Discussion

Technical and educational measures

Tests with ordinary drivers have revealed substantial differences in braking performance on icy/snowy road surfaces among various types of tyre and cars with or without ABS. Results indicate that technical and educational measures offer a great potential of safety improvements.

The distinctive superiority of ABS brakes both in deceleration performance and in directional stability put serious doubts on the claims from believers of risk homeostasis (RH). They consider ABS and other technical improvements useless, and believe that drivers adapt their behaviour to keep risks constant.

Negative results (no significant differences found) from various studies, are taken as proofs of RH, though many other scientists disagree on the principle; see e.g. Stottrup Hansen et al. (1990). Though biased in annual mileage, insurance statistics from the U.S. (HLDI, 1994 & 1995) have been considered proofs of RH. That seems to delay the transition from uncontrolled brakes to ABS in the car population.

Both driving and crash safety

Not only accident avoidance but also injury prevention may be effected, since crash speed is reduced with better braking performance. On the lane (B) with polished ice (no studs allowed), used summer tyres and disconnected ABS gave an average deceleration less than 50% of that with ABS and new winter tyres.

In a situation where both cars start braking from the same point, the inferior car will go with 75% of the initial speed, where the superior car has stopped completely; see Strandberg (1995b, figure 2).

Whiplash injury prevention

Individual braking performance on each type of surface varied substantially between cars and drivers in many test sessions. A time lag of 3s between the vehicles would have been sufficient in less than 10% of the 66 cases, if the best car+driver was braking in front of the worst one from 70 km/h. Such time lag requirements are much greater than what is needed on dry or wet tarmac surfaces. The demand of greater gaps in car following situations is probably unexpected to most drivers of inferior vehicles. It may be responsible of many rear-end collisions and whiplash injuries.

Spread

The poor braking performance of certain vehicles on icy roads is a serious safety problem. The variations lead to dangerous mis-matches in the traffic flow. The great spread indicates that skid mark length only is insufficient for assessment of initial speed in accident investigations.

Unstudded tyres need studs to increase road adhesion

Irrespective of ABS, the average deceleration was greater on lane C than on lane B for all eight types of tyre without studs; see Fig.12 & Fig.13. Lanes B&C were covered with ice treated in the same way, except of that the surface of lane C was scratched and made harsh by the studded tyres. Since the lateral inclination of lane C was greater than of lane B, the advantage of harshness is probably underestimated in this study.

Road adhesion is increased by studs on ice-free road surfaces, too. This has been observed as a seasonal variation in Germany, which faded out after the winter of 1975/76, when studs were banned. Before that, the enhancement of road adhesion during spring and summer (due to traffic with studs in winter) may have reduced crash speeds and prevented some accidents, which would have occurred with more polished road surfaces. The advantage of stud scratching is well known among tyre development professionals and should be taken into account when prohibition of studs is considered.