, data collection test conditions, speed data collection and processing, and statistical met hod utilized for this analysis. Study Design The study first evaluated the effectiveness of SFT in reducing drivers' speed while approaching and entering a freeway lane closure. The position of the SFT was varied to identify the optimal location for driver speed reduction while entering and traveling through the lane closure. The SFT utilized in this study was a solar -powered trailer -mounted radar speed feedback sign with a high - definition full -matrix display. The sign was capable of displaying real -time speed information (in mph) and feedback messages to the approaching vehicles. The sign assembly, as shown in Figure 1, includ es a static 60 mph speed limit sign , which was the work zone speed limit at the freeway lane closure study site when no workers were present , a 35 -in by 36 -in feedback display capable of displaying 20-inch speed display digit s, a smaller black -on-white “YOUR SPEED” panel on top of the display panel, and a solar panel on top of the sign. The sign assembly was mounted on a trailer t hat allowed the sign to be quickly moved to different areas within the work zone. During the operation, the sign was positioned on the left shoulder behind orange barrels, keeping an adequate lateral buffer from the open travel lane on the right. The sign uses Doppler radar capable of detecting vehicles up to 2,000 ft in advance of its location . For the purpose of this study , the feedback sign was programmed to display the speed of the approaching vehicles alternating with a “SLOW DOWN ” feedback message , wh ich is consistent with the Michigan Department of Transportation ( MDOT )’s draft special provision for dynamic speed feedback signs. This feedback messaging strategy was found to be very effective in prior evaluations at freeway exit ramps and rural highway curves (35–37). 4 Mahmud et al. FIGURE 1. Speed feedback trailer positioned at the end of taper at WB I -69 work zone A second field study was performed to evaluate the effect of law enforcement presence on the behavior of drivers traversing a freeway lane closure. The enforcement a nd corresponding data collection areas were positioned near the end of the work area. Workers were present during the entire da ta collection period, and consequently, the 45 mph speed limit was in effect during the enforcement operation. To remain covert, the officer responsible for monitoring work zone travel speeds was seated in an MDOT work truck positioned near the end of the work area, approximately 600 ft upstream of the end of the work zone traffic control , as displayed in Figure 2 . A total of four additional Michigan State Police vehicles were parked on the shoulder 150 -ft downstream from the end of the work zone and were visible to motorists traversing the work area. The speed monitoring officer, who utilized LIDAR to measure speeds, would relay information on speeding motorists to the downstream officers. The downstream officers would then pursue, stop, and potentially cite the offending vehicles. The cluster of police cars was positioned downstream of the work zone so that motorists could be stopped beyond the end of the work zone to minimize interference with work zone operations. 5 Mahmud et al. FIGURE 2. SB I -75 work zone data collection setup and law enforcement vehicle locations Study Sites Two freeway work zones with lane closures were selected for this evaluation, including one on WB I -69 and the other on the SB I -75. The fir st work zone on WB I -69 is a two -lane limited -access freeway with a speed limit of 75 mph for passenger cars and 65 mph for heavy vehicles. The left lane was temporarily closed using orange barrels for road maintenance work (Figure 1) . The work zone contai ns all the typical traffic control elements according to the MDOT. In addition to that, three sets of transverse rumble strips were installed prior to entering the single -lane operation segments. The spacing between the individual rumble strip decreased wi th the proximity to the work zone start, providing drivers with additional alerts to reduce the speed before entering the work zone. The other work zone was on SB I -75, which is a four -lane limited -access freeway in Saginaw County with a non-work zone speed limit of 70 mph for passenger cars and 65 mph for heavy vehicles. The work zone consisted of closure of the rightmost lane for road maintenance work , leaving the three left lanes open. Similar to the WB I -69 site, this work zone also contained all the el ements according to the MDOT. Data Collection Test Conditions WB I -69 work zone was utilized to evaluate the effectiveness of the SFT and identify the optimal location of the SFT for sustained speed reductions after entering the work zone. To assess the effects of SFT position, the SFT