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High way Tunnels 6.1 HIGHWAY REQUIREMENTSThe special construction features for highway tunnels include geometrical configuration,road construction, lighting,ventilation, traffic control,fire precautions,general facilities for cleaning and maintenance, and cable services of public utilities. The requirements will vary according to the tunnel situation and character, whether urban, motorway, subaqueous or mountain and whether long or short.A highway tunnel is part only of a larger highway system. Its function in providing a crossing under an obstacle may be of major importance in linking up areas with poor communications, as under an estuary or through a mountain range. In some instances, tunnels may be relatively minor elements as in a road following a rocky gorge Road systems are designed on the basis of their traffic capacity, taking account of lane widths and clearances, gradients and traffic composition. The same formulae can be applied to tunnels, but it has been found that in tunnels where regular commuters constitute a substantial proportion of peak hour traffic the peak hourly how may exceed the nominal capacity by 50% or more.For a number of two-lane one way tunnels actual flows of fromvehicles/hour have been observed as compared with nominal capacity fromIn contrast rural and mountain tunnels, in whose peak traffic the proportion of regular users is small, may have an actual capacity of 80—90% of the nominal capacity.The principal factor determining the traffic capacity of a tunnel is the geometry of its alignment and cross section, but its lighting and ventilation may impose lower limits. Special factors influencing traffic flow include freedom from side turnings, absence of cyclists and pedestrians, and the regularity of use by a substantial proportion of drivers, or, alternatively, unfamiliarity. If capacity is found less than adequate the geometry cannot be readily altered, but Deficient lighting can usually be improved to any necess