Such vertical markings are indicated by:

Explanation

The lower edge of a bridge or a tunnel.

The black-and-white chequered vertical marking drawn on the underside of the bridge opening in the picture has a specific job: it shows the lower edge of the bridge or tunnel structure; that is, the very surface that limits the permitted height of passage. That is why this band is applied exactly on the line of impact, across the full width of the opening, so that it is seen immediately from behind the wheel — just as in the picture, where even in the dim light beneath the bridge that band catches the eye more than anything else.

The safety logic is clear: the real danger at these points is the roof of a lorry, a bus or a tall load striking the bridge opening. The driver of a passenger car does not feel that danger, but the driver of the green lorry in the picture must judge the height of his load against this very edge. The crashes this marking prevents are usually severe: structural damage, a load spilled across the carriageway and a road completely closed.

Worth memorising: in vertical marking, "where it is drawn" determines its meaning. A band on the lower edge of an opening means a height restriction; the same pattern on piers and columns has a different meaning and warns of a dangerous element of the road structure. Do not swap these two in the exam.

• The entrance to an unlit tunnel is announced by a sign, not by a chequered band across an opening; and besides, there is no tunnel in the picture, but a bridge under which the road passes. • "Any dangerous element of the road structure" describes the vertical marking of piers, columns and parapets; a band on the lower edge of an opening shows only a height restriction.

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