was first installed at the start of the taper and then moved approximately 800 ft downstream to the end of the taper. Data were collected for a total of three test conditions, which included: • Inactive SFT, • Active SFT at ta per start, and Speed measurement vehicle (also used for upstream speed data collection) Pursuing enforcement vehicles (up to four vehicles) Downstream speed data collection vehicle Work area 6 Mahmud et al. • Active SFT at taper end. Data were collected for all three conditions within the same day. This allowed for controlling external factors such as weather and work activity that may otherwise contribute to speed variation. Speed data were col lected using a sequence of three handheld LIDAR guns operated by technicians from within separate vehicles parked just beyond the shoulder. This method allowed for continuous measurement of speeds for vehicles approaching and entering the work zone. Vehicl es were tracked for over 4,500 ft covering the approach, tapered section, and inside of the work zone. Locations of the data collectors, rumble strips, taper start, taper end, and SFT (both locations) are displayed in Figure 3. FIGURE 3. WB I -69 work zone and data collection setup The other work zone, SB I -75, was used to evaluate the effect of law enforcement presence on driver behavior. This evaluation was conducted near the end of the work zone area, with workers present during the entire data collection period. Data were collected before and during the police enforcement. During the enforcement period, at many times, none of the downstream police cars were present at the site due to the frequency of traffic stops for vehicles caught speeding in the work zone. This allowed for the collection of data with no visible police present during the enforcement period. Data were collected for a total of three test conditions, which included: • Before enforcement • During enforcement, at least one downstream police car present • During enforcement, all downstream police cars absent For this evaluation, d ata were also collected within a single day for all three enforcement conditions. Vehicle speed data at this site were colle cted by the research team using a sequence of two handheld LIDAR guns operated by technicians from within separate vehicles parked just beyond the shoulder . Details on the data collection setup are displayed in Figure 2. Speed Data Collection and Processi ng Speed data were collected using a series of handheld LIDAR guns operated by a team of technicians positioned within unmarked vehicles on the roadside within the work zone. The LIDAR guns were used to continuously track individual vehicle speeds througho ut the entire target area at the work zone . The LIDAR guns utilized in this study were ProLaser III manufactured by 7 Mahmud et al. Kustom Signals Inc. These devices are able to measure vehicular speed and distance three times per second with an accuracy of ±1 mph at a ra nge of 6,000 ft. The LIDAR data collection vehicles were positioned on the roadside at strategic locations that were away from any critical speed measurement points (e.g., start of taper, end of the taper, work area) to minimize the influence of the data c ollection vehicle on drivers. The same data collection procedures were utilized across all data collection periods for a given evaluation. Most of the data were collected under dry daylight conditions on weekdays between the hours of 10:00 AM and 4:00 PM. During the data collection, the upstream data collector would begin to track each subject vehicle and continue tracking at least 100 ft beyond the downstream LIDAR technician. At this point, the tracking responsibilities were then transferred to the down stream technician, who would track each subject vehicle until the next vehicle or over the remaining distance. The data collectors communicated via cellular communications to ensure a seamless “hand‐off” of the LIDAR speed tracking as each subject vehicle proceeded through the site. In doing so, the upstream technician would convey the type and color of each subject vehicle to the downstream LIDAR collector. To isolate driver response to the traffic control devices, only freely flowing vehicles (e.g., minimum 3-second headway) were tracked. Each LIDAR gun was connected to a laptop using a data transfer cable, which allowed for all measurements to be recorded in real -time using proprietary software. The computer LIDAR recordings included timestamps, distances, and speeds for each measurement. After completion of the LIDAR tracking for each subject vehicle, all data collectors entered remarks on the type and color of the vehicle, in addition to any other comments. This information was later used to combine the data sets into a continuous speed profile for each subject while traversing throug h the site. a) Raw LIDAR data ( n=100 