Assessment of Dynamic Speed Limits on Freeway A20 near Rotterdam, Netherlands

On Freeway A20 near Rotterdam, Netherlands, a trial with dynamic speed limits began on June 28, 2011. On a 4.2-km stretch, the speed limit increased from 80 to 100 km/h as soon as congestion appeared to set in and at night. The aim of dynamic speed limits was to improve traffic operations and to avoid deterioration in the local air quality. This paper presents an assessment of this trial with respect to traffic operations, air quality, noise level, and traffic safety. Traffic operations on A20 appeared to have improved significantly as a consequence of the dynamic speed limits, which produced a reduction in the number of lost vehicle hours by 600 (20%). This improvement was the result of a 4% increase in the free-flow capacity at the main bottleneck on the freeway stretch. The dynamic speed limits caused a change in driver behavior: the median lane was better occupied when flow increased. Air quality deteriorated slightly. The effects varied along the stretch with a maximum increase in nitrogen oxides and particulate matter10 emissions of 3.7% and 3.6%, respectively. However, the effects on the average concentration of nitrogen oxides per year were limited. The noise level appeared to increase slightly with 0.2 dB. This increase occurred mainly during the two peak periods. The indicators for traffic safety showed sometimes a (possibly) positive and sometimes a (possibly) negative effect. However, it was not likely that dynamic speed limits had a significant negative effect on traffic safety.