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George White, Az Roundabouts, Arizona Roundabouts, Roundabout Design


Hello and thank you for visiting my page.  My name is George White and I am a transportation design specialist with a focus on modern roundabout design.  On this page I will share my knowledge of modern roundabouts and related questions in an effort to promote safety and consistency in the design and to provide general education on them.  Email your questions to

Question: Can the curbing on the splitter islands be mountable?

Answer: The purpose of the splitter island is to separate opposing traffic, provide a pedestrian refuge area at the crosswalk and to slow vehicles down. 

Both curb types, mountable and barrier can separate opposing traffic. 

Both curb types can provide a pedestrian refuge area. However, if an errant vehicle were to drive onto the splitter island, the curb type selected would be important for deflecting the vehicle or reducing the impact before it reached the pedestrian refuge area. A barrier type curb could redirect the wheels back towards the roadway or possibly take some of the impact at reducing the vehicle speed, whereas a mountable curb would offer little to no resistance to the vehicle and it could reach the pedestrian area at a high speed. Additionally, it could continue across the splitter island and meet an opposing vehicle, street light, street sign etc…. 

Both curb types can also be used to slow vehicles down, however the barrier curb type would be more effective as it offers less forgiveness if the driver were to enter too fast. The fear of hitting the barrier curb at a high speed is what will ultimately reduce the speed of the approaching vehicle combined with the entry deflection of the roundabout. Better that they hit the curb on the approach if they don’t slow down, rather then hit a pedestrian or another vehicle at the circulatory roadway. Most mountable splitter islands are found in smaller traffic calming or min-roundabout type locations and are used by emergency vehicles that are too large to travel through the smaller intersections. These locations tend to be 20 to 25 mph intersections though.

 I hope this is helpful. Please let me know if I can provide additional information.


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