vehicles ) b) LIDAR data interpolated at 50 -ft increments FIGURE 4. Raw and interpolated vehicle speed data from LIDAR After completion of the LIDAR tracking data collection from the field, all files from the LIDAR technicians were joined using the vehicle information recorded in the comments. As the relative distances between the LIDAR collectors and the fixed reference points at the sites were known (e.g., start of taper, end of taper/beginning of lane closure) , all distances were converted to be relative to the fixed point on the road. An example representation of the output of this process is shown in Figure 4a for vehicles approaching WB I -69 when SFT was at the taper end . Because LIDAR speeds can not be measured at the same locations on the roadway for every vehicle, it was necessary to convert this data to a series of spot speeds using an interpolation techniq ue, thereby allowing speeds to be assessed at specific reference points. The combined raw data were linearly interpolated at 1 -ft increments using the adjacent speeds. Interpolated speeds were then calculated Average speed 8 Mahmud et al. at 50 ft interval s using a reference point on t he road, as shown in Figure 4b. Compiling the data in this manner provide d a robust array of spot speeds throughout each study site. Statistical Methods Vehicle speed profiles were analyzed to determine the effects of SFT, law enforcement presence , and the various conditions of their use. First, to determine any obvious trends in the data, sources for potential bias, and data distributions, a preliminary comparison of the descriptive statistics (i.e., mean, standard deviation, percentiles, etc.) and grap hical representations (i.e., frequency distribution, box plot, scatterplot) for the vehicular data was performed across the data collection periods. From there, statistical models were developed to estimate the speed at different locations to evaluate the effectiveness of these speed reduction strategies. All the analyses were performed using statistical software RStudio. Speeds were analyzed using multiple linear regression. The general form of the multiple linear regression is shown in Equation 1: 𝑌𝑖= 𝛽0+𝛽1𝑋𝑖1+𝛽2𝑋𝑖2+⋯+𝛽𝑘𝑋𝑖𝑘+𝜀𝑖 (1) where Yi is the measured speed for vehicle i, 𝑋𝑖1 to 𝑋𝑖𝑘 are independent variables affecting the dependent variables (including test condition), β0 is an intercept, β1 to βk are estimated regression coefficients for each independent variable, and 𝜀𝑖 is a normally distributed error term with variance 𝜎2. RESULTS AND DISCUSSION Effect of SFT Positions on Work Zone Speeds The LIDAR speed data from the WB I -69 work zone were joined, organized, and coded into a single file . The final data set included complete speed profiles for 297 vehicle observations for the three conditions evaluated . The average vehicle speed profiles for all three test conditions are displayed in Figure 5. The figure reveals a few important insights on the effect of SFT location within the work zone, which are summarized as follows: • SFT Po sitioned at Start of Taper : When the SFT was positioned at the start of the taper, vehicles began to decelerate more rapidly in advance of the taper compared to the other conditions. By the time vehicles had reached the start of the taper, average speeds were approximately 1 mph lower than the inactive test condition . Vehicles continued to decelerate through the taper, with minimum speeds achieved by the end of the taper. These speeds were generally sustained through the end of the LIDAR tracking range (i.e., more than 1 ,300-ft beyond the end of the taper). • SFT Positioned at End of Taper: When the SFT was positioned at the end of the taper, rapid deceleration did not begin to occur until the start of the taper, which was further downstream compared to the start of the taper. However, deceleration was sustained for a longer duration, and by the time vehicles had reached the end of the taper, average speeds were approximately 1 mph lower than the inactive test condition . Additionally, vehicles continued to decelerate beyond the end of the taper, reaching a minimum speed approximately 350 -ft beyond the end of the taper. These speeds were generally sustained through the end of the LIDAR tracking range (i.e., more than 13 00-ft beyond the end of the taper) but did begin to increase gradually . 