In France, the A31 motorway liaises the Metz-Nancy area to Luxembourg. With heavy local, national and international traffics, traffic efficiency, safety and pollutant emission are high stakes. Therefore DIT (the Direction des infrastructures de transports) of the French Ministry in charge of transport, via its local road operator, the DIR Est (Direction interdépartementale des routes Est), decided to deploy variable speed limits on 26 km of A31 within the Arc Atlantic framework.
A few figures: 26 km with variable speed limits, 6,9 M€, including 1,4v M€ of European funds, 80 000 vehicles per day,45% of time with traffic difficulties.
As the local road operator for DIT (the Direction des infrastructures de transports) of the French Ministry in charge of transport, the DIR Est (Direction interdépartementale des routes Est) has in charge the toll-free national road network of the "Sillon Lorrain" area, i.e. the Metz - Nancy area. DIR Est has developed a dedicated traffic management system, called Myrabel, covering 756 km of the Sillon Lorrain network.
The second stage of the Myrabel extension focuses on 240 km with the objective to improve the network efficiency and the daily mobility. This extension includes dynamic measures, one of these being the variable speed limits on A31.
The A31 section between the A30 and the French / Luxembourg border (26 km) supports both transit and local urban / peri-urban flows that lead to heavy traffics, including high commutting traffic towards Luxembourg. Therefore traffic efficiency and safety are high stakes on this congested highway.
In response, the variable speed limits measure is deployed in order to:
- improve traffic conditions: increase in flows, decrease in congestion and travel times,
- improve safety: more homogeneous speeds, decrease in accident and in seriousness,
- decrease pollutant emission.
An a priori evaluation has been conducted in 2011 by the Cerema. Based on modelling, it assessed traffic, safety and environmental impacts and confirmed the socio-economic benefits of the measure.
In the field, the variable speed limit measure leads to equipement deployments :
- for traffic measure and monitoring : 23 traffic sensors (loops and magnetometers) and 14 CCTV cameras,
- for communication toward drivers : 25 variable message signs, on the shoulder or on gantries above the lanes.
In addition to in-the-field deployment, the measure requires the traffic management system to evolve. The new algorithm integrates three parts: congestion prevention (before congestion appears), end-of-queue protection (in case of congestion) and event protection (e.g. vehicle breakdown). A spatial and temporal harmonisation ensures that the final speed limits respect the legal framework and is still consistent from the user point of view.
A first section (5km south to Thionville) was put into service at the end of 2013. An evaluation was conducted by DIR Est in 2014. It focused on the system technical reliability and on the traffic management centre operating. It resulted in the algorithm improvement for the other sections deployments. The global measure will be put into service in 2015, and should provide first evaluation results in 2016.