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Author Topic: Left Turn Lane, or Right?  (Read 417 times)

webny99

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Left Turn Lane, or Right?
« on: July 10, 2018, 09:02:35 PM »

On approach to a four-way, stoplight-controlled intersection, with right-of-way for two lanes in your direction, which do you prefer:
(a) Left turn lane and shared thru/right lane (cannot use shoulder to complete right turn), or
(b) Right turn lane and shared thru left/lane

What are some factors that play into this, and how do we analyze what's preferred at a given location?
Some potential factors are volumes of turning traffic, volumes of cross traffic, and length of light cycle.

Eth

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Re: Left Turn Lane, or Right?
« Reply #1 on: July 10, 2018, 09:31:06 PM »

In almost all cases, I'd go with option (a); the dedicated left-turn lane prevents traffic waiting to turn when oncoming traffic is present from impeding through traffic. There are a few cases where option (b) could be appropriate, though, mostly when little traffic proceeds straight through the intersection and much more of it turns right instead.
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NoGoodNamesAvailable

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Re: Left Turn Lane, or Right?
« Reply #2 on: July 11, 2018, 01:06:26 AM »

I agree that (a) is safer. (b) puts back pressure on left turning vehicles, one of the main issues that turning lanes are supposed to eliminate.
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jakeroot

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Re: Left Turn Lane, or Right?
« Reply #3 on: July 11, 2018, 02:47:37 AM »

B shouldn't exist. That would have the straight lanes pointing directly at each other, assuming three lanes across for both directions.

That said, if the straight/left lanes were offset (road basically four lanes wide but only three striped lanes -- through lane on each side is about two lanes wide), B is definitely OK with the right scenario. The straight movements at this intersection (which resembles the former example) pale in comparison to the left and right turns, so it works well.

Even at a T-intersection, if the top-left approach has a dedicated right turn lane, the straight movement points right at the oncoming left turn lane. That type of situation occurs here in Federal Way, WA, and can be pretty awkward when someone is waiting to turn left:

« Last Edit: July 11, 2018, 04:22:53 AM by jakeroot »
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webny99

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Re: Left Turn Lane, or Right?
« Reply #4 on: July 11, 2018, 09:04:14 AM »

B shouldn't exist. That would have the straight lanes pointing directly at each other, assuming three lanes across for both directions.

How is that even possible?
Imagine a standard approach with no turning lanes at all. Then add a right turn lane, and voila! you have scenario (b). Very similar to your first link above; here's two examples in my area.

jakeroot

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Re: Left Turn Lane, or Right?
« Reply #5 on: July 11, 2018, 01:42:02 PM »

B shouldn't exist. That would have the straight lanes pointing directly at each other, assuming three lanes across for both directions.

How is that even possible?
Imagine a standard approach with no turning lanes at all. Then add a right turn lane, and voila! you have scenario (b). Very similar to your first link above; here's two examples in my area.

I was thinking you were talking about a road like this. A and B are not interchangeable with pavement markings. They require different road layouts to avoid head-on collisions. In the link, if the left lane of each approach allowed straight-ahead movements, drivers may end up crashing into each other.
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jeffandnicole

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Re: Left Turn Lane, or Right?
« Reply #6 on: July 11, 2018, 01:55:12 PM »

A definitely preferred, B in some instances.  We had a B near me that worked fairly well, but I'm amazed at how many people refused to go around a single left turning vehicle, even when they were in the intersection, when the right lane was clear.

The situation changed mostly due to the opposing direction which only had a single lane and a fairly heavy left turning movement.  If two cars were waiting to turn left, it was impossible for other vehicles to go around them.  That side was widened and now has the standard A situation, and the side I mentioned above has separate lanes for each movement.

Before:  https://goo.gl/maps/2WX6TsXi5r92

Current:   https://goo.gl/maps/LbMnHvdCLEG2
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jakeroot

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Re: Left Turn Lane, or Right?
« Reply #7 on: July 11, 2018, 02:00:44 PM »

A definitely preferred, B in some instances.  We had a B near me that worked fairly well, but I'm amazed at how many people refused to go around a single left turning vehicle, even when they were in the intersection, when the right lane was clear.

Yeah, that's odd. I would have definitely passed on the right. I don't think you can legally go straight from a right turn lane, but you could go around if you were past the stop line and in the intersection.

