Automated speed photo enforcement (SPE) systems have been extensively used in Europe; however, in the U.S. Illinois was the first state to use the system in work zones.
The Illinois Automated Traffic Control Systems in Highway Construction or Maintenance Zones Act authorized the use of automated speed enforcement on interstate work zones. The SPE initiative is in line with the strategic highway safety plans, state and national plans that are developed to improve traffic safety and to reduce the number of traffic-related fatalities.
Previous studies in other countries have shown that SPE can significantly reduce the average speed and the percentage of vehicles exceeding the speed limit in the locations where it has been deployed. However, these applications have been limited to SPE in regular highway sections and not in work zones. The study of the University of Illinois is unique in this aspect and also presents a direct comparison of SPE and different speed-reduction treatments.
Shutter speeding
The Illinois SPE system is contained in a full-size van. The van is equipped with two high-resolution cameras and sophisticated radar for speed measurement and is parked on the shoulder and parallel to the roadway. As a vehicle approaches the van, a down-the-road radar determines its speed and displays it on an LED board located on top of the van, providing feedback to the approaching driver. This gives an opportunity for speeders to comply with the speed limit without being caught. If the vehicle continues speeding, a more advanced radar determines the speed of the vehicle as it is about to pass the van. This second radar operates at a specified angle to the path of vehicles and accounts for the angle effect. If the speed of the vehicle, as measured by the advanced radar, is greater than a specified value, the radar activates two on-board high-resolution cameras to take pictures of the vehicle. A camera at the rear of the van captures the face of the speeding driver and the front license plate of the vehicle. The picture is stamped with basic information such as the speed of the vehicle, date, location and time. Then, a second camera at the front of the van takes another picture to capture the rear license plate as the vehicle leaves the van location.
The SPE vans are staffed by Illinois State Police officers trained to use the system. The officers oversee the process from inside the van through computer monitors and receive audible alerts of oncoming speeding drivers. An officer in the van can issue a citation for speeding vehicles if the officer decides it is a clear case of excessive speeding. In such a case, the SPE system vendor processes the approved citation and mails it to the registered owner of the vehicle within 14 business days. The system also provides the option of activating warning messages to alert workers of a speeding vehicle as a preventive measure. In addition, nighttime operation is possible thanks to two 140-W bulbs installed at the rear of the van, which provide the necessary amount of light when the pictures are taken.
Thus, the SPE system is capable of providing automatic identification of speeding drivers and to issue citations through the mail, serving also as a deterrent for speeders in work zones. It has the potential to improve safety not only for workers, but also for drivers and police officers as well. Through the SPE system, it also is possible to identify a greater number of speeders (and more safely) than police officers in a patrol car, since it does not require a police officer to pull the driver over and to be exposed to live traffic conditions.
Tracking traffic
The effects of the SPE system on the speed of vehicles were compared to a base case and to the following commonly used treatments for speed reduction in work zones: (1) speed feedback trailer, (2) police patrol with emergency lights on, (3) police patrol with emergency lights off and (4) the combination of methods 1 and 3 (speed feedback trailer and police patrol with emergency lights off).
A total of three sets of field data were collected from two work zones in Illinois on I-64 and I-55, near St. Louis and Chicago, respectively. The data sets were collected during off-peak hours in the morning and afternoon. In both work zones the posted speed limit was 55 mph, and two lanes per direction were open to through traffic. For each data set, the speed of vehicles was estimated at two locations: (a) at several hundred feet downstream of the speed treatment location (e.g., the SPE van); and (b) at about 1.5 miles downstream of the first data collection point, to determine potential spatial effects.
A video camera was used to record the vehicles as they traveled on a section of the roadway. Later, the videos were processed in the lab to obtain information about the vehicles.
In addition to the speed of vehicles, the vehicle type (passenger car or heavy vehicle/trucks), the vehicle lane (shoulder or median lane) and whether the vehicle was in platoon or free flowing also was recorded. Free-flowing vehicles were those that had the freedom to travel at their desired speed and were not closely following another vehicle. To distinguish free-flowing vehicles from in-platoon vehicles, a four-second criterion was used. This means that if the headway between a vehicle and the vehicle in front was more than or equal to four seconds, the following vehicle was considered traveling at free flow.
