a filter lane for left-turning traffic at slough-central roundabout (m4/a355)

experimental markings have been used in the roundabout at slough-central motorway interchange, to ease the problem of evening peak-hour queues in the westbound exit slip road from m4. an exclusive lane was marked in the existing roundabout carriageway to allow left-turning traffic from the slip road to filter through the roundabout, without having to give way to circulating traffic. this movement accounted for two-thirds of the slip road flow. with similar before and after levels of circulating traffic, a 36 per cent increase in slip road capacity was recorded, and a corresponding increase in total flow through the "weaving section" of 12 per cent. queues of left-turning vehicles, which had previously blocked back onto the motorway main carriageway, have largely disappeared. no personal injury accidents were reported in the first seven months of operation. (a)