Traffic Calming Measures - Sample: Roundabout

After a crash course on "Effective Roundabout Design" with the University of Wisconsin, and a lot of research and learning, I started with my first roundabout design, and then two more. After that, I put together a visual guide on designing a roundabout, which follows on the next few pages. Since there are many good materials that can guide designers on the feasibility and calculations needed for roundabout design, there was no need for me to reinvent the wheel on those subjects.

However, I saw there were not enough guidelines addressing the various physical elements to consider for a roundabout design and actual construction. Next is my attempt in doing just that.

The distance between flow lines (Inscribed Circle) should be 120 feet or more.

Splitter islands are essential parts of a roundabout. The width and length of them are dictated by several factors like street width, design speed, design vehicle, fastest path, and the size of the central island. Often times, particularly in urban settings, pedestrian facilities are part of the design. Therefore, refuge islands need to be incorporated into the splitter island, with needs of disabled pedestrian in mind.

In the case of roundabouts with pedestrian crossing, the distance between the yield line and the edge of the crosswalk shall be no less than 20 feet. The reason for this gap is to provide enough space for one vehicle to be in the yield position without blocking the crosswalk. Also, installation of a yield sign or signs, yield line and yield legend are necessary.

Installation of a few regulatory or warning signs on each approach is mandatory, like:

1) Pedestrian Crossing Advance Sign

2) Crosswalk Sign

3) Yield Sign

4) Central Island Sign such as W1-8

5) Median Sign R4-7

An apron is a mountable curb around the central island. Its width may vary depending on location. The reason for an apron is to create more deflection for passenger cars entering and traveling around the center island while accommodating larger vehicles like buses and trucks. The width of the apron, among other factors, is directly dependent on design vehicles (vehicles that are expected to drive on the street), both large and small. Since it is likely that large heavy vehicles will travel over the apron, it must be constructed in such way that can withstand the load, so it must be adequately thick and reinforced. The detail on this page provides some guidelines in that regard. It should also be noted that the height of the apron above the roadway at the edge is no more than three inches with a fillet edge (R 1” to 3”). This is to ensure a smaller car will have a rather safe pass should it travel over the apron.

Bike lanes, should they exist on approaches to roundabouts, are normally terminated several feet before the median striping. A “share the road” sign is installed where the bike lane ends to warn drivers and encourage them to be considerate to bicyclists. Where sidewalks are at least 10 feet wide, it is possible to move cyclists from bike lanes onto sidewalks on approaches to a roundabouts, and ramp back down to bike lanes on exit sides. The drawing on this page details bike ramp up/down in these kind of situations.

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