Variable Speed Limits at Intersections - Effects and Experiences
Mohsen Towliat, Helena Svensson, Swedish National Road Administration Consulting Service, SE; Gunnar Lind, Anders Lindkvist, Movea Trafikkonsult AB, SE
Variable speed limits were implemented at six rural intersections in Sweden to increase traffic safety and improve observance of speed limits. Evaluation at one site shows very positive effects on car drivers? speed behaviour and economical benefits.
Background, problem and aims
The Swedish Road Administration (SRA) initiated an experimental work during 2004-07 aiming at implementing variable speed limits (VSL) at six rural intersections and carry out studies of effects. The overall problem is the poor observance of speed limits in Sweden, which has a large negative impact on traffic safety.
In the particular intersection presented as an example here - Fogdarp - a number of serious accidents have occurred when vehicles from the side road have turned to the main road and collided.
Description of test site and studies
The intersection is situated on a 13-meters wide (single carriageway) rural road with a speed limit of 90 km/h. It is a T-junction with a west-bound right turn lane of the main road and a east-bound left turning lane.
VSL was introduced to increase the observance of the speed limits in the intersection, improve the traffic safety and the accessibility. The speed limit through the inter¬section was 90 km/h before, but with variable speed signs it can be set to 70 km/h when a vehicle is turning either from the side road to the main road or vice versa.
Different kinds of effects of VSL were investigated by before and after studies. The before study was carried out in autumn 2003, the VSL measures implemented in autumn 2004 and the after studies almost 8 months later.
Car drivers? experiences
According to the results from the attitude study, VSL have made a troublesome rural intersection much better. Both effects of VSL, an increased possibility to turn to the main road and better observance of the speed limits, have been fulfilled. A majority of the drivers state that it has become easier especially to turn to the main road. Some drivers did however feel an uncertainty about the approach speed of the cars on the main road.
Fewer drivers were exceeding the speed limits since the VSL were introduced and the drivers claimed they had more respect for the VSL than the ordinary speed limits. Those who admitted they drove faster than the speed limits claim this was more due to the traffic flow and traffic environment as a whole than the fact that they did not know the actual speed limit.
Drivers who experienced that the technique was working properly, was considerable more fond of the system than the others and more willing to keep the speed limits than the others. Thus it is very important that the drivers feel that the system work properly.
Behavioural changes and Socio-economic analysis
The expected average speed on a main road such as E22 is in Sweden 91,5 km/h for passenger cars. Speed measurements show a very good accordance. Average speed is 91,6 km/h for passenger cars on E22, eastern direction.
The compliance of the variable speed limit was expected to be 3 km/h better for passenger cars and unchanged for trucks compared to local static speed limits. This would have resulted in speed reductions of 0 km/h with a 90 km/h speed limit and 8 km/h with a variable speed limit of 70 km/h. The actual speed change was surprisingly substantially greater.
Incoming traffic from the side road (1065 vehicles/day) results in VSL 70 km/f activation 530 times per day during 44 seconds in average. That means around two vehicles per period. The active time per day is hence ca 6.5 hours. Also the 500 left-turning vehicles from the main road activates the lower speed limit.
Estimated safety effects
As the expected number of accidents at the intersection is a low as 0.7 accidents per year, a statistical analysis can not be made for Fogdarp. The safety effects are instead estimated with guidance from Swedish Guidelines for Cost-Benefit Analysis. This is used together with the traditional power model, in this case with power 3.7 for the number of severe injuries according to recent experience.
After the introduction of VSL the speed is reduced both at 90 km/h and 70 km/h. At VSL 90 the reduction is 7.3 km/h and at VSL 70 the reduction is 16.6 km/h. VSL 70 km/h is valid for 27% of the vehicles and VSL 90 km/h for 73% of the vehicles.
The measured speed reductions with application of the traditional power model results in 0.067 fewer severe injuries and kSEK538 yearly in reduced accident costs. Compared to the expec¬tations in the feasibility study, the effects are around twice as great. The number of dead and seriously injured road users is expected to halve by introducing VSL in the intersection of Fogdarp, which of course is a very good and promising outcome.
Construction costs at Fogdarp have been estimated to MSEK 2.1. The operating costs were roughly estimated to 10% of the investment. This results in a total preliminary life cycle cost during a 20 year period of MSEK 5.8 (C).
To the benefit should be added increased comfort and the value of improved compliance that cannot be estimated in the current model. With this reservation, the illustrated speed changes and the assumed cost results a probable net benefit/cost ratio of -0.2.
Recommendations and conclusions
The overall conclusion in this study is that variable speed limits contributes to improved traffic safety at the intersection at the cost of a few seconds increase in travel time for drivers on the main road (E22). Speed violations have decreased after introduction of variable speed limits during the time (70 %) when the speed limit is 90 km/h for all drivers, while more than 50 % of passenger car drivers and 60 % of the truck drivers do not respect the lower speed limit 70 km/h.
Variable speed limit at intersections can therefore give very good results with suitable conditions. With high demand on equipment and central control it seems however that the costs will be too high. Savings in construction and operating costs are therefore necessary and work has therefore started to investigate various possibilities. The final profitability can therefore not be estimated in the current stage of the project
Association for European Transport