The situation changed mostly due to the opposing direction which only had a single lane and a fairly heavy left turning movement.  If two cars were waiting to turn left, it was impossible for other vehicles to go around them.  That side was widened and now has the standard A situation, and the side I mentioned above has separate lanes for each movement.

Before:  https://goo.gl/maps/2WX6TsXi5r92

Current:   https://goo.gl/maps/LbMnHvdCLEG2

An interim measure could have been to implement a leading left from Turkey Hill Road. At least they fixed it with pavement, and not split-phasing!
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webny99

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Re: Left Turn Lane, or Right?
« Reply #8 on: July 12, 2018, 08:49:04 AM »

B shouldn't exist. That would have the straight lanes pointing directly at each other, assuming three lanes across for both directions.
How is that even possible?
Imagine a standard approach with no turning lanes at all. Then add a right turn lane, and voila! you have scenario (b). Very similar to your first link above; here's two examples in my area.
I was thinking you were talking about a road like this. A and B are not interchangeable with pavement markings. They require different road layouts to avoid head-on collisions. In the link, if the left lane of each approach allowed straight-ahead movements, drivers may end up crashing into each other.

I see what you're saying now. Should have specified that the two aren't interchangeable with pavement markings alone.
(A) requires ROW for three lanes through the intersection itself, while (b) only requires ROW for two lanes through the intersection itself. Either way, ROW for three lanes on approach is required, it's just a difference of how wide the road is within the actual intersection.

jakeroot

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Re: Left Turn Lane, or Right?
« Reply #9 on: July 12, 2018, 01:48:46 PM »

There was an intersection near University Place, WA with offset through movements. As you can see in this street view image, drivers would often continue straight ahead in the intersection, only to find themselves driving into the oncoming left turn lane. I don't know if there were any head on collisions, but it was an awkward intersection until finally being fixed recently. Oddly enough, it was like this for a long time.

So I guess the moral of the story is that you can have through lanes pointing in odd directions, but do expect some odd behavior! :-D

There's also this intersection near me (a freeway off-ramp) which has three approach lanes: left only, straight and right, and right only. Because it's an off-ramp 99% of vehicles turn left or right, but when someone accidentally exits but needs to re-enter, they have to occupy one of the right turn lanes. Because right on red arrows are allowed here in WA, this double right turn has a near-constant flow most of the day. When someone does arrive to go straight, it really screws with traffic. Always a symphony of horns. This is a situation where the through lane and the left turn lane should be the same, as neither can go on red.
« Last Edit: July 12, 2018, 01:52:07 PM by jakeroot »
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Eth

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Re: Left Turn Lane, or Right?
« Reply #10 on: July 12, 2018, 05:46:15 PM »

Because right on red arrows are allowed here in WA

Huh? What's the point of the red arrow then? I'm guessing that's supposed to convey the meaning of "stop before turning", in which case surely a red ball would be more appropriate, and if it's not that, wouldn't that make it functionally equivalent to a flashing yellow?
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jakeroot

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Re: Left Turn Lane, or Right?
« Reply #11 on: July 12, 2018, 09:11:11 PM »

Because right on red arrows are allowed here in WA

Huh? What's the point of the red arrow then? I'm guessing that's supposed to convey the meaning of "stop before turning", in which case surely a red ball would be more appropriate, and if it's not that, wouldn't that make it functionally equivalent to a flashing yellow?

Because left on red onto a one way from a two way is permitted here, I guess the idea with allowing turns on red arrows was so that they could have green orbs and red arrows lit at the same time, without having to have "LEFT TURN SIGNAL" signs.

It also helps to use red arrows so that you don't need to use lane-use signs all over the place. The arrow in the signal tells you what your lane does. In the rare occasion that turning on red shouldn't be permitted, they'll put up a sign.
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doorknob60

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Re: Left Turn Lane, or Right?
« Reply #12 on: August 06, 2018, 04:29:58 PM »

In 90% of instances I would choose A. Makes way more sense. However, specific traffic flows can make B make more sense. This one comes to my mind.

At that light, probably 60-70% of traffic is turning right on Vista to access I-84 or the airport (that's usually what I'm doing). Probably 25-30% is turning left on Vista towards downtown. And only 5-10% at most are going straight. Straight only accesses a gas station, hotel, and one neighborhood with no other outlet. The lane that gets the most backed up is still the right turn lane, even though it has its own lane, and a protected green arrow when SB Vista has a green left arrow (so it gets more green time than the straight/left lane does; plus of course you can RTOR).

 


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