This is part two of a four part series of “Traffic Signal Trivia”, dealing with some history and interesting facts concerning traffic signals. Part 1 covered vehicle signals in general, but left turns are a whole story in themselves, so here we continue.
It didn’t take long after the introduction of traffic signal until left turning traffic became an issue. These were the days before dedicated turn lanes became common, so stuck traffic unable to turn would back up through traffic behind that would otherwise be able to go straight. To solve this, a left turn indication was added that would light for a few seconds
Note how there’s no warning, the “clearance interval” in traffic signal speak, that the turn phase is going to end, Later came phases designed to let a significant amount of traffic through, not just a few stuck cars. Also there were longer delays after the left turn phase ended before releasing oncoming traffic, but there was initially no indication for this. This “blind clearance” has been banned for many years but is still present in some older installations in other parts of the country. (This absolutely freaked me out the first time I encountered it, in Keokuk, IA).
Early Indications
With the introduction of protected left turns, it took a while for this indication to be standardized. Some very early indications were white balls, rather than arrows. The indications were originally 8”, but soon grew to the 12” standard. Before the standard pattern came about, there was an alternative style (called the “Chinese Arrow” in collector / enthusiast slang) that had notable curves to the strokes, and another style with the arrow heads filled in. There was an arrow that was neon and even a white arrow has turned up, no doubt related to early white ball indications.
Early left turn indication with a white ball in the middle section.
http://i431.photobucket.com/albums/qq35/TwinGreentraffic/000_0611.jpg
Also of note was the “Arroway”, which was a panel that replaced the green indication. There was a 8″ green ball in the center and red bars and green arrows extending outward would light up indicating prohibited and protected turns.
Later Indications
Later, with left turn lanes becoming more common and roads getting wider, left turns indications started being handled by a dedicated head. The yellow and red were standard 8” balls. Uniquely, California used visors on the red and yellow sections so through lanes would not see them. Red and yellow arrows came much later, in the 1970s, and red balls in left turn displays were still permitted until the most recent standards update. At one time red arrows were under consideration for being banned due to drivers not understanding them, but this is no longer a problem, drivers have since gotten used to them.
Classical California left turn arrangement.
Protected vs Permitted
Permitted operation is simply the default, where you have a single green light for all traffic, and left turning traffic has to yield to oncoming traffic. Protected is where you have the three left turn arrows and traffic is not allowed to turn when oncoming traffic has the green. The tradeoffs are efficiency vs safety. With protected only you remove 4 conflicts from the intersection, but dedicated arrows take up time in the cycle, and turning traffic has to wait even if there’s obviously no-one coming. A hybrid approach is protected/permitted, where both modes operate during the same cycle. This is a split the difference approach between efficiency and safety. Left turning drivers are less likely to take chances if they know they’ll get a protected movement eventually, and the permissive phase allows gaps in the oncoming traffic to be utilized.
Typical protected / pemissive signal as used in Minnesota
Lead vs Lag
There’s also various trade-offs as to whether arrows should come on at the beginning or end of the cycle. (Lead vs Lag Lefts) Lead lefts have the advantage if there’s no or inadequate left turn lanes and the traffic tends to back up, blocking the through lanes. A lead left allows them to clear before they’re in the way of through traffic. Second, if there’s an unusual volume of left turning traffic, extending a lead left impacts traffic operations less because you can still release the through traffic the same direction during the left turn phase. (And they actually did a survey and found lead lefts are what drivers want.)
Lag lefts have the advantage in that drivers have less of a tendency to continue going through as the arrow turns yellow and red. I’m sure everyone has witnessed it where left turning cars keep going through until oncoming traffic notices they have a green and start edging forward. Secondly, the overall left turn phase can be shorter, as more turning traffic has an importunity to find gaps before the phase begins. Finally, pedestrians have the tendency to step into the street as soon as they see a red on a side street, which would put them in conflict with left turning cars. (I see this all the time at Calhoun parkway and William Berry Parkway; this would be a good location to modify to a lag left just because of pedestrian behavior).
Split phase
Sometimes traffic one direction will have both a through and protected left turn simultaneously, then it’s the other sides turn. The left tun traffic signal heads will be four lights, as in lag lefts the arrow always ends at the same time as the ball so a yellow arrow isn’t needed. This usually isn’t particularly efficient but is used where there’s a high volume of turning traffic relative to through (like a pair of shopping mall exits at a wide suburban road), or there simply isn’t room for adequate left turn lanes and it was determined a protected phase was necessary. In this setup it’s really easy for lanes to be used for both through and turning traffic as cars will never wait to turn blocking through traffic.
Split phase display, can also be used for lag protected / permissive as neither require yellow arrows.
The Problems with Permissive
Over time permissive turns became an increasing safety issue, as traffic volumes in general grew, and drivers becoming used to protected turns didn’t realize they had to yield on green ball. (Yes, this is a real issue, a permissive phase had to be yanked from a traffic signal in Stewartville because too many left turning drivers were not yielding on green) Another issue is that and lead/lag protective/permissive phasing (the most efficient way to operate coordinated intersections) creates an extremely dangerous situation called the “Yellow Trap”. Various local solutions were tried, including “Dallas Phasing”, (in Texas), flashing red arrows (in Delaware), flashing red balls- (in Michigan- where they were treated like a yield in practice, and Maryland), and flashing yellow balls (Washington state). So it was obvious a national solution was needed. This led to the development of our new friend, the flashing yellow arrow. The next post in the series will deal with the flashing yellow arrow and the various issues with permissive phasing.