Small curves should be avoided, especially if they are connected to a straight alignment. A minimal curvature of 550-600 m should be observed. The lateral clearances must also allow for longitudinal visibility in curves.
In urban tunnels, it should be adequate to consider design speeds close to the actual speed in fluid and uncongested traffic flow.
Due to the influence on speed, longitudinal downgrade profiles cause more accidents, especially with high traffic volumes (increase of speed downwards).
Reduced cross-sections are dangerous and may cause accidents.
Attention should be given to the point that, if the width of the carriageway and/or off-carriageway area in the tunnel and in the approach to the tunnel is less than on the open road, these changes should be implemented well before the tunnel portal and as smoothly as possible: see Chapter 4.7 "Design of tunnel portals" of Report 2008R17.
Accidents involving oversized vehicles are often recorded in rectangular tunnels or tunnels with a ceiling for ventilation purposes.
It is advised to install outside the tunnel, ahead of each portal, a signed escape route as well as a system to stop physically oversized vehicles.
More information is available in Section IV.2.6 "Height clearance" of Report 05.04.B.
Bidirectional tunnels cause more accidents than unidirectional ones. Nevertheless users observe fairly well the prohibition to overtake in tunnels with average longitudinal gradients. In case of steep gradients it should be adequate, however, to plan an additional lane for slow vehicles.
It is strongly advised against changing the traffic direction to absorb daily traffic peaks.
Bidirectional tunnels may be economic for the phased construction of motorway tunnels, where economic considerations require bidirectional traffic operation to be planned at a first stage, then unidirectional at a second stage. However, this is under the proviso that the usable tunnel width is designed with bidirectional traffic requirements in mind and is thus wide enough, in order to absorb a number of traffic peaks (e.g. summer or winter holidays). Even if such an arrangement may be acceptable from a safety viewpoint, it must be avoided as often as possible. For urban tunnels it must be prohibited.
Underground interchanges (slip roads in and out) may cause accidents. They must thus be correctly designed. The lighting equipment should lay emphasis on these singular points and on the geometric challenges faced by the driver. Consideration must be given to the driver’s visual perception.
Inside the tunnel, the traffic exits must be located at a distance from the portal. A number of accidents, mostly injury accidents, have occurred in tunnels where the slip road is located directly after the tunnel. In the case of tunnels with restricted space conditions, it should be adequate to plan an additional lane inside the tunnel for the exit slip road.