THE INFLUENCE OF LANE WIDTH ON TRAFFIC FLOW ON MOTORWAYS

Since 1974 the recommendations (ral-q) contain a reduced motorway cross-section (rq 26) which has a lane width of 3,50 M instead of 3,75 M. To determine the influence of lane width on traffic flow and safety test stretches with lane widths of 3,25 M, 3,50 M and 3,75 M were selected and traffic data such as local speed, time intervals, traffic density, traffic composition, lateral distance keeping of drivers were collected for traffic densities of up to 1200 vehicles/hour. As expected the average speed of goods vehicles fell significantly (7 km/h) on stretches without hard shoulder, when the lane width was less than 3,50 M. The speed spectrum of passenger cars was very non-uniform, but showed a marked reduction even for 3,50 M lane width. The frequency of dangerous time intervals (t less than/equal to 1 s) on 3,25 and 3,50 wide lanes without hard shoulder averaged 3 to 4% more than stretches 3,75 M wide. The lateral distance keeping of drivers was governed mainly by the presence of a hard shoulder, the lane width, the speed and the occupation of the lanes. The frequency of accidents through errors during overtaking, being overtaken or lane change on stretches with narrow lanes (3,25 M) was above the average. On the basis of the available results the cross-section rq 26 provides sufficient manoeuvring latitude even for goods vehicles for design speeds V (index e) less than or equal to 120 km/h. Lanes 3,25 M wide with hard shoulder are suitable only for traffic with a preponderance of passenger cars for V (index e) less than or equal to 120 km/h. /TRRL/