Design Related

This interchange pair of roundabouts has a single lane overpass requiring a merge taper for the exits from the multilane roundabouts.  Some folks think that a lane drop on the roundabout exit would result in poor lane utilization and merge collisions.  This video illustrates that a high volume exit doesn’t congest; instead, in this case the overpass being a single lane poses the bottleneck.  The concept is what Leif Ourston coined as wide-nodes and narrow roads.   It works quite effectively for staging road widening.



This roundabout has crossfall sloping out from the central island while most roundabouts should have a crown line on the lane line with the inside lane sloping back into the central island. The site, Happy Valley Road, and I-17, Phoenix, AZ, has had 5 truck rollovers in 3 years.  There’s no advantage to sloping the entire circulatory roadway, except to save a catch basin or two.  Drivers find it uncomfortable and it puts trucks at greater risk.


Most multilane roundabouts have circulatory markings.  Watch the first minute of video and note the entry vs. circulating conflict.  Would this conflict occur if there was no lane line in the circulatory roadway?  Before 2003 when we started adding lane lines in the circle, we didn’t see this conflict pattern.  There are 25 or so multilane roundabouts without lane lines in the circle that don’t have this conflict pattern and don’t have truck car sideswipe issues either.  MUTCD says ‘should’ for lane lines in the circle but there was not a lick of research that led to the decision to endorse stripes on the circle.  Most roundabouts lack good approach signs and markings in the first place.  Think of that as vitamins and circulating stripes as medicine.  Which would you rather take?  Keep an open mind on this issue it isn’t going to go away.


Gate Circle in Buffalo, New York is an unconventional traffic circle or rotary.  Note the flat entry angles that produce weaving sections on the circle.  The circle is large and has no clear rules of priority, fast entries, no lane definition on approaches etc.  The central island is accessible and aesthetic but hazardous for pedestrians to access it.


Exit Conflicts at Long Beach Circle.  In 1992 it was modified from being a rotary with geometry and yield signs but the circulating and exit conflicts persist due to the large circle size (400+ft.)  Note the weaving at exits and some drivers not being able to exit due to fast circulating traffic.  The next round of improvements to this high capacity roundabout is to reduce its geometry and place lane lines and spiral markings in the circle.  This is one instance where circulating lane lines and spiral markings are essential.


Also the Long Beach roundabout but showing entry path overlap. When the entries were modified in 1992 extra capacity was added by flaring the entries and adding short lanes, but this introduced some path overlap. Note the slight hesitation of drivers in the far left lane. What is the effective, versus theoretical, capacity of this inside lane?


Oakland County Michigan three-lane roundabout lacks adequate approach signs for correct lane choice to prevent lane changes in the circle (first); and, the entry-circulating crash in (second) might be an indication that education and better design might have helped this condition.

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