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Slip roads in need of a redesign


I refer to the news item (NCE 14 January) concerning problems with the slip roads on the Newbury bypass. This is not something new, arising from environmental considerations. It is a fundamental flaw which has existed in the design for many years.

The stated purpose of acceleration/deceleration lanes is to allow vehicles to change speed to match that of traffic on the main line.

Many layouts have a tight radius (25m to 30m) 90degrees bends at the end of the merge/diverge lanes. These are often joined to a further tight 90degrees bend of the same or opposite hand to provide a link to the minor road.

On the Quorn Mountsorrel bypass, many vehicles negotiate these bends at about 50km/h. The slip roads on to the A6, particularly those approached uphill, provide little time for drivers to assess the speed of traffic on the main line, and the short and direct nature of the taper forces them to quickly decide whether to merge or take alternative action, such as slowing or stopping, which may be equally hazardous.

Drivers on the main line also have little forewarning of traffic merging and quick lane changes are the order of the day. Fortunately,

the traffic flows on the road are not too high. Drivers leaving the main line have little forewarning of the tightness of the bend and it is impossible to negotiate the bends safely from normal nearside lane speeds without slowing considerably before entering the deceleration lane.

Surely acceleration/ deceleration lane lengths should be related to the speed change required and the gradient. This implies that layouts which do not have long straight slip roads should have longer acceleration/ deceleration lanes than those that do; the opposite of current practice.

Perhaps a visit to France is called for. Their acceleration lanes are nearly always adequate and bends on single carriageways often have a curvaceous alignment but one that allows speeds to be maintained comfortably at 90km/h to 100km/h.

John Marriott (M), 66 Latimer Road, Cropston, Leicester, LE7 7GN

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