Left Turn Patterns for Automobile Traffic

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Left Turn Patterns for Automobile Traffic

Bruce Perens

Wed Oct 15 15:46:00 -0700 2008

Science.

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On Long Island, New York, where I learned to drive around 1976, the pattern of making a left turn was to enter the intersection, pass through the entire intersection on the right, and make a left turn once you'd entered the appropriate lane. If two cars were making left turns opposite each other, they'd pass on each other's left. Intersections were often built with islands in the middle, either made of concrete or just painted orange circles, to enforce the proper way to pass through.

In California, vehicles generally start the left turn as they enter the intersection, and two vehicles making left turns from opposite each other will pass each other on the right. They don't generally put circles in the middle of the intersection here.

I think the New York way was better at preventing head-on collisions. I wonder why they don't use it in California?

Remember that we drive on the right in North America.

Roundabouts, traffic circles with a large island in the center, which enforce a circular pattern - right turn in, right turn out, are rare here. When used properly, they make traffic lights less necessary, although I have no idea if they actually make traffic any faster. There's one big roundabout in Berkeley that is usually a madhouse, because people don't know how to handle it.

Left Turn Patterns for Automobile Traffic

Uncle Entity

Wed Oct 15 16:14:44 -0700 2008
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Roundabouts are just evil.

How would this prevent head-on collisions?

I guess if the car coming the opposite direction with their blinker on were to change their mind or wasn't really intending to turn you wouldn't pull right out in front of them but would wait until they passed before making your turn?

And what would you do if the left turn lanes are directly opposite each other like is pretty much standard everywhere?

Left Turn Patterns for Automobile Traffic

Charles E Hill

Wed Oct 15 16:54:20 -0700 2008
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Roundabouts are just evil.

I used to have this opinion as well. After a couple of weeks of driving in England, I changed my mind. Roundabouts seems to work rather well, once you get the hang of them.

Left Turn Patterns for Automobile Traffic

Uncle Entity

Wed Oct 15 17:09:40 -0700 2008
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I'm guessing you've never had to drive 70+ feet of truck around one with your front tire rubbing on the outside curb and your trailer tires up in the landscaping on the inside circle...in traffic.

Left Turn Patterns for Automobile Traffic

Charles E Hill

Wed Oct 15 17:13:50 -0700 2008
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No, I had some wind-up toy that the Brits called "mid-sized". When I told the guy at the rental counter I could fit it in the trunk of my "mid-sized" car at home, he just smiled and said something about "crazy Americans".

Left Turn Patterns for Automobile Traffic

John Dalton

Wed Oct 15 19:28:12 -0700 2008
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Small diameter roundabouts are evil as you have to look in all directions at once. We have lots of them here in Sydney. My theory is that they work by creating enough confusion that everyone gets scared and slows down, or stops, to give themselves a chance to work out what everyone else is doing. You all do a Mexican standoff, then someone makes a decision to go for it. Hopefully not everyone decides at the same time.

Large roundabouts can work well since they reduce the intersection to a simple "look right" (or left for the US). In this context "large" means large enough that the traffic flow for each entry/exit does not interact with its neighbour.

Each driver seems to have their own theory on how indicators work in roundabouts. For a four way intersection with a small roundabout, the law here says indicate left if going left, right if going right or nothing if going through. For a large roundabout though, one is turning left into it and left out of it, thus you indicate left on entry and exit. The law doesn't say where the cross over is between large and small. Consequently people just make it up. In practise it's not that bit a deal though. It just adds to the existing sense of confusion and is dealt with be people adding another level to their alertness/caution.

Having said all of that, even a small roundabout seems less evil than the idea of having oncoming traffic flows crossing each other in intersections. Surely the keep left/right rule should be of highest precedence?

Left Turn Patterns for Automobile Traffic

Calum

Thu Oct 16 05:08:50 -0700 2008
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I've never seen anything more confusing than The Magic Roundabout in Swindon. You can go round it overall in the wrong direction, as long as you're going around the mini ones that make up the large one the right way.
However, I think that roundabouts are fine.
I was surprised to learn that in some countries, you have to give way to traffic entering the roundabout, which seems very stupid to me. Keeping the roundabout flowing so it doesn't seize up is the best solution.

Left Turn Patterns for Automobile Traffic

ColaMan

Wed Oct 15 19:29:36 -0700 2008
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Roundabouts work until you reach a certain density of traffic, and then they become a great hinderance as one route through the roundabout tends to 'hog' it, leaving no break in the traffic for other people to enter.

