In this survey the main objectives were to find information on unfastening of tires in heavy vehicles and separations of towing vehicle and trailer. The main source of information was data base of Finnish Police Force (RIKI) from year 1998-1999. Fatal accidents were also looked from Finnish road accident investigation teams records. In tire unfastening accidents in the case if heavy vehicles has to be pointed out always the possibility of large and even fatal damage for other road users. In typical tire unfastening accident twin wheel from left side of trailer rolls over to the lane of oncoming vehicle. Tire unfastening accidents seem to be more common during the winter time. This is probably because of snow banks. Snow banks prevent tire from rolling freely from the road after this tire hits oncoming vehicle and afterwards the accident comes to police records. The vehicle of which tire has been unfastened will often keep its stability and normally damages are minor. There are also signs that temperature and inclination of road surface has an effect on tire unfastening accidents. In Finland there are yearly about 150 accidents of this type reported to police. All together there are several hundreds of tire unfastening accidents yearly. Fatal accidents happen approximately every second year. Tire unfasten more often from trailer than from the truck itself. This is most likely because the driver has relatively poor sense of driving behaviour of trailers as compared tot the sense of the truck itself. Foreign trucks and trailers have very few tire unfastening or trailer separation accidents in Finland. Tire fastening in trucks is under extremely heavy stress and needs to be monitored regularly. This is why there should be a tire diary type of solution to make sure that tires are maintained regularly. Trailer and truck accidents are more common among full trailers. Most common reason is loosening of pulling coupling. Pulling couple is very difficult to inspect without disassembling it. This is why there should exist an official document of verification of full trailer drawing vehicle pulling coupling. This document should be renewed yearly and checked during inspection. Screw nut of pulling couple has to be renewed after inspection. All repairs of draw bar should be considered to be prohibited. Trailer separation accident from passenger cars have connection to maintenance of trailers. Small and light trailers are relatively cheap and seldom used. This is why its maintenance is easy to neglect the mechanism of an accident is simply that trailer breaks off from drawing vehicle. In most cases the to wing car has been relatively new (age under 10 years) and it is suitable for towing. There is a need for bulletin on maintenance of light trailers. Passenger car trailers should be inspected also by roadside inspections. (A)
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