The finalised manual for streets reveals how much highways engineers need to narrow carriageway widths and restrict visibility to bring speeds down.
TRL found that, on a 6m carriageway, forward visibility would need to be no more than 60m, if the Manual for streets’s aim of bringing speeds down to 20mph through the design of roads was to be achieved. On a 5m-wide road, visibility could be increased to 100m.
The guidance, which replaces the 30-year-old Design bulletin 32 on residential roads, presents the evidence to embolden highways engineers to reject the dominant, post-War thinking, that wide, open roads are safer. TRL’s Stuart Reid, who led the research programme for the manual, told Surveyor: ‘The message is that the intrinsic design of a street should make it clear to people what speed they should be travelling at. ‘It’s designing for the desired speed, not the predicted speed.’ He could see an end to ‘designing roads where drivers can easily do 40mph, then introducing a 30mph limit and vertical deflection to bring speeds down’. However, the Manual for streets team hopes that practitioners will not slavishly follow the new guidance. ‘The width of the road, for example, will not just be determined by the speeds that we want motorists to travel at, but a large number of factors,’ stressed Reid.
The needs of buses or refuse vehicles, for example, might require wider roads, he said. However, the document stresses that, where service frequencies mean that it is unlikely that two buses would meet, a bus could cross the other side of the carriageway when turning. Refuse vehicles could possibly be excluded, providing householders and waste collectors do not have to move bins too far. Other factors would also slow speeds – the presence of planting and parked vehicles; the proximity of buildings; and the materials used for the road.
‘We want to get engineers to think. We spend millions training them, so why should they be reduced to reading numbers out of a table?’ Reid added. As such, the guidance is deliberately not prescriptive. As well as having ‘no fixed rules’ on widths, there are pros and cons to straight roads. They better serve pedestrians, who prefer direct routes, but they could also encourage faster vehicle speeds.
View the manual here: http://www.dft.gov.uk/pgr/ sustainable/manforstreets/
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