Lag Left Turn Signals 2

[FLisa]

Just a general question a driver for you traffic engineers from a Manufacturing Engineer. The city of Tucson, AZ instituted "Lag Left" turn signals a number of years ago. The story was that it would move more vehicles thru the left turn lanes since some would get a chance to turn even before the left green arrow came on. Tucson has stuck with it but I haven't heard or seen any one else pick it up. Any insight on whether this was a good move, bad move or had no effect?

 

[teeman]

By using a lagging left turning traffic can take advantage of existing gaps in traffic instead of waiting for an artificial gap created by the arrow. This minimizes delay to both left turn traffic and opposing thru traffic because the left turn should only come in when there are no natural gaps in the traffic stream.

Lagging lefts can be useful in some circumstances, less helpful in others. If there are never natural gaps in traffic, lags may increase delay to left turning vehicles. However a lead left can't service vehicles that aren't already there at the beginning of the green.

It's hard to generalize about traffic because every intersection has unique characteristics. Sometimes corridor considerations (maximizing thru greenbands) take precedence over servicing turns.

 

[BJANDERS40]

lagging left turns are very helpful in maintaining bidirectional progrssion in coordinated systems.however they become quite restrictive towards the use of protected / permissive 5 section left turns.if there is a permisive (protected or not) turn opposite the lagging turn, there is a danger of catching that driver unable to know what opposing traffic is doing as their signals turn amber then red. the opposing may well have a green/green arrow and that trapped driver will think all are stopping thereby turning into the path of that traffic.