![]() Unfortunately, somewhere along the line UK highway engineering has lost its link to ingenuity and independent thinking and become an exercise in picking the desirable minimum value out of the right table in the DMRB. Plus the unnecessary lane width makes crossing to the island in the first place a bit of a lottery. This leads to overly wide entries which many designers then seem to create by reducing the size of the splitter island so that it can barely accommodate a child, never mind someone with a pushchair or on a bike. Anyone who has been involved in transport planning in the slightest way knows which factors will notionally increase a roundabouts capacity - the entry width and the flare length - even if the changes are a few 100mms. The other significant problem is that to get a roundabout design accepted in the first place it must be shown to have sufficient capacity, worked out using ARCADY. For a normal roundabout this value should be no larger than 100m, which, when all's said and done, is still a very large, relatively flat path, which is going to have limited effect on drivers' speeds. This is the flattest path a vehicle will take if the driver tries to straight-line through the roundabout. ![]() The governing factor is the 'entry path radius'. It is very easy to drive through the typical UK roundabout." "Where traffic flows are light, you'll see some drivers pass through the roundabout taking as straight a line as possible, switching between lanes as they do so. ![]()
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