This report describes the collection and examination of traffic information at 27 highway maintenance sites selected from the 1989 and 1990 highway maintenance programme. Sites were selected to provide as representative a range of layouts and traffic composition as possible subject to the requirement that traffic operation was likely to be at or near capacity. At each location activities at the merge and crossovers were recorded using camcorders erected on overbridges. Data collected on site were supplemented by a questionnaire survey and discussions with site personnel. In the absence of incidents flow breakdown was found in the traffic flow range of 1600 to 2300 pcu/h/lane. The average sustainable flow was found to be 4050 pcu/h for a 2 lane section which does not include a hardshoulder, and 3900 pcu/h for a 2 lane section including a hardshoulder. The available evidence identified the merge area as the primary source of congestion induced by excess demand flow. Under heavy flow conditions forced merging from the closed lane served to initiate and propagate flow breakdown similar to that observed on motorway sections downstream of entries from intersections. Observations confirmed that incidents are the major cause of congestion at motorway maintenance sites with successive incidents likely to cause continuous congestion over long periods at sites with high demand flow. A reduction in throughput was also noted on the opposing carriageway as drivers slowed to view the incident. Recommendations are made for further action to reduce the effect of late merging and to minimise the effect of incidents.

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