eSafety Compendium

Influencing vehicle speeds with the help of variable speed limits has been tried especially inconnection with weather-related traffic management systems by lowering speed limits in adverseconditions. A variable speed limit system integrated with a fog warning system reduced thenumber of injury accidents on a German motorway by around 20 % (Balz & Zhu, 1994), and avariable speed limit system integrated with a slippery road warning system on a Finnish motorwayby around 10 % (Rämä, 2001). Both studies reported significant reductions in mean speeds (3 to 9km/h) in adverse weather conditions, and the latter also a significant decrease in speed variation.An accident study showed that weather-related speed control reduced injury accidents by 13 % inwinter and 2 % in summer on sections, where the control system was automatic and of goodquality. Manually operated systems, however, were estimated to result in increased accident risks(Rämä & Schirokoff 2004).A Dutch fog warning system including a text warning (“fog”) and dynamic speed limit VMS signson a motorway, reduced speeds in fog by 8 to 10 km/h, although in extremely dense fog, thesystem had an adverse effect on speed. This was due to the too high “lowest possible speed limit”display in the VMS (60 km/h). A more uniform speed behaviour was obtained due to theintroduction of the system (Hogema, van der Horst & van Nifterick, 1996). Variable speed limitshave also been applied by schools, resulting in a 20 per cent accident reduction (Elvik et al., 1997).In addition to speed control, the high accident risks caused by adverse weather conditions can bedecreased by providing information, warnings and support to road users, but also by combatingweather problems with the help of winter maintenance. A Finnish study (Rämä et al., 1996)showed that slippery road warning VMS decreased mean speeds by around 1–2 km/h when thesigns were lit. The system was also shown to affect the direction of attention to find cuesshowing potential hazards, and to make passing behaviour more careful indicating an even largerpositive impact on safety than that due to lower speeds (Luoma, Rämä, Penttinen & Harjula,1997).The automatic fog-warning system on the M25 motorway in England displays the “Fog” legendon roadside matrix signals. The assessment of this system showed that the net mean vehiclespeed reduction was around 3 km/h, when the signals were switched on as a result of theformation of fog (Cooper & Sawyer, 1993). Collision warning systems are probably beneficial toroad safety in the fog (Saroldi, Bertolino & Sidoti, 1997).Speed AlertThe largest study so far on Intelligent Speed Adaptation (ISA) systems have been carried out inSweden (Biding & Lind 2002). These studies involved 5000 equipped vehicles driven by over10000 drivers (from different age groups) in urban areas as well as an accident study. Speed alertwas one of the systems studied. The studies found out that all ISA systems result in better roadsafety without increasing travel time, and that there were quite minor differences between theimpacts of speed alert and those of other types of ISA. The estimate was that if everyone hadISA, there could be 20% fewer road injuries in urban areas.Page 368 of 490