Report numberRA-2015-005
TitleDynamic speed limits on Flemish motorways
SubtitleEffects on crashes and the balance against the costs
AuthorsEllen De Pauw
Stijn Daniels
Laurent Franckx
Inge Mayeres
Published byPolicy Research Centre for Traffic Safety 2012-2015
Number of pages23
Document languageEnglish
Partner(s)Universiteit Hasselt
Work packageWP5: Ranking and evaluation of the measures
Dynamic speed limit systems are widely applied in order to account of the real time traffic, road and weather conditions. On the basis of the traffic volume and other environmental conditions, the speed limit can be adapted, which is denoted by electronic signs that are housed within gantries situated above lanes. Dynamic speed limits try to harmonize traffic flows, which can improve capacity. Furthermore these systems try to improve traffic safety, through the reduction of manoeuvres.
The present study evaluates the traffic safety effects of dynamic speed limit systems in Flanders, Belgium, and compares the benefits of this measure in terms of traffic safety effects against the costs of these systems. In order to analyse the traffic safety effects, the number of crashes after the implementation of the dynamic speed limits were compared with the number of crashes before the implementation. General trend effects and chance were taken into account in this comparison. At first all crashes were studied (irrespective of the severity of the injuries); secondly, a separate analysis was applied for severe crashes (i.e. fatal and serious injury crashes). Furthermore, the injury crashes were subdivided according to the type of crash and the three main crash types were analysed: (1) rear-end crashes; (2) side crashes; (3) single-vehicle crashes.
Five road segments with dynamic speed limit systems were analysed, which were located at motorways and had a total length of almost 60 km. The before-and-after study of the crash numbers showed that the injury crashes significantly decreased by 18%. A non-significant decrease of 6% was found in the number of serious and fatal injury crashes. A distinction according to the crash type showed an almost significant decrease of 20% in the number of rear-end crashes. The number of single-vehicle crashes decreased non-significantly by 15%. No effect was found for side crashes.
In addition to the analysis of the effects, a cost-benefit analysis was applied. The costs of the implementation of these systems were compared with the benefits of crash prevention. Using unit values from the international literature for the valuation of crash prevention, the cost-benefit analyses of the crash effects showed a benefits-to-costs ratio of approximately 0.7, which means that the costs exceed the benefits. Taking into account the important margins of uncertainty with respect to both costs and benefits, we have also explored how the net benefits are affected by some key assumptions. The general conclusion is that there is no convincing evidence that the cost of the system outweighs the expected benefits in terms of crash prevention. However, as this analysis is based on an ex post assessment of systems that were implemented over the course of the last decade, one should be careful in drawing conclusions regarding the expected benefits of new systems that would be based on current state-of-the-art technology.
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The Policy Research Centre for Traffic Safety carries out policy relevant scientific research under the authority of the Flemish Government. The Centre is the result of a

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