Slow vehicles (tractor, farm or forest machine, etc.) which go on the roads significantly slower than the traffic flow, obstruct road traffic and become a source of hazard. This fact must be taken into consideration both by the drivers of slow vehicles and drivers of fast vehicles. For ensuring traffic safety, it is essential that the drivers be able to estimate the speed of vehicles and distances, when approaching a slow vehicle. In practice, drivers estimate separation distance and speed of vehicles "by feeling", intuitively. It is necessary for the drivers to cognize safe separation distance and know the influencing factors.
The minimal distance from a slow vehicle (safe separation distance), at which the driver, approaching the slow vehicle, has to decide if it is possible to pass the slow vehicle at once or if he should start slowing down, has been studied in this paper.
Dependence of safe separation distance upon the speeds of vehicles, on road adhesion factor and on the reaction time of the driver have also been studied. Dependence of safe separation distance on the speeds of vehicles and on road surface adhesion factor is non-linear. Safe separation distance is 66–52 m in case of real speeds of vehicles (5–40 km/h for slow vehicles and 90 km/h for fast vehicles). As it is very difficult for the driver of a fast vehicle to estimate the exact speed of the slow vehicle, the driver of the fast vehicle must become observant with regard to the slow vehicle at the distance of 70 m and estimate the possibility of passing the slow vehicle.
The speed of the fast vehicle has greater impact than the speed of the slow vehicle on safe separation distance. In case of varying the speed of the fast vehicle in range 50–120 km/h, safe separation distance varies in range 20–100 m (speed of slow vehicle is equal to 20 km/h).
Road surface adhesion factor has as essential influence on safe separation distance as the speed of the fast vehicle. If on clean dry asphalt surface of the highway (speed of fast vehicle is 90 km/h and speed of slow vehicle is 20 km/h) safe separation distance is 64 m, then on wet surface of the highway, safe separation distance is 6–10 m longer, on muddy or slimy surface, safe separation distance is 15 m longer and on slippery road surface (adhesion factor φ ≤ 0.4), safe separation distance is over 100 m at the given speeds (Figure 4).
Dependence of safe separation distance on the reaction time of the driver is linear. Safe separation distance is equal to 64 m when driving on the highway at given speeds and the reaction time of the driver is 0.8 s. Safe separation distance varies in range 55–72 m, if reaction time of driver varies in range 0.5–1.2 s. For a drunken or drugged driver whose reaction time is 2–3 s or more, safe separation distance is 100–120 m or more (Figure 5).
Approaching a slow vehicle, the driver of a fast vehicle must decrease the speed proportionally to the decrease of separation distance for ensuring traffic safety (Figure 6). At separation distance of 60 m, the safe speed is equal to 75–90 km/h, at distance of 40 m the safe speed is equal to 55–70 km/h and at the distance of 20 m the safe speed is 25–45 km/h, when the speed of a slow vehicle is 7–40 km/h.
The driver of a slow vehicle ought to indicate at least 6–10 s before left turn and changing to the left lanes.
Keywords: slow vehicle, fast vehicle, separation distance, stopping distance, adhesion factor, reaction time of driver.