Differentiated Speed Limits - How to Implement This Safely?
H Ludvigsen, Danish Road Directorate, DK; J Mertner, COWI A/S, DK
How to identify road sections where the speed limit could be increased from 80 km/h to 90 km/h without jeopardising road safety and where only minor and cheaper safety measures are necessary for implementing the higher speed limit.
Differentiated speed limits allowing higher speed at certain road sections whilst maintaining the safety standards are presently being applied in Denmark. The typical odds that higher speed limits will increase the number of accidents must thus be beaten by the project. The Danish Road Directorate has been asked by the Ministry of Energy and Transport based on a request from parliamentarians to suggest an approach to assess the potential for introduction of differentiated speed limits on the Danish state road network.
A pilot project was carried in late 2006 and the entire state network will be assessed during the first half of 2007 - first of all to identify where speed limits may be raised.
The paper will present the methodology and findings of a project carried out by the Danish Road Direc-torate and COWI aimed at identifying potential sections where the speed limit could be increased from 80 km/h to 90 km/h without jeopardising road safety and where only minor and cheaper measures are necessary. Thus it will be described how to systematically assess the road network when the speed limit is to be increased.
The methodology to identify sections with a potential for increasing speed limits are divided in the fol-lowing activities:
? criteria for assessment of speed limits
? data collection
? screening of road network
? inspection and assessment of network
? identification of potential sections and measures needed
? assessment of consequences.
Speed dependent criteria that should be fulfilled at speed limits 60 km/h, 70 km/h, 80 km/h and 90 km/h were identified. These include road class, cross sections, road side (safety zone), existence of non-motorised traffic, horizontal and vertical alignment, land use, density and type of junctions, sight dis-tances, number of accesses, traffic level and density and frequency of accidents.
Data on the road network was collected from the road data bank (VIS) and the data were inserted in the database where the road data were compared to the above criteria. This screening resulted in a list of road sections fulfilling the criteria for different speed limits and the results are presented on digital maps. The advantage of screening the network in office is that road sections that are far from the criteria for 90 km/h and thus generally too costly to upgrade can at an early stage be sorted out. Focus can then be on the roads with a potential for higher speed limits and resources may then be used more efficient during the more time consuming inspection in the field of the network.
The inspection of the roads is used to confirm data from the road database and to register issues not in the road database such as too narrow safety zone, accesses, sight distances in junctions, etc. Based on the screening and inspection potential road sections are identified which have the potential for higher speed limits. Also measures to avoid negative consequences on road safety are identified and costs of the measures are assessed.
The consequences of the changed speed limits are assessed. These indicators are traffic accidents, time, emissions and noise. When the effects are assessed these may be applied a cost. This result may be used to describe the consequences for society by raising the speed limit, thus whether it is cost-effective and it could be used to prioritise among the sections if more resources are needed than available.
The expected conclusions of the project is a number of road sections which have been identified to be appropriate for higher speed limits, the measures and associated costs needed to ensure that they are as safe as before the speed limit is increased and finally the consequences of the changed speed limits are assessed. It is thus expected that the project will identify a number of km roads with a potential for higher speed limits, but there will be certain costs to implement measures to avoid reduction of road safety.
Association for European Transport