On a plate below this sign should be stated the permissible height of the loaded vehicle to avoid danger and if a gauge exists this should also be indicated in the plate below the sign in advance of the gauge as in figure 65a.
123. The Protocol prescribes a sign to indicate 'the approach to a section of the carriageway which, under certain conditions, may have a slippery surface'. Such conditions would include a carriageway worn smooth and in need of resurfacing or one which, for example, is liable to be made slippery by clay from the wheels of farm tractors. Ideally, no permanent sign should be necessary in the first instance since road surfacing should, and in this country probably would, be carried out as soon as it became slippery. This may, however, be a counsel of perfection and we would think this sign would be useful in the United Kingdom to warn against certain road surfaces, especially on bends, and that it should therefore be prescribed. We think it should also be used where road surfaces are liable to be fouled, for example, by agricultural or industrial operations. We have referred to this also under temporary signs (paragraph 206) . We recommend adoption of the sign in the form shown at figure 53 for limited use in circumstances which should be defined by the Departments.
124. A Protocol sign for this has recently been adopted. It is to be used to give warning that aircraft taking off or landing at an airfield may fly low over the road. In such circumstances, or where sudden aircraft noise near a road may startle drivers, we recommend use of the sign at figure 67.
125. Where a road leads to a quay or river bank we recommend use of the sign at figure 59, which is similar to the Protocol sign recently adopted.
126. On roads where there may be danger from falling rocks we recommend the use of the sign at figure 68 which has also recently been adopted for inclusion in the Protocol.
127. It has recently been agreed by the Working Party to the Economic Commission for Europe that warning of sudden side winds should be given by a wind sock, and that wherever possible this should be placed so that it can be seen by drivers from some distance. Where this is not possible, advance warning of the existence of the wind sock should be given by a sign containing the appropriate symbol within a red triangle.
We believe that both these signs would be useful in this country and we have suggested at figure 66 a design for an advance warning sign. The symbol should be reversed where the prevailing wind is in the opposite direction.
128. The Protocol prescribes a sign containing a vertical black bar symbol as
a warning of any danger for which there is no specially designed sign. It makes
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