Why does KDOT typically put taller vegetation blocking visibility through the central island of their roundabouts?
Thought and planning goes into the native landscaping found in the central islands of our roundabouts. While there is nothing fundamentally wrong with seeing the upstream approach of cars from a long ways off and timing your own approach accordingly, the real value in sight distance restrictions provided by taller vegetation in the roundabout is the positive effect they have on those vehicles getting ready to enter the roundabout.
As vehicles are getting ready to enter the roundabout, restricting sight distance across the central island with strategic taller landscaping may enhance the safety of the intersection by encouraging lower speeds and making only the left-hand side of the roundabout the focal point. In addition to the increased aesthetic value, a well-landscaped island will make both circulating and entering vehicles more cautious by limiting how far ahead they can see. A circulating vehicle likely will be a bit slower around the curve if its view ahead is restricted (see graphics below). That makes it easier for a car entering the circulating lane to receive a reasonable gap. If sight distance restrictions can limit circulating vehicles to 20 mph, you need less than 120 feet of pavement to provide a four-second entry gap – which is big enough for just about any passenger car.
An additional characteristic of taller central island landscaping is that it shields the headlights of oncoming vehicles making it easier to see the circulating lane as a driver approaches a roundabout.
Graphic credit: WisDOT