Complex Traffic Signals Make for Dangerous Intersections

Ben Ross decries the growing complexity of traffic signals at intersections all over the country, which might be better at distracting drivers than they are at directing them.

Ross selects the "intersection of doom" in Arlington—where Mount Vernon Trail turns into Custis Trail at the foot of the Key Bridge in Arlington—as a case study of unnecessary complexity in traffic signals and intersection design. There, explains Ross, "Drivers must simultaneously watch for cars coming from the left, cyclists and pedestrians entering the crosswalk from the right, and an overhead signal that went in in January that flashes a no-right-turn graphic for a few seconds during the leading pedestrian interval." Moreover, "the no-right-turn graphic is hard to see in bright light, and it is flanked by highly visible signs that seem to say turns are allowed."

According to Ross the arrangement of this intersection is common around American cities—the only thing remarkable about it is the large number of bikes and pedestrians travelling through the intersection.

The article goes on to discuss larger concepts that inform an understanding of how intersections like this fail to meet a goal of reliably keeping roads safe. Ross notes especially that two concepts are missing from traffic engineering guidelines: redundancy and parsimony. "Redundancy means backups for missed signals and improper actions. Parsimony means signals aren't excessively complex."