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Fogging of windshields in road tunnels - synthesis (ASTRA2001/013)

Switzerland Flag
Complete with results
Geo-spatial type
STRIA Roadmaps
Transport mode
Road icon
Transport policies
Transport sectors
Passenger transport,
Freight transport


Background & Policy context

When a car enters a tunnel, the occurrence of water condensation on the outside of the front shield is possible if air temperature and/or humidity in the tunnel are higher than outside. Due to cooling of the tunnel air on the surface of the vehicle, the temperature can fall below the dew point and therefore condensation on the surface occurs. Because of sudden appearance and reduction of the driver's view, this phenomenon presents a remarkable potential for danger.


The main objective of the project was to develop tools for predictions  and by means of surveys with tunnel users, theoretical studies, model calculation and measurements in three affected tunnels to propose measures reducing the appearance of windshield fogging.

Beside the suggested measures, the final report shall contain recommendations which have been made to be included in the Swiss guideline "Ventilation of Road Tunnels".


The project was worked out in three steps:

1. Through a survey of the Cantonal (Provincial) Authorities, it was investigated in which tunnels windshield fogging occurs in Switzerland. The phenomenon is reported to occur in a total of 19 Swiss road tunnels.

2. In selected tunnels indoor-air climate measurements were carried out

3. Measures were suggested to reduce the appearance of windshield fogging.


Parent Programmes
Institution Type
Public institution
Institution Name
Swiss Government: State Secretariat for Education and Research
Type of funding
Public (national/regional/local)


The following statements can be derived from the measurements and surveys: 

1. On the basis of stronger emission regulations, or in other words, decreasing noxious emissions from the traffic, a lower air-change rate by the mechanical tunnel ventilation system is to be expected for the future.

2. When employing a midpoint-extraction ventilation system the dew-point temperature at the tunnel entrance is significantly reduced. This minimizes an abrupt climatic change when entering a tunnel and, as a consequence, reduces the sudden occurrence of windshield fogging. The midpoint-extraction system is well suited to reduce the hazard. Also of critical importance is the correct operation of the dew-point control system. 

3. With a dew-point controlled semi-transverse ventilation system even with a large expenditure of energy, the fogging hazard can be marginally improved. This system reduces the dew-point temperature at the tunnel entrance just slightly.

Various measures are suggested in order to reduce the incidence of accidents caused by the sudden fogging of windshields. A measure can reduce the risk through an improved reaction of the tunnel users and/or through a reduction in the occurrence of fogging windshields.

On the strength of the findings, different recommendations and measures for safety standards are suggested for both road tunnels in the planning stage and for tunnels in service. For one-way traffic tunnels no safety measures are necessary. For bidirectional tunnels longer than 1400 m, measures are to be implemented.

Innovation aspects

Measures for lowering of windshield fogging in tunnels.

Policy implications

Enhancing of safety in tunnels.


Lead Organisation
EU Contribution
Partner Organisations
EU Contribution


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