Streets across the United States are often difficult and dangerous to walk on because wide lanes invite drivers to speed. That isn’t all that makes them dangerous, though: many also have signals that distract drivers and draw their eyes away from the road.

Arlington’s Intersection of Doom. Drivers who want to turn right in order to travel north from the eastern side of the intersection have to account for oncoming north-bound cars, people crossing from all directions, and confusing signs. Photo by the author.

A case in point is the “intersection of doom” where the Mount Vernon Trail turns into the Custis Trail at the foot of the Key Bridge in Arlington.

Drivers exiting I-66 are allowed to make right turns on red, except during a brief “leading pedestrian interval” when a walk signal gives pedestrians and cyclists a head start across North Lynn Street before the traffic light turns green.  But it’s hard to see how drivers can make this turn safely without extra eyes on both sides of their head.

Base image from Google Earth.

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.

To make things worse, 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.

Diligently watching the short-lived no-turn signal while looking for a gap in oncoming traffic from the left would make it nearly impossible to look to the right. A driver trying to legally turn right on red has no time to look toward the sidewalk on their right and can’t see whether someone is about to enter the crosswalk and pass in front of their car.

Overhead signs at intersection of doom, with the no-right-turn sign illuminated.

Photo by the author.

Dangers like these are widespread on American streets. What makes this intersection stand out is the heavy bike and pedestrian traffic, not the arrangement of the signals.

Tell drivers what they need to know, and repeat it

From an engineering point of view, the information traffic signals send to drivers is part of a control system that must operate reliably to keep roadways safe.  So is the drivers’ reaction to that information.

The traffic engineering establishment certainly recognizes that human behavior affects road safety.  But two concepts are conspicuously missing from its guidelines on human factors in signal design: redundancy and parsimony.

Redundancy means backups for missed signals and improper actions.  Parsimony means signals aren’t excessively complex.

It’s easy to see redundancy’s value. Intersections with simple red-yellow-green traffic signals are full of redundant information: The movement of vehicles and pedestrians is a cue to when the light changes, so drivers don’t need to stare at the signal and can keep watch on the street.

More complexity — turn arrows, walk signs, rules that allow right turns on red — means less redundancy.  Demands on the driver’s eye and brain increase, and the inevitable moments of inattention do more harm.

Parsimony is a less intuitive idea, but an equally important one. This principle, which originated from the statistical analysis of time series, warns against using too many input variables to control decisions.  Adding complexity, when there aren’t enough data to do it right, makes outcomes worse.

Consider the countdown clocks attached to walk signals.  The Federal Highway Administration mandated them when research indicated that when pedestrians know how much time is left before cars start to move, they get hit by drivers less often. But this information changes the behavior of drivers too.  Whether the drivers speed up to beat the light or simply get distracted is not clear, but the effect is real. A recent study in Toronto found that countdown timers cause more collisions than they prevent.

Complex signals have other costs.  Turn arrows make signal cycles longer.  This gets more cars through the intersection, but everyone waits longer for a green light and the street becomes a barrier to walking.  The delays to pedestrians and non-rush-hour drivers may exceed the time saved from reduced congestion — but the traffic engineers won’t know without data on off-peak travel, something they rarely measure.

And slow lights create safety hazards of their own.  The longer pedestrians are asked to stand and watch a don’t walk signal, the more likely they are to ignore it.

It will never be possible to govern traffic with the mathematical precision of electric circuits.  But they are both control systems afflicted with random noise, and similar principles apply.

Solving problems by adding more gadgets to an already complex system can do more harm than good.  Traffic control operates most reliably when everyone on the road knows that green means go and red means stop.

Turn arrows and separate walk signals should be used sparingly.  They squeeze more cars through an intersection in rush hour, but they exact a price in safety, dollars, and travel delay.

Traffic engineers need to balance vehicle throughput against the benefits of redundancy and parsimony.  In any control system, and especially in one that relies on the actions of human beings, simplicity has to be a priority.