The effects were analyzed through an empirical Bayes before-and after study, which compared the crashes after the implementation of the measure with the number of crashes before, and controlled for confounding variables. The before period consisted of crashes from 1999 up to 2002 and the after period included crashes from 2006 until 2011. All data were made available from the national crash statistics database. In total, five road segments with DSLs were assessed, covering a total distance of 59.54 km. These five segments were all located at access roads of the ring road of Antwerp.

The empirical analysis was further complemented by a cost benefit analysis of the applied DSL system.

Findings

• The number of injury crashes decreased significantly (by 18 percent) after the introduction of the system. A separate analysis for serious and fatal injury crashes revealed a non-significant decrease of 6 percent. A distinction according to crash type showed an almost significant decrease of 20 percent in the number of rear-end crashes whereas the number of single-vehicle crashes decreased by 15 percent. However, no effect was found for side crashes.
• The cost-benefit analyses of the crash effects showed a benefits-to-costs ratio of approximately 0.7, (meaning the costs tend to exceed the benefits). Taking into account the important margins of uncertainty with respect to both costs and benefits, the study also explored how the net benefits are affected by some key assumptions. The general conclusion was that there is no convincing evidence that the costs of the system currently outweigh the expected benefits in terms of crash prevention.