9 Mahmud et al. FIGURE 5. Average vehicle speed profile for different locations of speed feedback trailer In order to confirm the graphical observations presented here, t he vehicle speed data were statistically analyzed to determine the effects of SFT operation and i nstallation location on drivers’ speed selection while approaching and entering the work zone . Prior to a nalyzing the data, the speed measurements were binned at 50 -ft increments, which covered from 2,350 ft upstream of the start of the work zone taper to 2,150 beyond the start of the work zone taper – for a total tracking distance of 4,500 ft . Binning the d ata in this manner allowed for the speed -reduction effects of the SFT to be statistically analyzed at various locations of interest throughout the work zone. Separate multiple linear regression models were generated for vehicle speed measured at the follo wing locations of interest within the work zone: • Speed at the start of taper; • Speed at the end of taper (800 ft beyond the start of taper); • Speed 1,300 ft beyond the start of taper; • Speed 1,800 ft beyond the end of taper; and • Speed 2, 150 ft beyond the end of taper. The primary independent variables entered into each regression model were as follows: • SFT operation and location within the work zone: o Inactive; o Active and positioned at the start of taper; o Active and positioned at the end of taper; • Vehicle t ype: o Passenger vehicle; o Heavy vehicle; and • Speed 2,350 ft upstream of the taper start. While evaluating the effects of SFT, the vehicle speed at the furthest upstream point ( i.e., 2,350 ft upstream of the taper start) was treated as an independent variable (covariate) in the regression models. This allowed for variations in the normal speeding tendencies of drivers between the data collection periods to be controlled for within the models. Controlling for 10 Mahmud et al. variations in upstream speed between the data collection periods was important, as the upstream speeds were found to be slightly higher during the two active SFT test conditions (see Figure 5), which suggested a slightly faster sa mple of drivers during the two active SFT test conditions. Analysis of the data in this manner allowed for a direct comparison of the speed reduction effects of each SFT test condition at various locations within the work zone while controlling for vehicl e type and speed measured upstream of the work zone. The m ultiple linear regression results for speed s across the three SFT conditions are presented in Table 1. The parameter estimates from Table 1 can be directly interpreted as the difference in mean spe ed compared to the base condition (i.e., the inactive SFT ). For example, compared to the inactive SFT , mean speeds at the taper start were 1.9 mph lower with the SFT positioned at the start of the taper and 0.5 mph lower for active SFT at the end of the t aper. The results displayed in Table 1 suggest that the SFT operation and location had a statistically significant effect on driver speed selection while traversing the work zone. Speed at the start of the taper was significantly lower when the SFT was po sitioned at the start of the taper compared to the inactive SFT and SFT at the end of the taper. Similarly, speed at the end of the taper was significantly lower when SFT was positioned at the end of the taper. These findings indicate that the speed redu ctions were the greatest at or near the SFT itself. This finding has implications on the positioning of the SFT with respect to the work area, which is described in further detail in the paragraphs that follow. Assessment of driver speed selection beyond the end of the taper found that the SFT positioned at the end of the taper provided a more sustained speed -reduction benefit compared to the SFT positioned at the start of the taper. With the SFT positioned at t he end of the taper , speeds continued to decrease beyond the end of the taper, with the lowest overall vehicle speeds in this condition occurring approximately 350 ft beyond the end of the taper. Speeds measured 500 -ft beyond the end of the taper were 1.2 mph lower with the SFT positioned at the end of the taper compared to at the start of the taper. Similarly, when vehicles had reached 1000 -ft beyond the taper, speeds were 1.0 mph lower with the SPF positioned at the end of the taper versus at the start of the taper. The results indicate that while the SFT positioned at the start of the taper resulted in an early reduction in speed, the effectiveness of the SFT began to diminish earlier than when the SFT was positioned at the end of the taper. B y the time vehicles had reached 500-ft beyond the end of the taper, speeds with the SFT positioned at the start of the taper were not statistically different than those measured with the inactive SFT. On the other hand, the SFT placed at the end of the taper resulted in later driver response, but with speed reduction s that were significantly greater by the end of the taper, and sustained a much greater distance into the work zone, continuing to the end of the measurement area (2,150 -ft beyond the start o f the taper). This finding suggests that the SFT (or series of SFTs) be positioned near the work area so that the speed reduction effect of the SFT is maximized near the workers. An interesting aspect of this evaluation was the magnitude of speed red uction. While earlier studies have reported a reduction of 2-10 mph in the average work zone speeds with the SFT present (15, 18, 20, 28, 38), this study found a decrease of up to 1.5 mph in the average speed. This may be due to the presence of three sets of temporary rumble strips at the site, which follows MDOT standards for long -term freeway lane closures. As the rumble strips were present from the initial implementation of the work zone and associated traffic control, it was not possible to discern the effects of rumble strips across the various SFT test conditions. 