Reducing Speed Limits on Highways: Dutch Experiences and Impact on Air Pollution, Noise-level, Traffic Safety and Traffic Flow
M Stemerding, P van Beek, Goudappel Coffeng, NL; P F Havermans, Ministry of Transport, NL
The Air Quality Order ('Besluit Luchtkwaliteit') was introduced in the Netherlands in June 2001. This order was a result of European rules (PbEG, L163 and directive 96/62/EG) and refers to air quality standards for the compounds sulphur dioxide, nitrogen dioxide (NO2) and nitrogen oxide (NOx), particulates (PM10, black smoke) and lead. Authorities are bound legally to take measures when standards of air quality are exceeded.
Near freeways in particular, NO2 and PM10-concentrations are important factors for air quality. Emissions from traffic contribute significantly towards these concentrations. One of the measures to decrease the concentration of NO2 near freeways is to reduce the maximum speed from 120 or 100 km/h to 80 km/h. The effect is reasonably well-known in the Netherlands because of the introduction of this measure near Overschie (A13 Rotterdam) in 2002, accompanied by strict enforcement. This takes place by route control in which case the mean speed of every vehicle was measured, and vehicles with higher speeds were fined immediately. Evaluation studies showed that the speed limit led to a significant decrease in NOx-emissions and also therefore, (although to a lesser degree) in NO2-concentration. A speed limit of 80 km/h and more homogeneous traffic flow as a result of route control were the most important causes of the decrease in air pollution. Besides the improvement in air quality, noise-level, traffic flow and traffic safety also improved.
The National Institute for Health and Environment (RIVM) pointed out nine other locations along freeways where the air quality standards would also not be met after 2010. Given the positive results near Overschie, the question arose as to whether the 80 km/h speed limit would also lead to an improvement of air quality, noise-level, traffic flow and traffic safety at these bottlenecks.
To answer this question, a detailed study was conducted to determine the effects of an 80 km/h speed limit with route control on air quality, noise-level, traffic safety and trafic flow. The effect of the speed reduction was determined for three years: the current situation (2002) and two future years (2010 and 2015). The year 2010 is relevant for the Air Quality Order and 2015 is important because of the introduction of Euro IV and V.
To determine the effects for air quality, noise-level and traffic flow, we used highly detailed 'state-of-the-art'-models. The models were necessary to determine the effects of a control measure in locations where it is not yet introduced. Simulation of traffic flows was done using the microscopic dynamic model VISSIM. NO2 and PM10-concentrations were determined using the model KEMA-Stacks. Air quality was determined using a range of factors. Important factors were background concentrations, emission data and meteorological data. For noise-level, we used Silence, a model from the Ministry of Traffic and Transport in the Netherlands. Finally traffic safety effects were determined using well-known relationships between speed and number of accidents.
First the impact of the control on the Overschie A13 highway was modelled, and the results were compared with measurements. It appeared that for this location the model and measured results were very close. Second, the effect for the other locations were then modelled.
The results showed that an improvement of air quality near freeways of up to 5% is possible by decreasing speed from 100 to 80 km/h (for two locations the speed decreases from 120 to 80 km/h). However, the effect varies by location. The maximum effects were found around the large cities Amsterdam, Rotterdam and Utrecht. The average improvement for NO2 was about 5%. The effect of the measure on emissions was much larger than on concentrations. The effects for PM10 concentrations were generally small, mainly because of the smaller contribution of local traffic to total concentrations. The effect is clearly larger for NO2 than PM10. The absolute effects in 2010 and 2015 were comparable with those for 2002, although the relative effect was even larger.
The introduction of a speed limit with route control had, in general, a positive effect on noise levels. The effect was influenced by:
-traffic composition (more truck traffic - less effect); -road surface type (quiet road surfaces - less effect)
-presence of noise screens (noise screens - less effect).
The average effect varied between 0,2 dB(A) and 1,3 dB(A). In 2010 and 2015, the effect was similar to the current situation.
The safety-effects near Overschie were very positive: the number of accidents and victims decreased substantially (around 60 and 90%). The positive effects were caused by lower speeds and more homogeneous traffic flows because of route control. The effects for the other locations were estimated with knowledge based on empirical studies . On average, the total number of accidents decreased by 35% and the total number of injury accidents by 47%. In 2010 and 2015, the expected effects were less because of increasing intensity and congestion.
An 80 km/h speed limit lead to a decrease in the average speed of traffic. The largest decreases were observed in free flow situations. In congested situations, the decrease was very small and in some cases there was even a slight increase. For almost every location, the variation in travel times decreased. Again, the largest decreases were at free flow. Network effects were minimal. The future effects were the same for most of the locations.
Despite the growth in traffic volumes across the Netherlands, the speed reduction measure resulted in a significant improvement in air quality. This improvement was due to technical innovations such as the more common use of catalytic converters, cleaner fuel and better quality roads. Further progress might be expected with the introduction of Euro IV for personal vehicles and Euro V for vans. As stated in the introduction, problems remain however with concentrations of NO2 and PM10. High concentrations were found mainly around heavily used freeway and street 'canyons'. Which measures could local authorities apply to lower these concentrations? Our study shows that reducing speed is a successful measure: it is possible to reduce the air pollution significantly at relatively low costs. But what if the reduction levels are not met? We think that in this case, a more comprehensive traffic management scheme should be applied. Possible additional measures include zoning measures (i.e. time slots), route measures (i.e. special routes for freight transport), traffic signal controls (i.e. exclusion of polluting vehicles) and price measures.
Association for European Transport