Shown below are simulations that approximately demonstrate operation of a concept three-lane Compact Urban Roundabout with Narrow Lanes (inscribed diameter 37m or 121 ft), using directional Raised Platforms for speed control (ramps indicated by white triangle marking). Car lanes are in the order of 2.6 metres wide (8 ft 6 in), which for a four-lane approach allows for three large vehicle lanes at 3.5 metres wide (11 ft 6in). As usual for narrow lane designs, circulating and departure lanes are more generously wider to allow for clearances between turning vehicles. This example uses coloured road marking to delineate large vehicle lanes, which are believed could be helpful for driver interpretation (orange used here). Off-road trials are suggested before real world application, and could include experimenting with lane widths and road marking configurations. Driving simulators might be an expedient tool for doing this.
It is considered that the use of varying lane widths for different categories of road user can potentially achieve greater benefits at a roundabout than other forms of intersection. This is mainly due to the speed control and large vehicle swept path requirements at roundabouts, as opposed to traffic signals which rely on time separation with less restrained geometry. Even should autonomous vehicles become prevalent as is widely predicted, ideas to maximize available road space will still be important – and perhaps also relevant for mid-block higher speed applications.