Sicherheit in Autobahn- und Strassentunneln - Fonds für ...

84 Zusammenfassung / Résumé / Riassunto / Abstract
In fact, the accident figures only increase by a factor of 1.68 (=2-0.32) and the number of
casualties by a factor of 1.80 (=2-0.20).
The ADT influences shown in Table Z.1 are adjusted for exposure, which means that the effects
calculated are attributable to traffic density. An increase in traffic density increases the risk of
collisions and the risk that persons will be injured. To put it another way - the results mean that
an increase in ADT will lead to a disproportionate rise in accidents and casualties. Thus, twice
the amount of ADT will not result in twice the number of accidents but will lead to an increase
in accident figures by a factor of 2.77 (=2+0.77) or to an increase in the number of casualties by
a factor of 2.38 (=2+0.38).
Roughly speaking, when compared with single tunnels with bi-directional traffic, twin tunnels
have half the risk of accident and casualties. However, the fact that any extra traffic can have
negative repercussions should be taken into account.
The proportion of heavy goods vehicles in ADT merely has a marginally significant influence
on the risk of injuries but not on the risk of accidents. This difference points to the fact that a
large proportion of heavy goods vehicles has no influence on the frequency of accidents but on
their severity.
The width of the shoulder has turned out to be a significant accident predictor. However, in
contrast, the width of the shoulder has no influence on the number of casualties. These results
indicate that, while wide shoulders can reduce the probability of an accident, they cannot reduce
the consequences of accidents.
Other features investigated appear to have no significant influence on road safety in tunnels. In
particular, the features of "posted/permissible top speed", "number of bends", "longitudinal
inclination", "height of shoulder", "lane width", "tunnel height" and "light density" are irrelevant in
this connection.
This study basically reveals that, from an objective standpoint, the problem of accidents in tunnels
should not be dramatized. However, it is important that tunnels are constructed and operated in
such a way that the optimum level of safety can be assured.
To supplement the measures proposed by the Tunnel Task Force, the following recommendations
can be made (Table Z.2):