Headlight glare screens on motorways.

In order to take full advantage of an unlighted motor road at night it is necessary to protect drivers from the glare of headlights of vehicles approaching in the opposite carriageway. This can be done in the initial design by widely separating the two carriageways. When however the carriageways are built close together the median strip is sometimes planted with shrubs, but until these have grown sufficiently it is desirable to erect some form of fence; where the median is too narrow for shrubs a permanent fence may be necessary. The report deals with the design of such fences, which need not be completely opaque to reduce headlight glare to a tolerable level. This is achieved if the fence cuts off light up to an angle of about 20 deg. from the axis of an approaching headlamp. The maximum height and the depth of the fence required are determined by the eye height of the drivers, the headlamp heights of vehicles using the road, the transverse position of vehicles the crossfall of the road and its undulations. Neglecting changes in gradient it is shown that for the london-Birmingham motorway all drivers could be protected from serious glare, whatever the position of their vehicles by a fence 7 ft 2 in. high with a gap 1 ft 9 in. at the bottom both measurements made in relation to the plane joining the edges of the carriageways adjacent to the median. The bottom clearance not only effects a saving in cost but is necessary in order to prevent the formation of snow drifts on the lee of the fence. Taking into account the fact that commercial vehicles will normally travel in the slow lanes it is shown that a fence height of 5 ft 8 in. Will provide protection for their drivers in 95 per cent of their meetings with oncoming vehicles and this is the minimum height recommended in practice for flat sections of road. A fence 5 ft. high would only afford protection in 22 percent of such meetings. The properties of various types of fence are discussed and it is concluded that glare protection is best achieved by a metal mesh fence whose characteristics are described.