11 Mahmud et al. TABLE 1. Multiple linear regression results for speeds of vehicles traversing the work zone as a function of SFT location and operation Parameters Estimate Std. Error t-value p-value Speed at Start of Taper Intercept 20.395 3.061 6.662 <0.001 Upstream Speed 0.648 0.043 14.994 <0.001 Passenger Cars Base Condition Heavy Vehicles 0.120 0.646 0.186 0.852 Inactive Speed Trailer Active Speed Trailer at Taper Start -1.886 0.578 -3.261 0.001 Active Speed Trailer at Taper End -0.539 0.579 -0.931 0.353 Speed at End of Taper (800 -ft Beyond Start of Taper) Intercept 28.902 3.103 9.314 <0.001 Upstream Speed 0.494 0.044 11.274 <0.001 Passenger Cars Base Condition Heavy Vehicles -0.019 0.655 -0.030 0.976 Inactive Speed Trailer Base Condition Active Speed Trailer at Taper Start -1.013 0.586 -1.729 0.085 Active Speed Trailer at Taper End -1.457 0.587 -2.483 0.014 Speed 1,300 -ft Beyond Start of Taper Intercept 33.804 3.223 10.490 <0.001 Upstream Speed 0.419 0.046 9.208 <0.001 Passenger Cars Base Condition Heavy Vehicles -0.439 0.680 -0.645 0.519 Inactive Speed Trailer Base Condition Active Speed Trailer at Taper Start -0.345 0.609 -0.567 0.571 Active Speed Trailer at Taper End -1.584 0.610 -2.599 0.010 Speed 1,800 -ft Beyond Start of Taper Intercept 36.826 3.240 11.366 <0.001 Upstream Speed 0.377 0.046 8.234 <0.001 Passenger Cars Base Condition Heavy Vehicles -0.943 0.684 -1.379 0.169 Inactive Speed Trailer Base Condition Active Speed Trailer at Taper Start -0.276 0.612 -0.452 0.652 Active Speed Trailer at Taper End -1.233 0.613 -2.012 0.045 Speed 2,150-ft Beyond Start of Taper Intercept 38.362 3.374 11.369 <0.001 Upstream Speed 0.349 0.048 7.334 <0.001 Passenger Cars Base Condition Heavy Vehicles -0.927 0.712 -1.300 0.194 Inactive Speed Trailer Base Condition Active Speed Trailer at Taper Start 0.092 0.637 0.145 0.885 Active Speed Trailer at Taper End -0.787 0.638 -1.233 0.219 12 Mahmud et al. Effect of Law Enforcement Presence on Work Zone Speeds A total of 320 vehicle speed profiles were collected between the three different enforcement test conditions during the I -75 work zone evaluation. The average speed profiles for three test conditions are presented in Figure 6. It can be observed from the figure that speeds were approximately 5 mph lower within the work zone when at least one law enforcement vehicle was present , and this reduction was sustained beyond the end of the work zone. Interes tingly, when police vehicles were not visibly present at the site during the enforcement period , the average speed profile was very similar to the before enforcement period . Note that the upstream law enforcement car was not readily visible to the drivers . These findings suggest that visible police presence has a substantial speed reduction effect on work zone speeds. Future deployment of this enforcement strategy should consider leaving at least one police vehicle near the work zone site at all times to achieve a sustained speed reduction effect. FIGURE 6. Average vehicle speed profiles for police enforcement test conditions To confirm the graphical observations presented in the prior section, t he vehicle speed data were statistically analyzed using linear regression to determine the effects of law enforcement presence on drivers’ speed selection while traversing the work zone. Prior to analyzing the data, the speed measurements were similarly binned at 50 -ft increments, which covered from 350 ft upstream of the end of the work zone (250 ft beyond the speed measurement vehicle) to 1,700 ft beyond the end of the work zone – for a total tracking distance of 2,050 ft . Similar to the SFT analysis, b inning the data in this manner allowed for the speed -reduction effects of the enforcement activity to be statistically analyzed at various locations of interest throughout the work zone and beyond. It should be noted that , unlike the SFT linear regression analysis, this regression model did not control for drivers’ normal speed selection tendencies as it was not possible to collect speeds upstream of the work zone. Thus, to simplify the analysis, a single multiple linear regression model was generated with vehicle speed as the dependent variable, along with the following independent variables, each of which was coded in