Thousands of crashes occur in work zones each year, leading to numerous fatalities and injuries. Texas Department of Public Safety data indicate that approximately 9,500 crashes occurred in work zones on the Texas state highway system in 2000, leading to 143 fatalities and almost 9,900 injuries. Nearly 42% of these crashes were attributed to excessive speed, which emphasizes the need to motivate drivers to comply with speed limits in work zones.
To address these concerns, the Texas Department of Transportation (TxDOT) sponsored a recent project conducted by the Texas Transportation Institute (TTI) titled “Development of Measures for Motivating Drivers to Comply with Speed Limits in Work Zones.” The goal of the project was to determine effective measures to improve compliance by:
- Evaluating traditional measures including speed-limit enforcement and traffic-control devices;
- Identifying new technologies that may be suitable for work zones;
- Field-testing two to three promising devices; and
- Developing guidelines for recommended work-zone designs based on literature review and field testing.
The following is a summary of the research efforts on this project, focusing on treatment selection, field testing and analysis of results.
Slowing down traffic
Researchers at TTI reviewed recent studies within Texas and across the country and conducted a survey of engineers within TxDOT and other state DOTs. Previous studies indicated that motorists tend to slow down in the presence of police enforcement or at the risk of collision or injury. A variety of treatments have been tested to increase driver awareness and influence a reduction in speed, including portable changeable message signs (PCMS), speed-display systems and innovative static signs. The tests indicated that these devices had the potential to produce modest to significant reductions in speed and improvements in speed-limit compliance.
The survey of DOT personnel led researchers to the following conclusions concerning the state of the practice for work-zone speed limits.
In Texas, and nationwide, the methods for determining the appropriate value for work-zone speed limits vary. Engineering judgment and a simple 10-mph reduction from the regulatory speed limit were the most common approaches, but the former can be fairly subjective while the latter may be a blanket approach that incorporates little or no consideration of conditions in the work zone.
The majority of PCMS users display a speed-related message, either paired with work-zone information or as a stand-alone message.
The use of PCMS with radar (PCMR) is increasing in other states, but use in Texas is less common. Those who do use PCMRs are likely to use them in a manner similar to a speed trailer, often with an alternating message displaying the posted speed limit. Occasionally, the speed display is supplemented by or replaced with a text message encouraging drivers to reduce speed.
Use of innovative messages on static signs is very common, particularly in states other than Texas. However, the innovative message is almost always “Give ‘Em a Brake” or some variation thereof.
Testing the speeds
Researchers used the findings from the DOT surveys and literature review to develop a list of treatments with potential for field testing. Ultimately, three devices were tested in this project: a speed-display trailer, a PCMR and an orange-border speed-limit sign.
Speed-display trailers are used to notify drivers of their current speed as they approach the location of the unit. In addition to making drivers aware of their current speed, trailers commonly have a psychological effect on drivers who may think there is active enforcement nearby and improve their compliance. Therefore, speed-display trailers can have a substantial effect on work-zone speed limit compliance even in the absence of enforcement.
PCMR displays often consist of up to three fixed-height rows of minibulbs or LEDs. More recent PCMRs have a full matrix of LEDs or bulbs to permit text of different sizes or even graphics.
PCMRs may be used to send specific messages to speeding drivers. When the radar unit detects a vehicle traveling at a speed above a preset threshold, the display changes from the default message to a message urging the driver to slow down to a compliant speed. Once the vehicle either sufficiently slows or passes the PCMR, the display reverts to its default message.
This method has an advantage over basic PCMS operation, because the change in message helps to catch the driver’s attention. It also signals to speeding drivers that they are being watched by the sign and may create a concern that there is active enforcement nearby, which further motivates them to slow down.
Recent TxDOT-sponsored studies explored the effectiveness of conspicuity treatments on nonwork-zone speed-limit signs, speed-limit reductions that are often at locations that are unexpected by drivers. In many cases, the initial reduction in posted speed limit is unexpected because it occurs with no physical indication of a need to slow down, such as a change in cross section or environment. In addition, work-zone speed-limit signs are often posted close to other signs, most of which are orange, and they have a light blue or white sky as a background. These conditions reduce the visibility and conspicuity of many signs used to notify drivers of a reduced speed limit.
Results from those studies led researchers on this project to test the use of an orange-border speed-limit sign (OBSLS), anticipating that it would attract motorists’ attention and reinforce the message that there is an active work zone. Sign borders are not currently a treatment identified by the Texas Manual on Uniform Traffic Control Devices; however, if the borders are applied properly, they may be used without altering the basic design of the sign.
In this study, the orange border could not obscure the face of the sign, including the black border around the sign’s legend. The orange sheeting met the standard of work-zone orange sheeting (fluorescent, microprismatic). A border width of 3 in. on all sides was used for nonfreeway signs and 6 in. on all sides for freeway signs.
Researchers tested the devices at two sites in Texas: a rural interstate highway (Site 1) and a U.S. highway within the city limits of a small town (Site 2). Field studies involved installing a device, recording speeds and then removing the device, installing another and repeating the procedure. In some cases, speed data also were recorded during the period between removal of one device and installation of another.
What works and what doesn’t
It should be noted that, except for trucks upstream of the work zone at Site 2, the 85th percentile speed was never below the posted speed limit, regardless of the device installed. It also should be noted that, regardless of treatment, speeds at both sites increased as traffic approached the end of the work zone and anticipated a return to the increased regulatory speed limit.
Using a PCMR at the beginning of the work zone at Site 1 coincided with a reduced 85th percentile speed for both passenger cars and trucks at the two nearest measurement locations downstream of the device. The greatest reduction was 2 mph for passenger cars and 1 mph for trucks at Location 3. Using a second PCMR at the midpoint of the work zone showed additional improvements, as much as 2 mph lower for passenger cars at Location 4. The OBSLS showed a reduction of 1 mph for passenger cars at Location 3, but there was no significant effect on the 85th percentile speed of trucks.
The speed-display trailer at the beginning of the work zone at Site 2 showed the most potential for reducing speeds at that site, some 4 mph below the 85th percentile speed at Location 2 for no treatment or OBSLS. Informal discussions with some drivers in the area indicated that they thought the presence of the speed-display trailer meant that enforcement was active nearby, even though no enforcement was employed for this study. While the overall speed trend declined as distance into the work zone increased, the speed display had the most effect early in the work zone. As with Site 1, the OBSLS at Site 2 showed little to no improvement in compliance for the locations where data were available.
Based on findings from the field studies, researchers concluded that devices that display an approaching driver’s speed show considerable potential for reducing speeds and improving work-zone speed-limit compliance.
Orange borders are a low-cost method of substantially improving the visibility and conspicuity of speed-limit signs, but their effects on improving compliance were minimal. They should be used in conjunction with other devices to obtain the greatest benefit.