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Traffic Signals: Yellow before Green May 8, 2008

Posted by federalist in Open Questions.

Many countries use a “4-state” traffic light pattern which shows a red-and-yellow signal before the light turns green again. As an efficient driver I think this is a great idea. For one thing, it gives distracted drivers in front of you extra time to notice that they will have to begin driving again soon, so that they do not waste precious seconds standing still with a green light. For another, it allows skilled drivers approaching a red light the ability to pace their deceleration so that they coast through as it turns green, instead of coming to a complete stop.

Regrettably, the United States Federal Highway Administration (FHWA) regulates traffic signals; their Manual on Uniform Traffic Control Devices (MUTCD) specifically prohibits the use of a “pre-green” signal. I contacted the FHWA to find out why and got the following explanation:

The yellow-red pre-green interval is not a European standard but it is still being used by some European countries and by some former colonies of such countries in other parts of the world. It is by no means an internationally-standard practice.

The practice of a yellow-red “pre-green” interval relates to the prevalence of manual transmission vehicles. [T]he pre-green interval is intended to give drivers a heads-up to shift out of neutral and into first gear before the green appears, in order to reduce the otherwise long (compared to U.S.) startup delays.

I’m not aware of any studies or consideration of this for the U.S. or Canada. The reason there is no interest in pursuing such a display is safety-related. Drivers in North America are already overly aggressive and, with the large majority of vehicles having automatic transmissions, there is no need to provide any incentive to “gun it” when the signal changes to green. Also, the startup delay does provide some small measure of a “safety cushion” for cross street vehicles and pedestrians caught in the intersection at the end of their phase.

Counting on drivers to be slow off the line as a safety cushion doesn’t seem like the most prudent policy.  If that’s really important then lights can be programmed with a delayed green.  Therefore, I stand by my desire to convert the United States to 4-state traffic lights!


1. Sam - December 17, 2008

I am familiar with both 3- and 4- phase traffic control systems. The pre-green signal (at least in Israel) causes drivers to prematurely cross into the intersection before they’re legally allowed. In some instances, if they cross the entire intersection before a green signal, they might receive a violation from a red-light camera. Vehicles entering the intersection from opposite ways are in danger of colliding with other vehicles (and this happens a lot in Israel). The 4-phase system is intended for manual transmission vehicles, and I believe it will one day disappear entirely, or will be replaced by something very different Check out the traffic lights in southern Brazil… nothing like you’ve ever seen. (The signals are “visually timed” by a vertical column of lights… neat).

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