That's your cue

Jan. 5, 2015

Speed-limit signs prep motorists for work zone, but may not slow them down much

Transportation professionals view the setting of appropriate regulatory speed limits on public roads, including those under repair or reconstruction, as an important tool in promoting safe and efficient operations.

To aid this decision-making process, state highway agencies have adopted policies and procedures for determining if a reduced regulatory speed limit should be established in a work zone. Most of these policies indicate that lowering the regulatory speed limit in work zones should be avoided as much as is practical. However, in some work-zone situations, such as when workers are present in a lane closure without positive protection, a reduced regulatory speed limit may be needed.  

Slower isn’t always better

It is generally accepted that the safety of both workers and road users is improved in work zones when traffic is going slower. Logically, crashes and their resulting injuries are likely to be less severe at lower speeds. In addition, a driver traveling at a slower speed has more time to react and recover in emergency situations, should the need arise. Slower speeds also reduce wind effects of large trucks and provide workers more time to react should a vehicle intrusion occur. However, slower speeds in work zones also may reduce the roadway capacity and cause localized congestion, which in turn can increase the potential for rear-end crashes.

While the speed of traffic can affect crash frequency and severity, speed variance also is an important factor. Vehicles all traveling close to the same speed, albeit somewhat faster, may be safer than vehicles traveling at different speeds (some low and some high). Likewise, abrupt reductions in speed, especially if unexpected by drivers, increases the risk of rear-end collisions and other conflicts, such as vehicles swerving out of their travel lane.  

When and where?

Overall, the goal is to identify a speed that is safe and reasonable for conditions. Slower speeds are needed in work zones when workers are exposed to moving traffic without positive protection or when roadway restrictions and work operations present increased risks to roadway users. 

Reduced speed limits should only be used in the specific portion of the work zone where conditions or restrictive features are present. The Manual on Uniform Traffic Control Devices (MUTCD) recommends a maximum speed-limit reduction of 10 mph unless restrictive conditions in the work zone justify a greater reduction. Individual state laws and practices vary considerably regarding the amount of allowable speed-limit reduction, and the conditions under which reductions may be used.  

Reduced speed limits should only be left in place after the work shift is over if roadway restrictions still present a hazard. Failure to remove these signs when they are not needed leads to:

  • Reduced credibility of speed limits; 
  • Decreased compliance with speed control and other temporary traffic-control devices in the work zone; 
  • Greater variation in vehicle speeds; and 
  • Negative public opinion of work zones.  

In order to maintain the credibility of work-zone speed limits, signs and other devices and techniques used to reduce speeds should be covered or removed when no work is occurring and other hazards are not present. It also is important to keep accurate records of when reduced regulatory speed limits and advisories are installed or removed in a work zone. This information may be needed for effective speed enforcement, and also may be relevant in evaluating safety and mobility issues in the work zone whenever a crash occurs or congestion develops.

Changed to match conditions?

Covering or removing signs can be a very tedious task to complete on a daily or weekly basis. This task also may interfere with normal traffic flow and expose workers to traffic hazards. In recent research projects sponsored by the Texas Department of Transportation and the Ohio Department of Transportation, researchers evaluated the use of digital speed-limit (DSL) signs to vary the work-zone speed limit. When conditions warranted, the reduced work-zone speed limit was displayed. Otherwise the original posted speed limit was displayed. In Ohio, alternating flashing beacons also were activated when the reduced work-zone speed limit was displayed.  

The research in Ohio showed that drivers reduced their speed near the first DSL sign when it was within view of a lane closure. In addition, drivers further decreased their speed inside the lane closure and continued to travel at reduced speeds near the remaining DSL signs and where workers were present. The Texas study also found a positive impact on speed. In addition, Texas drivers understood the DSL signs and recognized that the speed limit on the DSL signs changed based on conditions.

Based on these results, the use of DSL signs should be considered at work zones on multilane highways where the work-zone-speed-limit reduction will vary within a 24-hour period in order to accurately reflect the work-zone conditions present. Typically, this need arises for two reasons:

  • The work-zone condition (e.g., lane closure) remains in place 24 hours a day but the speed-limit reduction varies based on whether or not workers are present; and 
  • The work-zone condition (e.g., lane closure) is removed when workers are not present.  

For both of these situations, the use of DSL sign assemblies allows the contractor to more easily change the speed limit to reflect the work-zone conditions present. It also does not require the contractor to install/remove or uncover/cover existing and temporary speed-limit signs on a daily/nightly basis.  

The credibility of the DSL signs, and the associated compliance, are dependent upon the contractor remembering to use them properly. Therefore, the reduced work-zone speed limit should only be displayed when the conditions for which the reduced speed limit was approved are present. Otherwise, the original posted speed limit should be displayed.

Is it enough?

One of the biggest misconceptions in work-zone speed management is that simply lowering the speed limit will actually reduce drivers’ speed. Unfortunately, numerous studies have shown that posting a reduced speed-limit sign by itself does not slow drivers down. Drivers only reduce their speeds through the work zone when they perceive a need to do so, based on conditions in the work zone or the perception of enforcement activities. Typically, drivers slow down when large equipment and work crews are located close to moving traffic and for roadway restrictions, such as temporary median crossovers. It is the situation they see, and not the reduced speed-limit sign itself, that causes drivers to reduce their speed. However, such voluntary speed reductions are typically less than 10 mph and often closer to 5 mph (Table 1). Thus, when normal operating speeds on the roadway are high, these voluntary speed reductions alone may not produce the desired speeds through the work zone.  

Unfortunately, simply posting even lower speed limits does not necessarily further reduce speeds. The presence of law enforcement in work zones can yield up to a 15-mph decrease in some situations (though average speed reductions of about 5 to 10 mph are more common). When law enforcement personnel are not available, other speed-management techniques can be used to encourage compliance. These techniques include speed-display trailers, portable changeable message signs (PCMS) with radar, transverse rumble strips, drone radar, narrowing lanes with channelizing devices, and transverse pavement marking. Most of these techniques only reduce speeds by a few miles per hour; although, in some cases radar-activated speed displays have been shown to reduce speeds by 10 mph (Table 2). Even when only very small speed reductions are achieved, these techniques may effectively alert drivers to an upcoming change in the highway environment, and thus achieve the safety benefit of an alerted driver. R&B

About The Author: Finley is a research engineer at the Texas A&M Transportation Institute, College Station, Texas.

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