Protected and unprotected lefts revisited: theory and practice

Anonymous has left a new comment on your post ""WHY" L.A. traffic sucks #4: Pesky annoying quirks: Unprotected Lefts":

Before you add protected left-turn signals to all major intersections, try reading the following document: Left-Turn Arrows.

Left-turn arrows can definitely help improve the flow of traffic at certain intersections, but not all. Have you ever thought of the cycle length of an intersection to be that of a pie of time that needs to be shared? When you add left-turn phasing, the more pieces you'll have to cut out of the "pie." The slices of time designated for the other movements, especially through movements, may have to be made smaller to accomodate the new slice of time for a protected left-turn. The pie can only be made so big sometimes.

Posted by Anonymous to L.A. traffic sucks: Let's fix it! at 5/23/2006 04:26:00 PM

italianesco replies:

Thank you for your interesting comment on protected left turns and for the link to the "Left Arrows" document.

What you and the source document you refer to say may be true. As I keep saying over and over again, though, everything having to do with traffic, especially in L.A.'s complicated, congested and confounding situation, is a trade-off. The theoretical "pie-sharing" properties of protected left turns may be one thing, but my observation, my experience and my perception of unprotected left turns in L.A. leads me to believe that they are contributing to the traffic mess and congestion.

In a heavy traffic, highly congested situation like L.A.'s, drivers get "ansy," edgy, hasty. They know that the only way to make it to their destination is to get past each intersection they come to. As far as I know, a YELLOW light means: "Proceed with caution and be ready to stop: the light is about to turn RED!" In L.A. a YELLOW light means: "KEEP GOING AS FAST AS YOU CAN AND MAKE IT ACROSS BEFORE THE LIGHT TURNS RED AT ALL COSTS AND NO MATTER WHAT." What this translates into in actual practice is that L.A. drivers are RUNNING RED LIGHTS at unbelievable rates and they are not only getting away with it but blocking left-turning vehicles. This also means that ONLY ONE OR TWO cars at the most are being able to turn left, when they may actually be a row of 5 to 7 cars or more waiting to make a left.

This situation gets complicated by pedestrians crossing the very same street the left-turning cars are heading into. EVEN with protected lefts, cars sometimes have to wait for slow-moving pedestrians to cross before they can get out of the intersection and be on their way. In an unprotected left situation, if there are pedestrians crossing that street at the last minute, chances are NO cars may be able to make a left turn.

The so-called "RIGHT ON RED" allowing vehicles to make a right turn even though the light is red, complicates the situation even further.

If there are cars zooming by on a yellow light until a millisecond before the light turns red, if there are pedestrians crossing the street and cars making a "right on red," how in the world can ANY cars, ANY cars AT ALL, make a left turn in an unprotected left situation?

This is the problem.

Every single car that is not allowed to make a left turn by this situation is stuck on that left turn lane until there are SO MANY CARS wanting to make a left turn that the left lane is affected. Sometimes there are cars with half their bodies sticking out onto the left lane waiting to get into the left turn only lane. Sometimes the whole left lane for half a block is filled with cars waiting to get into the left turn only lane.

I am convinced that the ONLY SOLUTION to this problem is to put in ENOUGH protected left turn arrows that give pedestrians and left-turning vehicles enough time to cross and turn left without jamming the intersection. They should be put in WHEREEVER THEY ARE NEEDED. And that probably means AT EVERY SINGLE MAJOR INTERSECTION.

Since NO urban planner seems to be able to think BEYOND THE GRID and most - if not all - cities have already been laid out and built in GRIDS, then you have four-way intersections with vechicles and pedestrians competing for crossing and turning. Whatever the "time pie" available for them to do so, you need to give everyone ENOUGH TIME to cross or turn. Or you'll have a jammed intersection - a jammed intersection that in turn will most likely be jamming the one before it and the one before that one.

Left turns are one of the most mismanaged traffic resources in L.A. Unprotected lefts are one of the contributing factors to the messy and seemingly uncontrollable congestion in L.A., and they need to be put in if angelenos want traffic to start flowing [GO!] more smoothly in their city.