14.10.2009
Everyone has been annoyed by a traffic situation with a slow-moving lorry blocking the overtaking lane (Figure 1). Such overtaking manoeuvres can cause considerable delays and endanger other road users; when two lorries are involved, it results in a "lorry duel" (the Germans call it an "elephant race"). By extending the type approval which was granted to the VIDIT company's traffic-monitoring system, it is now possible to determine the differential speed of two vehicles during overtaking, so that such hazardous passing manoeuvres can now be punished.
The German road traffic regulations stipulate: "Overtaking is permitted only if the overtaking vehicle is travelling at a clearly higher speed than the vehicle to be overtaken". Non-compliance may lead to a fine amounting to EUR 80 and a point on the driver's record whereby the list of traffic offence penalties does not define a minimum differential speed. There are, instead, decisions of the higher regional courts (OLGs) according to which overtaking an HGV which is travelling at a speed of 70 km/h (approx. 43.5 mph) with a vehicle which is faster by only 10 km/h (6.21 mph) is considered as inadmissible.
Figure 1: Road duel ("elephant race") on a German motorway
The VIDIT company's computer-aided traffic-monitoring system is based on the analysis of a video recording of the traffic situation. It determines the differential speed of the two vehicles involved in the overtaking manoeuvre in the period between two selected individual images from which the two individual speed values of the respective vehicles are calculated. To determine the speed, the distance covered by the vehicle in question between the two imaged traffic situations is measured. The distance is measured by means of a perspective transformation based on the vehicle positions digitalised in the video images and focused manually on the screen. The corresponding time interval between the two traffic situations results from the refresh rate of the camera which has been checked at verification and the number of full video images determined automatically by the traffic-monitoring system.
Figure 2 shows the localisation of the overtaking vehicle (red lorry). The measuring line characterises the corresponding location (tolerance range: 121.9 m to 123.0 m – 132.33 yd to 134.51 yd). The time data which is automatically read out from the video image is 12h 57 min 07s 21F (image no. 21). The position of the overtaken vehicle is determined from the same video image in the same way.
Figure 2: Digitalised video image of the traffic situation at time 1 with the processing mask (the subsequent addition of the coloured frames around the respective lorries is for identification purposes)
Figure 3: Digitalised video image of the traffic situation at time 2 (the subsequent addition of the coloured frames around the respective lorries is for identification purposes)
Figure 3 shows the second traffic situation (time data: 12h 57min 11s 14F, i.e. 3.72 s later) in which the position of the two vehicles in question is determined again.
PTB's type examination and type approval must, besides the correct functioning of the camera or image frequency, ensure that the algorithm used to determine the speed difference is always realised in favour of the overtaking vehicle's driver. Therefore, the value of the speed of the overtaking vehicle is systematically increased, whereas the value of the overtaken vehicle is – correspondingly – systematically reduced. From these two individual speed values of the vehicles involved (91 km/h and 86 km/h – 56.54 mph and 53.44 mph, respectively), the measured value of the differential speed (5 km/h – 3.11 mph) results. The operational error limit for the differential speed is +3 km/h (1.86 mph) and is also taken into account in favour of the overtaking vehicle's driver (further deductions are not necessary). In our example, the driver of the overtaking vehicle is accused of a differential speed of 8 km/h (approx. 5 mph). A larger differential speed can be ruled out.
The possibility of checking compliance with the rules in the case of overtaking manoeuvres is an immediate and active contribution to increased traffic safety and to improved traffic flow.
Contact person:
Frank Märtens, Dept. 1.3, WG 1.31, e-mail: frank.maertens@ptb.de