Stratec à l’ETC Barcelone
Towards laminar traffic flows? Impact of adaptive speed limitation policies on road congestion, safety and environment on the Belgian roadway network
Belgium is the European country where drivers experienced the direst congestion in 2015. According to INRIX index for 2015, Belgian drivers waste on average 44 hours in traffic jams per year. The same index shows that in Brussels and Antwerp, the time wasted peaks up to 70 hours. Besides time delays, congestion and a non-optimized motorway infrastructure lead to impacts on fuel consumption, levels of pollution and accidents. A side analysis of Eurostat data from 2013 brings out that Belgium is one of the countries of Western Europe where the ratio of persons killed in road accidents – 65 per million – is the highest. This ratio is twice as much as in the Netherlands, and overshoots the same ratio for France and Germany.
The Federal Government of Belgium has commissioned the Belgian Road Safety Institute (BRSI) to evaluate the opportunity to modify the speed limits on the motorway network. The objective was to study the impact of increasing/reducing speeds on congestion, pollution emission and accident rate. Stratec has contributed to this study in modelling changes in driver’s behavior and/or speed regulations in ATKINS SATURN, using a hybrid model that gives good congestion results in urban areas, using the well-known tools that are blocking back and flow metering.
We identified four modeling parameters:
- The critical speed – speed at capacity – which can be increased from 20 to 30 km/h.
- The critical flow, the value of which can be stepped up so as to reduce the capacity drop.
- The power of the speed-flow curve, which reflects the flow turbulence.
- The theoretical speed limit on the network or in specific areas.
Based on these parameters, we investigated concrete policy leverages, such as a dynamic speed limitation in and near congested areas, or a better lane assignment and management. The former policy aims at reducing flow fluctuations and therefore increasing the critical flow and speed; the latter is meant to prevent the flow from turning too turbulent.
The simulations are carried for personal vehicles and lorries, for a morning peak hour, an off-peak hour and a night hour. Both motorways, national and (some) local roads are modeled, allowing the adaptation of itineraries depending on the speed on the network.
Decreasing speed limitation during peak hours has a positive impact on congestion, pollution and accident rate. The mean travel speed throughout the network is higher and congestion has a lesser spatial extent. The outlook shifts from personal improvement of travel time to a global one that benefits the whole community. Usual performance indicators were calculated, as well as COPERT-style pollutant emissions and fuel consumption. Accident rates were also applied, showing the impact of a better use of the (safer than conventional roads) motorway network.
The model could still be improved by a more detailed differentiation of driving behaviors, especially by including drivers committing speed violations or riding below the limitation. In addition, more tailored policies could be investigated. One the one hand, we could apply micro models on smaller areas experiencing the direst congestion where actions need to be undertaken. On the other hand, especially for the motorways, we could test higher speed limitations in areas with low population densities or less intense traffic flows countries. A cost-benefit analysis would help in combining the best changes.