the model as a series of binary indicator variables: 13 Mahmud et al. • Speed measurement location: o 350-ft prior to the en d of the work zone; o Speed at the end of the work zone; o Speed 150 -ft beyond the end of the work zone; o Speed 500 -ft beyond the end of the work zone; o Speed 1,000 -ft beyond the end of the work zone; o Speed 1,500 -ft beyond the end of the work zone; • Enforcement a ctivity: o Before enforcement; o During enforcement - no police car present; o During enforcement – at least one police car present; • Vehicle type: o Passenger vehicle; o Heavy vehicle; • Lane: o Left; o Center; and o Right. The m ultiple linear regression results for speed s across the three enforcement conditions are presented in Table 2. The parameter estimates from Table 2 can be directly interpreted as the difference in mean speed compared to the base condition . For the case of the law enforcement variable, a ll paramete r estimates were computed relative to the s peed measured 350-ft prior to the end of the work zon e and before the enforcement period. For example, compared to the before enforcement period , mean speeds at this location were 4.3 mph lower during enforcement when at least one police car was present and 0.9 mph higher during enforcement when no police car was present. It follows that the effects of the law enforcement presence are interpreted by taking the difference between the parameter estimates at each sp eed measurement location. So, for speeds measured at the end of the work zone , the parameter estimates would suggest that the presence of at least one police car at the site during enforcement had a -3.8 – 2.7 mph = -6.5 mph effect on speeds compared to wh en no police car was present during the enforcement period . These marginal effects on work zone travel speeds associated with the enforcement conditions are displayed graphically in Figure 7. 14 Mahmud et al. TABLE 2. Multiple linear regression r esults for speeds of vehicles traversing the work zone as a function of law enforcement activity Parameters Estimate Std. Error t-value p-value Intercept 60.686 0.412 147.323 <0.001 Passenger Cars Base Condition Heavy Vehicles -3.781 0.283 -13.363 <0.001 Right Lane Base Condition Center Lane 1.880 0.248 7.587 <0.001 Left Lane 7.824 0.697 11.224 <0.001 Speed 350 -ft Prior to the End of the Work Zone Before Enforcement Base Condition During Enforcement -No Police Car Present 0.898 0.734 1.224 0.221 During Enforcement -Police Car Present -4.280 0.511 -8.383 <0.001 Speed at End of the Work Zone Before Enforcement 1.467 0.497 2.949 0.003 During Enforcement -No Police Car Present 2.657 0.734 3.620 <0.001 During Enforcement -Police Car Present -3.819 0.511 -7.481 <0.001 Speed 150 -ft Beyond the End of the Work Zone Before Enforcement 2.204 0.497 4.431 <0.001 During Enforcement -No Police Car Present 3.315 0.734 4.518 <0.001 During Enforcement -Police Car Present -3.332 0.511 -6.527 <0.001 Speed 500 -ft Beyond the End of the Work Zone Before Enforcement 3.803 0.497 7.645 <0.001 During Enforcement -No Police Car Present 5.094 0.734 6.941 <0.001 During Enforcement -Police Car Present -2.103 0.511 -4.119 <0.001 Speed 1,000 -ft Beyond the End of the Work Zone Before Enforcement 6.132 0.497 12.327 <0.001 During Enforcement -No Police Car Present 6.891 0.734 9.391 <0.001 During Enforcement -Police Car Present -0.315 0.511 -0.617 0.537 Speed 1,500 -ft Beyond the End of the Work Zone Before Enforcement 9.153 0.497 18.401 <0.001 During Enforcement -No Police Car Present 8.940 0.734 12.182 <0.001 During Enforcement -Police Car Present 2.486 0.511 4.868 <0.001 15 Mahmud et al. FIGURE 7. Average speeds measured at various locations within and beyond the work zone as a function of law enforcement activity The results displayed in Table 2 and Figure 7 suggest that the visible presence of at least one law enforcement vehicle has a significant effect on vehicle speeds while exiting the work zone , and this reduction persis ted beyond the end of the w ork zone. Not surprisingly, during the enforcement , when the downstream police car was not present, the speeds at different locations were similar to the conditions prior to the enforcement. As the conditions were most similar during the enforcement period, an assessment of the effects of law enforcement presence was made by comparing the speeds with and without at least one police car present during the enforcement period. At the initial speed measurement location 350-ft prior to the end of the work zone, the presence of at least one police car resulted in a speed reduction of 5.2 mph, which had increased to 6.5 mph upon reaching the end of the work zone . The law enforcement e ffects on speeds were maximized between 500 -ft and 1000 -ft beyond the end of the work zone, where speeds were 7.2 mph lower with at least one police car present on the