David Noyce study: Drivers don't get left turns quite right

You're driving your car, and you approach an intersection with a left-hand turn lane and a traffic signal. You drive into the turn lane, and wait for the arrow that permits you to turn left. When the green arrow appears, you turn.

Simple enough, right? Not really, according to research by Assistant Professor David Noyce. In a paper prepared for the annual Transportation Research Board meeting in January, Noyce said surveys indicate there can be considerable confusion among drivers about left turns and left-turn signals. In particular, Noyce said drivers are often confused when confronted simultaneously with a red signal indicating "stop" and a left-turn arrow that permits a driver to move through an intersection. This is particularly true with elderly drivers, he said.

"Drivers are really confused by this," he said. "We kind of took it for granted that it wasn't a big deal. We can't give drivers two conflicting messages at the same time and expect them to understand what to do."

Noyce's research is part of a large, national research project into driver habits and understanding of turn signals. Part of the research project involved surveying nearly 2,500 drivers who were asked questions about 200 separate driving scenarios. All told, the survey asked about 25,000 questions specifically dealing with left-hand turns. Then researchers conducted an extensive study using driving simulation technology, the largest study of its kind completed nationally.

His research is already attracting the attention of national and international transportation experts. His paper received the Transportation Research Board's annual D. Grant Mickle Award for the best paper of the year in operation, safety and maintenance of transportation systems.

One key finding in the research, according to Noyce, is that variations in left turn signaling can mean different things to different drivers. For instance, some intersections stop traffic with a red stoplight, but allow traffic going the same way to turn left with a green arrow. The assumption on the part of the driver turning left is that any oncoming traffic will also be stopped by a red traffic light, Noyce said.

But some intersections also allow left turns with a solid green light. In this permitted left-turn situation, the solid green light must be interpreted as a "yield" message, unlike the solid green light for drivers going straight through an intersection. Traffic engineers assume drivers will know that they must yield to oncoming traffic when trying to turn left with a solid green light, because oncoming traffic will also have a solid green light, Noyce said. But that's not always the case.

"The problem we have is that the circular (solid) green light has two different meanings," he said. Because of this, traffic engineers throughout the country have implemented flashing red and yellow lights for left-turn drivers, in place of the green light, in an effort to clarify this meaning.

"We are working on eliminating this variability and identifying which type of indication for permitted left turns provides the most understandable message to the driver."

Noyce said one goal of the research is to provide detailed information to the Federal Highway Administration so it can recommend more uniform traffic control devices. That could in the long run help reduce traffic fatalities that this year will total more than 800 in Wisconsin and more than 40,000 nationwide, he said.

Noyce said he also hopes to build on this research by conducting additional driver simulation experiments ­ using real-time video taken from streets and highways ­ to get a better understanding of driver reactions to traffic signals. "We're making a lot of progress, but we still need to learn more about why drivers do what they do on the road," he said.