Skewed Intersections

Roads at most intersections meet at 90 degree angles.

When roads aren't perpendicular the intersection is described as "skewed." Skewed intersections are sub-optimal from a safety perspective.

Here's an example:

The intersection between Manke Rd SE and WA-507 creates one 55 degree angle and one 125 degree angle. That’s really skewed.

I don't know how much you remember about geometry, but an acute angle is less than 90 degrees and an obtuse angle is more than 90 degrees.

Traffic engineering standards recommend intersections be as close to 90 degrees as practical. The most authoritative reference is AASHTO’s Policy on Geometric Design of Highways and Streets (“Green Book”).

There are a number of reasons skewed intersections are dangerous.

• Drivers have to turn their head farther to look back toward the acute angle for cross traffic.

• Judging the speed and distance of approaching vehicles becomes more difficult.

• The crossing distance is longer (think "hypotenuse").

• Vehicle paths become more complex.

• Large vehicles may encroach into adjacent lanes.

Problems at skewed intersections are heavily biased toward the acute angle. But the effects can extend to both directions.

Things like challenges judging speed and distance and longer crossing distances apply in both directions.

One good example involves a driver turning from a side road onto a highway at a skewed intersection. Even if the driver is going to turn toward the obtuse, he still has to deal with traffic coming from the acute side. That's going to consume some or all of his attention bandwidth. He may spend 90 percent of his attention evaluating what's happening on the acute side and miss what's happening in the opposite (obtuse) direction.

The best “fix” is to reconfigure the roads so that they’re perpendicular when they intersect. These revisions are inexpensive and easy to implement. Here’s what that same intersection would look like with a simple reconfiguration.