But lighter-traffic (local neighbourhoods,etc), they're cheaper than traffic lights and much quicker to transit through.

And if you're scrubbing the tyres on your 70ft truck going through the roundabout, I respectfully suggest you try another route. But most small roundabouts built here in Australia have a smooth concrete lip on the inner circle that's wide enough to allow trucks to pass through relatively unimpeded.

Left Turn Patterns for Automobile Traffic

asdfg

Thu Oct 16 06:11:24 -0700 2008
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The state I live in seems to have gone all in for roundabouts. I dislike them.

There was an article in our paper about this a couple months ago. They have kept statistics on accidents at the intersections that have been replaced. The number of accidents has increased, but the number of serious ones (hospitalizations & fatalities) has decreased. The main problem I have with them is that incoming traffic does not yield to traffic in the circle as they should. This results in more side-swipe type accidents, I guess. In the traditional intersections, people blow through the stop sign/red light & there are more high speed T-bones.

Left Turn Patterns for Automobile Traffic

zuzulo

Wed Oct 15 16:15:37 -0700 2008
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To be somewhat cynical, i suspect that the lack of a large island makes the roads cheaper to build as they take up less real estate. Wouldnt surprise me at all if something like this were the case ...

Left Turn Patterns for Automobile Traffic

duhavid

Wed Oct 15 16:49:41 -0700 2008
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If I understand, the paths of the cars would cross one another. If that is the case, then if there is a line of cars, they will have to stop and mak way for one another. The "pass on the left" method seems like it would keep things moving better in congested situations.

What I dont understand is why roadway designers seem to prefer to use traffic signals when freeways intersect these days. Within the last couple years an intersection was done near my house. One of the freeways passes over the other, but instead of cloverleafing, they put in signals.

Left Turn Patterns for Automobile Traffic

Timothy Brownawell

Wed Oct 15 18:28:20 -0700 2008
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Starting the turn immediately means that the paths of two cars going opposite directions and making left turns don't cross. Sufficiently busy intersections here in Memphis TN generally give left-turn-arrow for both directions of a road at the same time, then go-straight for each direction when out of left-turn cars for the other direction, or after a timeout.

N, S: Left Turn; E, W: Stop
(no more cars turning left on the N side)
S: Left Turn, Straight; E, W, N: Stop
(timer expires)
N, S: Straight; E, W: Stop
...
N, S, E, W: Stop (for maybe a second)
E, W: Left Turn; N, S: Stop
...

I think the New York way was better at preventing head-on collisions. I wonder why they don't use it in California?

Avoiding a head-on collision only requires that people follow the curved lines that are sometimes visibly painted on the intersection. Avoiding side-on collisions in the NY model sounds like it would additionally require checking for oncoming traffic, the same as making a left-turn on green without the special left-turn light.

Left Turn Patterns for Automobile Traffic

Michael Smith

Wed Oct 15 18:51:29 -0700 2008
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You should try doing a Hook Turn.

Left Turn Patterns for Automobile Traffic

rubycodez

Wed Oct 15 19:01:39 -0700 2008
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hah, that article mentions Palentine Road somewhat south of me, which is a kooky aberration here in the midwest; I've lived here four years and I'm still trying to decide if I like it, think it would be better without the concrete seperator but then people not used to it would do even crazier apeshit confused manuevers.

Left Turn Patterns for Automobile Traffic

President4242

Thu Oct 16 08:44:46 -0700 2008
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And I thought it was horrid when ODOT did the thankfully temporary Canyon Rd LEFT LANE exit off of Hwy 26.

Speaking of ODOT- I don't have time today, but the check is in the bank, and I'll be writing a full confession/expose on what I think went wrong soon. It's long and complex, and I'm not sure if it belongs here or in my slashdot journal. Though an argument for it being here is one of the conclusions is that I was running into a clash of cultures where one generation felt that government employees are automatons who need to keep government secrets and "stay on message" as a unit, vs another younger culture in which government should not keep secrets, government employees are human beings with full rights, and that we get better government when such people are fully engaged in the culture they are serving.

Bet you can guess who had most of the power in my case.

Roundabouts are becomming more common

wowbagger

Wed Oct 15 19:50:35 -0700 2008
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Roundabouts are becoming more common, at least here in Kansas. There is on in downtown Wichita, one just off I-135 in Newton, and one going on on the intersection of US-400 and K66 (old Route 66) just outside of Riverton.

