Impact of human factor on speed choice

The paper presents results of an extensive study of speeds at night-time. The research was based on studies carried out over several years in which speeds were measured on two-lane rural roads (speed limit 90 km/h) and main network roads with suburban characteristic (similar to roads in small towns with various intensity of residential and commercial development along roads and a speed limit 50/60 km/h) in Poland. This paper shows results of comparisons made between free-flow speeds recorded during the day and night. These analyses showed that the worsening of perception of the road and its surroundings at night leads to speed reduction on two-lane rural roads. However, an opposite average effect of a speed increase by 1-4 km/h was observed on suburban roads at night (at specific sites the speed increase reached even 15 km/h). Limitation of the road observation area at night on poorly lit or unlit roads causes the influence of the road surroundings on the speed choice to decrease. Speeds observed during the day in a free-flow traffic tend to be very variable when compared with the estimated average value from the 24h. Such an effect should be taken into account when determining the appropriate speed to road design.