Data from free-flowing and in-platoon vehicles were analyzed separately, and the mean speeds of vehicles were estimated for each of the traveled lanes: one mean for the median lane, and one mean for the shoulder lane.
In general, speed reductions obtained from free-flowing vehicles were similar to those from the general traffic stream (vehicles randomly selected regardless of their headway), with a tendency for greater speed drops on free-flowing vehicles. This was expected, since their speed is not constrained by vehicles immediately ahead of them, as is the case for vehicles in the general traffic stream. Thus, for the sake of brevity, the speed reductions are presented in the next section for free-flowing vehicles only. Readers also interested in the results for the general traffic stream would have a sense of the magnitude of the speed reductions by slightly reducing the effects for free-flowing vehicles, but for further details they are directed to the full reports of the study, published by the Illinois enter for Transportation and available online to the public.
Free-slowing vehicles
Results showed that in all three data sets, the SPE reduced the mean speed of free-flowing passenger cars by 4.1 to 7.9 mph when SPE was deployed. Similar speed reductions were observed for free-flowing trucks ranging from 3.4 to 6.9 mph. In addition, some of the other treatments were as effective as the SPE in lowering the mean speeds. For example, a police patrol with the emergency lights off generated a speed reduction of 4.5 to 8 mph in free-flowing passenger cars, and about a 2.9- to 4.8-mph speed reduction in free-flowing trucks. The combination of a speed feedback trailer and a police patrol with emergency lights off also yielded positive results by reducing the speeds between 1.4 to 8.4 mph for free-flowing cars, and between 4.1 and 6.9 mph for free-flowing trucks. On the other hand, the use of the speed feedback trailer was much less effective and it reduced the speed less than 2.2 mph for both free-flowing cars and trucks.
The degree of speeding also was analyzed for all treatments, indicating the percentage of vehicles traveling above the speed limit. Using the SPE, the percentage of speeding cars was reduced by 40-51%, and that brought down speeding to 7-57% on the median lane. On the shoulder lane, the speeding was reduced by 7-51%, and speeding was lowered to 0-40%. For heavy vehicles, SPE reduced the speeding by 10-53%, and brought it down to 2-15% on the median lane. On the shoulder lane, the speeding was reduced by 0-56%, and the speeding was lowered to 0-11%.
In general, the SPE was as effective in reducing speed and the number of speeders as having a police car with flashing lights off present in the work zone. Similar or slightly higher effects were obtained from other treatments involving police patrol cars together with the speed feedback trailer. However, the speed-feedback trailer alone did not show, in general, significant reduction in the percentage of speeding drivers.
The spatial effects of SPE, measured at about 1.5 miles downstream of the treatment location, were smaller compared to those at the treatment location, as expected. However, data showed speed reductions between 0 and 3.8 mph for both passenger cars and trucks and reduction in the percentage of speeders by 0% to 41% depending on factors such as the mean speed without treatments, the type of vehicle and the traveled lane.
Snapshot of effectiveness
Field data has provided evidence that the SPE system can generate similar speed reductions and a similar decrease in the number of speeding drivers as those achieved using effective traditional speed-control methods such as police patrol cars and its combination with speed-feedback trailers.
Even though the initial cost of implementing SPE may be much higher than the cost of other traditional methods, the potential increase in safety for workers, drivers and police officers makes the SPE an effective option for speed control in work-zone environments. In addition, there could be significant advantages of using SPE in terms of the number of drivers that can be cited for speeding violations per unit of time, increasing the deterrent influence of the speed-reduction system, thus providing further safety benefits for workers.
This was the first study of SPE systems in U.S. work zones, providing quantification of the effects of SPE in the speed and the degree of speeding of drivers. However, further research is needed as many questions still remain open, including SPE effects on the traffic-flow characteristics, platooning and headway of vehicles and optimal location of the SPE van inside the work zone for maximizing safety benefits, among others.
