I’d say I’m having a Stupid Moment, but I’ve been trying to figure this out for weeks.
What is the advantage in terms of traffic flow to having stoplights that only let traffic from one direction proceed at a time? That is, instead of a green light that allows both northbound and southbound traffic to proceed simultaneously, only the northbound traffic gets the green light. Then they stop for the red, and the westbound traffic proceeds, and so on.
At first I thought it might have to do with too many drivers wanting to turn left, thereby overflowing the left-turn-only lane and gumming up the left-most straight-ahead lane. Instead of a left-turn arrow, the cars would just turn left throughout the green cycle, which is longer than the typical left-turn arrow phase of the cycle. But that doesn’t make any sense, either, first because the LT drivers would still need to wait it out, backed up beyond the left-turn lane, through the other three cycles of the light, and second, because I’ve seen these one-direction-at-a-time signals at intersections that have two left-turn-only lanes.
I’m sure there’s an explanation rational enough to appease me when I’m waiting an eternity at those lights. Would someone please enlighten me? (Ugh. Sorry.)
I’ve never seen this, and can only think it would make sense in unusual situations where a huge volume of the traffic makes a turning movement or the intersection has dangerous offsets. Can you give a specific intersection where this occurs?
I don’t know why they do this, but I can add that in my area (Orange County, CA) the signals will switch to this mode during rush hour, but will run on the standard (I think) system of N/S left turn, N/S straight, E/W left turn, E/W straight, repeat the rest of the time.
It seems like it takes longer to pass, but that’s because they use a longer cycle time overall. We see them at corner construction sites here now. The clear advantage is not having to wait for traffic in order to make turns. This is all important when there is only one lane in use at the time. Otherwise, a single vehicle could hold the line for one cycle. The other option is to not allow turns and that makes it difficult to get around. With the single direction green, the drivers know they can freely go forward and turn either way without delay so the line takes off and moves more cars per green cycle.
Where I live in New Mexico, we’ve got a couple of these sprinkled around our city. I hate them, and have always assumed they exist to discourage the use of certain thoroughfares running through neighborhoods and push traffic back on to the more accepted thoroughfares in the city and out of the residential areas.
When I’ve seen them (temporarily) in use in construction areas it was obviously with the goal of moving as many cars as possible through the intersection at a time to prevent gridlock.
If someone who knows what they’re talking about can explain my first instance (always one-direction-at-a-time for no seeming purpose intersections) I’d really appreciate it. As the OP said, these things are frustrating to sit at.
I think it’s already been mentioned. If there is no traffic coming from any other direction, there is no reason the left lane has to be blocked–not even if there are two left lanes. These people can turn immediately on green. This means that there is no backup on the left turn signals. In any widely used intersection, there is going go be a signficant number of people wanting to turn. And the old method of having a separate turn signal each cycle (as long as a car is there) has fallen out of favor.
I do wonder why they don’t shorten the cycle time to compensate, though. Sometimes it’s actually quicker to turn right, pull over at the nearest place, turn around, and then go through the green intersection. And that’s just ridiculous.
That makes no sense around here though, because virtually all intersections have two long left turn lanes, so no one is blocking traffic waiting to turn left, not even during rush hour.
There was a four-way intersection around here where every road had three lanes. The center lane could go straight or turn left on the green/left arrow. The left lane could turn left only, the right lane could go straight or turn right.
If you think about this a minute, you’ll see that the only way this intersection could function is if each way had a separate green light. That is, one of the four ways had green, and the other three ways were all red. Because a green for the middle lane had to allow cars to go straight and turn left at the same time.
These have been showing up around here in places where there isn’t space to put in dedicated left turn lanes, and traffic is heavy. Most often, this is in places were there is 2 lanes going each direction. By letting each direction go seperately, you have 2 lanes flowing for the whole cycle, rather than having one lane frequently blocked by someone waiting for a gap in the opposing traffic, so that they can turn left.
its really no different in wait time unless they use a different cycle.
Consider…
N/S left turn - 30s
N/S straight - 30s
E/W left turn - 30s
E/W straight - 30s
No matter what, you’ll be waiting at most 90 seconds to turn.
N - 30s
S - 30s
E - 30s
W - 30s
Still 90s
Now, if the left turn lanes where rather lightly used, they wouldn’t need equal time, in which case the more standard model would be used, so it might be shorter.
What I loathe are the lights that stay on the same timer 24/7. Usually loooooooong because its a tiny intersection going onto a major street. Staring at a red light for 5 minutes late at night is ridiculous. Everything should be blinking red/yellow from 11pm to 5 am.
I drove a cab in NYC, generally 5pm (height of rush hour) til 2am (free wheeling).
Your choice depends on time of day, traffic.
Rush hour, all at once 8 block dashes on Park Ave will get you up town pretty quick. The alternative is to move a block every second light change on 1st ave with its sequenced lights. Total gridlock.
After 8pm, you can drive from the lower west side to the upper east side on sequenced lights without stopping.
