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Effectiveness: 5 Star Cost: $
Use: High
Time: Short

Unstaffed speed display devices, also known as speed feedback signs, which can be portable (on trailers) or permanently installed, can show drivers that they are speeding and may encourage some drivers to slow down. These feedback signs (with radar to detect speeds) may also suggest to drivers that speeds are being monitored or enforcement is nearby. Portable changeable message signs (PCMS) are a similar device that can be triggered by speeding but display a message such as “Slow Down Now.”

Automated speed display monitors also provide a method to collect location-specific travel speed data. A meta-analysis of dynamic speed feedback devices found that these devices are effective at reducing speed at installation locations for different vehicle types across a variety of roadway contexts (Fisher et al., 2021).


Use of permanent installations seems to be growing but the actual number of displays and signs in use is unknown. Use of the displays tend to occur in work zones, school zone, transitional zones, and curves.


Several studies have shown these signs can slow speeds while in use. A high-quality multi-site study for FHWA has also documented crash reductions. However, speeds seem to rebound quickly downstream and as soon as the devices are removed (Donnell & Cruzado, 2008; Hajbabaie et al., 2011; Walter & Broughton, 2011), prompting recent efforts to evaluate permanent installations. Most studies have evaluated use of these devices in school zones, work zones, and other risky locations such as at curves.

Signs that provided either an implication that speeds were being monitored or a social norms message (“Average Speed” at the site; “Your Speed”) were effective at reducing speeds in a 50 km/h (31 mph) zone (Wrapson et al., 2006). Several U.S. studies have found promising reductions of speeds in school zones in response to permanent installations of speed display or changeable message signs (Lee et al., 2006; O’Brien & Simpson, 2012; Rose & Ullman, 2003), and little sign of driver “habituation” to the signs during school hours (O’Brien & Simpson, 2012).

Other studies have shown that speed trailers or portable changeable message signs, which may include speed feedback plus other messages such as “Slow Down Now” when triggered by a threshold speed, can also be effective in reducing speeds in work zones (Brewer et al., 2006; Mattox et al., 2007). In work zones, a combination of a parked police vehicle and speed feedback trailer reduced average and 85th percentile traffic stream speeds and free flow speeds to a similar degree as automated camera enforcement, whereas the effect of speed trailers alone was the same as no treatment. The presence of parked police alone was also effective, but to a lesser extent than the combination of police + trailer or the camera system. The number of speeders above 10 mph over the limit was essentially reduced to zero by both the automated enforcement and police + trailer combination. However, the treatment effects on speeds in work zones disappeared within 40 – 50 minutes of removal (Hajbabaie et al., 2011).

Permanently installed dynamic speed display signs also decreased speeds and crashes at rural, two-lane curves (speed limits 50 to 60 mph). A high-quality evaluation of dynamic speed display or curve warning signs installed at 22 rural, two-lane sites in 7 States estimated that crashes were decreased by 5 to 7% (Hallmark et al., 2015). The evaluators tested speed feedback signs and dynamically activated curve warning signs with the message “Slow Down” when motorists exceeded the 50th percentile speed on sites selected for speeding and crash problems. The speed sign displayed the vehicle’s actual speed, up to a certain threshold, which was selected to avoid the possibility that displaying actual speeds would encourage some motorists to test their speeds above this level. Once this maximum speed was displayed, the signs replaced the number or message with the actual speed limit or advisory limit. The evaluation found both sign types reduced the average mean speed and proportions of vehicles exceeding by 5, 10, 15, and 20 mph at 1 month, 12 months, and 24 months after installation at most locations. Although trends suggested the speed feedback signs were slightly more effective at reducing speeds at more sites compared to the “slow down” signs, statistical tests could not confirm this trend.

In summary, use of travel speed or other speed feedback messages displayed only when the motorist is exceeding a threshold speed can be effective at slowing speeds when used at locations where drivers can perceive the need to slow (school zones, curves, work zones). Use of visible law enforcement presence may enhance effectiveness. Some drivers may not reduce speed in response to these devices unless they perceive that law enforcement is nearby.


Hallmark et al. (2015), identified reliable, durable (would last at least 2 years) systems that cost less than $10,000 per sign for installation, support, and maintenance for a curve-based permanent speed feedback sign evaluation, but some types of signs did experience technical issues. (Signs may be powered with solar panels.)

Time to Implement:

Once law enforcement agencies and engineering safety partners have determined locations where dynamic speed display may help to control speeds, implementation time should be fairly short.

Other considerations:

  • Work zones: See NCHRP Report 746 (Ullman et al., 2013) for in-depth discussion of advantages, disadvantages, and deployment considerations for various methods of traffic enforcement in work zones. According to this report, there have been insufficient controlled trials to identify the optimal mix of enforcement types and other treatments for different highway types, geometries, and work zone situations. The report reiterates the importance of work zone speed limits that reflect the situation, including the presence of workers or alignment changes. A study of speed controlling strategies before freeway (repaving) work zones in Oregon recommended using a combination of reduced speed limit signs, portable changeable message signs, and speed feedback signs based on reductions in speed achieved with different combinations of these treatments (Gambatese & Zhang, 2014). There is more information about deployment in the report.