shoulder. Even 1,500 ft beyond the end of the work zone, where the 70 mph speed limit w as in effect, the average speed was 64.2 mph. CONCLUSIONS AND RECOMMENDATIONS Work zone speed limits and management of work zone speeds continue to be critical areas of concern for state DOTs. To address these concerns, this study conducted a series of field evaluations to determine the effectiveness of SFT in reducing drivers' speed while approaching 16 Mahmud et al. and entering a freeway lane closure and identify the optimal installation position for sustained speed reduction effects. Additionally, the study evaluated the effects of law enforcement presence on drivers near the end of the work zone. The speed feedback trailer was tested at the start and end of the taper within a freeway work zone single -lane closure. Results showed SFT to be an effective sp eed reduction strategy in the work zone. In general, the magnitude of the speed reduction effects was greatest in the general proximity of the SFT. Accordingly, positioning the SFT near the end of the taper led to lower speeds for a more sustained distance into the work zone compared to when the SFT was positioned near the start of the taper. The SFT positioned near the end of the taper reduced the average speed by approximately 1.5 mph at the SFT location and the reduction at the end of the measurement area (2,150 -ft beyond the start of the taper) was 0.8 mph compared to no SFT conditions. It was concluded that the SFT should be positioned near the location of greatest need for sp eed reductions, such as the work area. Note that , considering inactive SFT as the base line condition may have resulted in lower magnitude of speed reduction for active SFT conditions as presence of inactive SFT may have some speed reduction affects which could not be captured in this study . Future research in this area should seek to determine the optimal SFT location with respect to the work area, in addition to how worker presence influences the speed reduction effects of the SFT. Furthermore, future r esearch should also include an assessment of the distance that SFT effects are sustained within the work zone in an attempt to determine spacing guidelines for work zone SFTs. Additional evaluations may also consider the use of SFTs in combination with di gital speed limit sign, which has recently been approved for use in Michigan and allow s for the displayed speed limit to vary in real -time based on worker presence at the site. Finally , it is likely that the effectiveness of the SFT as a work zone speed -reduction strategy was dampened by the use of rumble strips in advance of the work zone. Future research should evaluate the effects of SFTs at work zone lane closures without rumble st rips. A second evaluation assessed the effectiveness of a specialized work zone enforcement strategy that included a covert speed measurement vehicle positioned near the end of the work zone along with four police cars positioned just beyond the end of t he work zone to stop speeding drivers. The visible presence of law enforcement at this location reduced work zone speed by approximately 5 mph, which increased to 7 mph shortly beyond the end of the work zone as motorists passed by the police cars positio ned on the shoulder. It must be emphasized that this speed reduction effect was only observed when at least one law enforcement vehicle was visibly present at the site. No speed reduction effects were observed during periods when each of the four patrol cars were pursuing violators downstream of the work zone. These findings suggest that visible police presence has a substantial speed reduction effect on work zone speeds. Future deployment of this enforcement strategy should consider leaving at least on e police vehicle in - place near the work area at all times to achieve a sustained speed reduction effect. Further, future work should also assess the effectiveness of law enforcement vehicles positioned at other locations within the work zone, including in advance of the work area, in addition to an assessment of whether the effects of enforcement vary as a function of work zone length and/or duration. CONTRIBUTIONS The authors confirm contribution to the paper as follows: study concept and design: Timothy Gates, Peter Savolainen , Md Sh akir Mahmud and Hisham Jashami ; data collection: Md Shakir Mahmud, Hisham Jashami , Megat Usamah Megat Johari, Anshu Bamney , and Nischal Gupta ; analysis and interpretation of results: Md Shakir Mahmud, Timothy Gates , Anshu Bamney, Megat 17 Mahmud et al. Usamah Megat Johari, Hisham Jashami, and Peter Savolainen; draft manuscript preparation: Md Shakir Mahmud, Timothy Gates , Anshu Bamney, Megat Usamah Megat Johari, Hisham Jashami, Nischal Gupta , and Peter Savolainen . All authors reviewed the results and approved the final version of the manuscript.