The larger ones are really nice as a non-blocking intersection. The smaller ones are just confusing.

That said, I'd like to ask any left-side-of-the-road types out there: is it as unnatural for you to enter a right-side roundabout as it is for me (who has been driving since 14, on the right) to enter a left-side roundabout?

Roundabouts are becomming more common

Chris Hopkins

Thu Oct 16 04:09:49 -0700 2008
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Well I drive in England, but my parents also have a house in France so I get plenty of practice at both left hand side and right hand side driving in both left hand and right hand drive cars and my motorbike as wel; so I can claim to have driven every combination that exists (they occasionally bring their French car back to England, and I have driven my car in France on several occasions)

In a word, no. It's very easy to swap between the two types of roundabouts as you swap sides of the road. Since you're on the right, I find now looking to the left becomes automatic in a matter of seconds. Equally as soon as I get back to England you're very obviously on the left so looking to the right jumps right back in.

Except of course for a few of the older style french roundabouts where traffic on the roundabout gives way to traffic entering the roundabout - which takes some getting used to and can cause some interesting moments if you haven't realised the roundabout you're on is the old variety...

Roundabouts are becomming more common

President4242

Thu Oct 16 08:46:41 -0700 2008
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A hint for swaping sides of the road in general:

The steering wheel is always next to the center line of the road for the majority of cars.

The corrollary for roundabouts in this case:

The steering wheel always goes next to the inside curb of the circle.

The Detroit Left

87C751

Thu Oct 16 03:54:07 -0700 2008
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Midway between the two examples, Detroit has many median-separated roads. To turn left there, one turns right and then executes essentially a U-turn through a median break halfway to the next intersection. That took a while to get used to.

The Detroit Left

Thornton

Thu Oct 16 04:46:31 -0700 2008
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Those types of "left" turns are common in New Orleans, too.

Left Turn Patterns for Automobile Traffic

The Baron

Thu Oct 16 06:05:24 -0700 2008
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Timothy basically has it correct. If you use the old 'pass on the left' path, left turn paths in opposing directions cross. What can happen is lines of left turn vehicles in opposing directions can block each others path and end up with grid-lock in the middle of the intersection, with no one able to move because a car is in their way.

It is also highly dependent on the layout of the intersection. In New York there are a lot of relatively narrow streets, where turning into them the pass-left path makes sense. In California there are a lot more 6 lane-6 lane intersections, with a lot of open space in the middle of the intersection for passing on the right. In a heavily congested turn, the engineer may want to have dual-left turn lanes, and that only works in pass-right configurations.

The thing is that, while pass-left follows the normal rules of the road, at left turns it really is generally more efficient to have everyone pass-right, but it is incredibly hard to convey that in an 'official' manner to the general public without causing tremendous confusion and upset.

In addition, I'll say that head-on, left-on-left collisions are extremely rare. Generally the speeds are low enough, and people are focusing on opposing traffic that at worst, there is a nose-to-nose confrontation where someone has to back off, but in 19 years of traffic engineering I've probably only seen an issue with it very few times and only at specific intersections with a poor layout. Pass-right can be more uncomfortable for the drivers, and in a permissive left situation the opposing left vehicles can block the view of on-coming through vehicles, so that is also a consideration we have when doing intersection design.

Left Turn Patterns for Automobile Traffic

Matt Fulkerson

Thu Oct 16 08:01:51 -0700 2008
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A lot of comments are in favor of large traffic circles, not small ones. In my experience, small roundabouts work great at intersections of two lane (a single lane in each direction) roads.

I find traffic circles with two or more lanes going around to be confusing and dangerous. There is a two-laner in Towson, Maryland that is atrocious in terms of safety, especially since there are a lot of pedestrians in the area. The right of way of the inner lane crosses the outer lane. If you are in the outer lane, you may be required to turn right immediately, but motorists who are not familiar with the rules will make mistakes. The rules still are not clear to me.

Left Turn Patterns for Automobile Traffic

Paul Jackson

Thu Oct 16 19:06:52 -0700 2008
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Our city had one intersection, Stone street and Pine Grove Ave that historically did left turns like that, Pine Grove crosses the intersection at a 45 degree angle So it was more nature to turn that way; the state finally came in and redesigned the intersection with lines and delayed each side so people wouldn't do it that way anymore. we had a lot of accidents from out of towners there. Ever see a Michigan left? Two divided Highways that intersect here normally do a Michigan left, to turn left you turn right, then do a U turn through the median.