Tuesday, September 8, 2009

Protected/Permitted Left-Turn Signals Changing

There's an endless list of problems we face: economic recession, wars in Iraq and Afghanistan, health care, global warming, immigration, federal debt. So, let's talk about traffic lights.

Richardson has been introducing something called the "flashing yellow arrow" to traffic signals for protected/permitted left-turn control. The research that led to this standard can be read here. Ian McCann explains how the Richardson introduction of the "flashing yellow arrow" is going. Judging from the reaction, you'd think the traffic engineers were rewriting the Bible, which I guess they are, as the Manual for Uniform Traffic Control (MUTC) is sometimes called the "bible" for traffic engineers. One reader says, "Almost 100 years of training and indoctrination are to be discarded on the whim of some doofus kids playing with lights." Putting aside the "old man yells at cloud" objections, what are the pros and cons of the new standard?

The researchers claim that the configuration that is most common in the country (not in Richardson), the 5-section cluster, results in a "yellow trap," a condition that leads drivers into the intersection when it is not safe to do so. The configuration common in Richardson, the so-called "Dallas Display," the 5-section horizontal display in the graphic above, alleviates the "yellow trap" but uses louvers or other devices to restrict visibility of the signals so that only drivers in the left-turn lane see them. The "flashing yellow arrow" alleviates the "yellow trap" without need for the visibility-limiting devices.

The biggest drawback to the "flashing yellow arrow" could be that some drivers will interpret it as meaning, not a permitted left-turn phase, but that the protected left-turn phase is coming to an end, so they speed up to complete their turn, leading to disaster because they didn't have the right-of-way they thought they did. After all, a solid yellow light during the transition from green to red means you still have the right of way, but not for much longer. And a flashing yellow light usually means you have the right of way but proceed with caution. Why should a flashing yellow arrow mean just the opposite, that you don't have the right of way?

Unfortunately, like the big problems of wars and the economy and global warming, sometimes even little problems like traffic signals have no simple, foolproof solutions. Most likely, the flashing yellow arrow won't become the single, universally understood permitted (but not protected) left-turn signal. It will become just one more such signal among many. And drivers will continue to get confused or make mistakes through careless inattention. Left-turn accidents will continue. So, be careful out there. You might have the right-of-way but that doesn't mean the other guy is going to yield. The safest course is to practice defensive driving. Always assume the other guy is not just careless, but actively out to get you.