Mini-roundabouts - Getting them RIght!
Detailed Drawings of a three-arm mini-roundabout

These three drawings illustrate in detail the process that you must go through in the design of your mini-roundabout. The first illustrates the possibility of "wrong paths" i.e. the routes that drivers could take if you do not discourage them by the details of your design.

 This has been seen extensively at more recent mini-roundabout layouts especially where the yield lines are set too far back.

 

1. Wrong paths

The design is based on placing the yield lines on or behind the circumference of the inscribed circle and the central island at its centre (orange). Note just how easy it is for drivers to cut to the wrong side resulting in severe conflict angles in the event of a collision (Purple paths).

This problem is exacerbated by kerbline build-outs which deflect entering vehicles to the right - even more likely to pass to the wrong side of the central island.

2. More realistic examination of vehicle paths.

Here the outer swept paths of larger vehicles are used to determine the approximate location of the yield lines. Now draw the paths of light vehicles turning right (left on RHD roads). If possible try to form two lanes well before entry such that the vehicle in lane 2 is driving parallel to the nearside kerbline at the yield line. You need to create a clear deltoid shape in the centre with convex curves around which all three movements circulate correctly. The deltoid determines the location of the central island. You can adjust the paths which should represent similar degrees of steering effort for each of the three movements. All must involve a radius which falls to or below around 60m radius at some point on approach or in the junction.

Note for UK designers: Former TD16/93 required deflection to be provided on the approaches where possible. On mini-roundabouts this is nearly impossible, but the deflection must be provided somewhere on all crossing paths unless physical vertical deflection is provided to a sufficient standard.

 3. The final design.

The details are based on realistic paths from drawing 2 and not drawing 1.

This is the way I have designed all my mini-roundabouts and it really works!

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Penntraff - Dec 2014
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