Trrl Lr 782

During the construction of a crawler lane at two gradients on M5 motorway in 1975, two lanes were kept open to traffic up the gradient by re-marking the carrigeway with sub-standard lane widths. The effects of the reduced lane widths on traffic flow were minimal, although advisory speed limits on the sites probably reduced the speed of traffic on the gradients and their approaches. The traffic delays averaged only 12 veh-h/h when the narrow lanes were operating. Later in the work, lane closure and two-way operation restricted the uphill traffic to only one lane and produced considerable delays as a result (up to 112 veh-h/h). Consequently, when it was necessary to close one of the carrigaeways during the final stages of the work, a system of two-way operation using narrow lanes was implementaed. This system provided two lanes for traffic up the gradients, and reduced the delays to veh-h/h. The effects of the different methods of operation on accidents and traffic diversion are examined. The completed crawler lanes resulted in journey-time reductions of 0.15 and 0.74 minutes (over a total journey length of 5.7 km, for light and heavy vehicles respectively).