APPROPRIATE HIGHEST SPEED IN DIFFERENT CRITICAL SITUATIONS

Speed adaptation in critical situations, such as wet/slippery roads, darkness, fog, at pedestrian crossings, intersections, school zones, road work zones, curves, are far from satisfactory while accident risks increase drastically in these situations. Today's speed limit system is rigid while it is supposed to work in a dynamic system. Dynamically changing speed limits, adapted to the prevailing conditions have a great potential to improve road safety and vehicle flow. The aim of this work has been to carry out theoretical work in order to define the appropriate highest speeds in situations where dynamic speed adaptation can be applied to increase safety and improve vehicle flow. The calculation of the appropriate highest speeds in critical conditions is based on the criterion of constant stopping distance, which means that the vehicle can be stopped within the same distance regardless of whether the road surface is dry or wet/slippery and within the visibility distance. The criterion for the calculation of the appropriate highest speed in darkness is that the stopping distance must be shorter than the lit-up visibility distance for the vehicle to stop in case of an unexpected stationary object ahead. In built up areas where motor vehicles and vulnerable road users share the same space vehicle speeds should not exceed 30 km/h in interactive situations with pedestrians and bicyclists. At road work zones and accident sites where road workers and rescue personnel are present vehicle speeds should not exceed 30 km/h. At intersections with only motorized traffic the maximum speed level should be 50 km/h. The calculation of the optimal speed for highest flow for different road types is based on the relations between speed, flow and density under ideal traffic and road conditions. To be able to establish and mathematically describe the actual conditions for a particular road stretch, observations of these parameters in field are necessary. The system for influencing the driver's speed choice can be based on road-side information with the help of variable message signs with the actual speed limit or recommended speed or vehicle based with information in the car via a display, feedback or speed limiter. Informative systems are less efficient than vehicle based systems with feed-back or systems intervening in case of erroneous speed choice. The draw-back with vehicle based systems is that they demand a relatively long and extensive implementation process. The report is available in fulltext as a PDF file ((630 kB). http://www.tft.lth.se/publ/7000/7196scr.pdf. (A)