|The M1 between junctions 7 and 8 has lanes to help prevent weaving|
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Weaving is the name given to the phenomenon when two grade-separated junctions are placed so close together that traffic entering from the first junction conflicts with traffic trying to leave at the second, and thus traffic can be prevented from either entering or leaving a road by traffic attempting the opposite manoeuvre.
The Design Manual for Roads and Bridges requires a minimum weaving length of 2km on a motorway, and 1km for other roads. On trunk roads and motorways, distances less than these require special approval through application for a Departure from Standard. Traditional cloverleaf interchanges have very short weaving lengths between loops, and cannot comply with this standard, therefore they are not widely used in the UK. Elsewhere, busy cloverleafs are being modified to remove one or more weaving lengths.
Weaving issues are common on roads with closely-spaced junctions like the Coventry Inner Ring Road, or the A38 through Birmingham city centre. They can cause congestion and may have a higher incidence of collisions.
A common way of trying to deal with the issue is by the construction of CD Lanes, for example between M1 junctions 7 and 8.