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A Catalogue of badly laid out Roads, Lanes, Lights and Signs.

Swept Path Diagram

Or "How Highway Authorities Could Avoid Getting It Wrong".

Swept Path Analysis is the jargon term for designing a road system so that vehicles which will use it can actually get round the curves in the design without demolishing signs, kerbs, cyclists and so on. There is a relevant Wikipedia article here.* The point is that when making a turn, allowing for the vehicle width alone is not enough. The back wheels do not exactly follow the front wheels - but tend to 'cut' the corner. This is espcially true when trailers or 'semi-trailers' are present. Furthermore, a reversing vehicle, using exactly the same movement on the steering wheel, will not take exactly the same path on exit that it did on entry. This is because tyres tend to 'walk' sideways with each vehicle movement.

Specialist software is not cheap but using it should avoid the many instances of bad planning illustrated on these pages. However, for simple or one-off jobs the expense may not be justified so appearing here soon will be a simple tool to enable readers to calculate the necessary radii for some of those curves. The output from this tool however must be used 'at your own risk' and no liablity is accepted, nor accuracy guaranteed by using it. What is more or less guaranteed however is that you will need to make more allowance for larger vehicles than you expect!