Dynamic Speed Display Signs

Dynamic speed display signs (DSDSs) measure the speed of approaching vehicles and communicate the vehicle’s actual speed to drivers on a digital display along the road, possibly also including pictures or verbal messages such as “Slow down” or “Thank you”. The underlying idea is that DSDSs help motorists self-enforce their speed. DSDSs should not be confused with dynamic speed limits (DSLs) which can impose different speed limits depending on traffic or weather circumstances. The essence of DSDSs is the individual feedback on driven speeds.
The number of studies on the effects of DSDSs published in peer-reviewed journals is limited. Most evaluations have been done by means of before-and-after studies, focusing rather on the resulting speed behaviour than on the (indirect) effect on crashes. No meta-analyses were found.
All reviewed studies consistently report significant decreases of mean speeds due to the presence of active DSDSs, although the size of the effect differs. The observed mean speed decrease ranges from 1km/h to 10 km/h. The observed decreases of the 85th percentile speed are of the same magnitude. The results of all the studies appear to be relatively homogenous which suggests that the measure is reasonably well transferable to other similar settings, including those in other countries. All studies also evaluated the proportion of drivers who exceeded the speed limits by some amount and reported considerable reductions in the highest exceedances of the speed limits. Some studies concluded that drivers become less responsive towards the DSDS over time, but the most elaborate study did not find significant evolutions in mean speeds at the DSDS locations during the period of use. All studies agreed that the speed reductions observed while the DSDSs were in place disappeared after the devices were removed from the study sites. It was also found that drivers increase their speed again after passing the DSDS. DSDSs that extend the numeric feedback with verbal messages tend to outperform the ones with only numeric feedback. One study calculated the effect on the number of crashes and found a significant overall reduction of 5%.