The Drive: Why we don't have more flashing yellow arrows?

Plenty of things annoy commuters, but perhaps nothing more than being stuck at a left-turn arrow with no traffic in sight.

On a recent Sunday night, Evelyn Nelson, of Hopkins, saw nary a car while she waited for more than two minutes for the left-turn arrow to turn green at the intersection of Hwy. 7 and 5th Avenue in Hopkins. She told the Drive in an e-mail that she entertained the idea of running the red light. She also wondered why the intersection doesn't have a flashing yellow arrow.

Flashing yellow arrows permit motorists to turn left after yielding to oncoming traffic and pedestrians. It's like having a solid green light, but the yellow is meant to remind motorists to use caution, in case oncoming traffic has a green light.

That's what's called a "permitted turn," as opposed to a "protected turn," when left-turning motorists see a solid green arrow while oncoming traffic sees red.

Approved for use in the mid-2000s by the Federal Highway Administration, flashing yellow arrows have reduced crashes, according to some research. They also have helped drivers avoid "yellow trap," which happens when motorists waiting to turn left assume that when their light turns a solid yellow, the light for oncoming traffic also is yellow — when in fact it may be green.

Flashing arrows also can keep traffic flowing by moving more vehicles through and eliminating long waits at signals with left-turn arrows, where it can take a long time to complete a cycle.

With all those benefits, why not put flashing arrows at every intersection?

More are on the way, says Jerry Kotzenmacher, a MnDOT traffic systems specialist. The agency's policy calls for them to be installed any time new traffic signals are put up or existing ones are replaced.

Intersections are dynamic places; traffic volumes, turning patterns and speeds change throughout the day. Some have good sightlines while others don't. All that has to be considered when deciding whether to use a flashing yellow arrow or stick with standard green, Kotzenmacher said.

Researchers at the University of Minnesota are developing a statistical model to help determine whether a flashing yellow arrow would be safe at a given place.

"There are certain times of the day that you don't need a protected turn because there is little opposing traffic," said Gary Davis, a professor of civil, environmental, and geoengineering at the U. The engineer can enter data and the model will show a graph depicting how the left-turn risk varies at this intersection.

"By simulating how crash risk changes as traffic conditions change, this model could help identify conditions when the risk would be unacceptable and you'd need to have a green arrow."