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Comment by rayiner | Hacker News Reader

Diagonals can be okay used sparingly, but roundabouts destroy traffic flow the way Americans do them (with lights).

We don't do them that way. In my area, roundabouts are put in where traffic occasionally bursts from one direction, and where traffic levels are low. Because they quit working once traffic increases, because it can be impossible to get on if there's continuous flow from another direction.

Maybe the lights reported are because the roundabout has outlived its usefulness? A stopgap before ripping it out.

Horses for courses....roundabouts are great for low to medium traffic conditions. Sometimes the lights are there for times of the day when the traffic is heavy. The lights go out when traffic calms down.

I think that the size of the round about matters. The larger it is, the more opportunity a driver has to get to an internal lane and not destroy on-ramping from other directions.

Also the timidness of drivers to enter a circle kills the flow and causes a backup that takes time to clear.

I hit 4 different traffic circles on my 40 mile trip home, two of them are 1 lane wide and only about 100 feet wide, the other two are a lot larger and have 2-3 lanes.

The narrow ones [1] back up much quicker during rush hour(s) especially when you have non-commuters using them (you can tell who they are).

The one that is 3 lanes wide [2] usually has a protected lane for only going to the first exit (on then off), then a middle lane that circles the entire roundabout and a middle lane that allows you to exit at any of the exits whenever you want. It works very well. I have never seen a backup on it.

The one that is only two lanes [3] backs up quite a bit during rush hour, when there is a West Point football game or because someone is too scared to enter. The backup clears relatively quickly, but I feel it wouldn't back up at all if there was protected on-ramp.

1: https://www.google.com/maps/@41.324487,-73.8829004,106m/data...

2: https://www.google.com/maps/@41.2981376,-73.935245,212m/data...

3: https://www.google.com/maps/@41.3202891,-73.9909974,212m/dat...

In countries where they're common place and part of the licensing curriculum, roundabouts are far superior for throughput and traffic flow than any other kind of intersection of comparable size.

To get any further improvement you start having to look at junctions with over head ramps and tunnels.

I think because the US only just started adding them, and various other intersection experiments, it will take us some time to become fully comfortable with them.

My wife's family comes from a small town outside of a small town in Louisiana and I remember when they opened the traffic circle there [1], the entire town hated it because they had never in their lives had such bad traffic, because no one knew how to use a yield sign -- stop meant stop, no sign meant yield, but now there is a new triangular stop sign, what the hell man.

Now days though, it flows amazingly, but it took close to a decade. I'm sure it will still improve over time, but now that the entire population has used it, they get it.

1: https://www.google.com/maps/@31.1609199,-93.267374,121m/data...

What you're describing is my experience with them as well. My parent's town in Ohio added one [1] and there was an uproar, but it's worked surprisingly well.

It replaced a very dangerous 4 way stop/traffic light intersection that very regularly saw accidents due to the geography (hilly) and the road layout (straight into the intersection after a mile of rural road).

1 - https://goo.gl/maps/NmyMpNEqn8A2

Then there's roundabouts that look like this:

https://www.google.com/maps/@27.9775503,-82.8273026,101m/dat...

Multiple two-lane entrances. An inner lane that goes all the way around but cannot exit. An outer lane that has a single break between the incoming and outgoing sides of the same road.

Oh, and cherry on the sundae, it's in a tourist-heavy area, so many drivers have little-to-no familiarity with it. People block the outer lane waiting to move inside when the outer lane disappears. And then block the inner lane waiting to go through the outer lane to exit.

Not all of this is on the drivers...

I'd actually say that is close to an optimal traffic circle. I'd add an even inner-er-er lane, make the outside lane a mandatory exit. If you enter in the right lane, you exit. The middle lane connects to the left of each entrance, and is able to exit at ANY of the exits, and the most inner-er lane would allow you to potentially bypass a traffic jam at another exit.

I think what you're describing is GGP's second example:

https://www.google.com/maps/@41.2981376,-73.935245,212m/data...

In which case I agree completely. And, from what I can tell, they're about the same size, so IMO this was a design miss, not a space constraint.

For me, it's all about the surprise factor. In my example, the outer lane almost goes all the way around. So you end up surprised after passing two exits and then being forced to merge to pass the next one. GGP's example is completely symmetrical, so nothing is surprising.

Yeah, that outer lane thing is weird, you should always be able to circle the roundabout (although you should be going on the inner lane, unless you know you're exiting next).

At some point someone has to bite the bullet a break the people don't know how to use roundabounts so no one builds them because no one knows how to use them vicious cycle.

It's finally starting to happen, at least in some parts of the country.

Even a mediocre roundabout is far better than that bane of American suburbia, the 4 way stop.

Look harmless. From the description you'd have thought this was the Magic Roundabout in Swindon!

It just reinforces the prejudice that Americans can't drive.

> Maybe the lights reported are because the roundabout has outlived its usefulness? A stopgap before ripping it out.

Yes, the roundabouts here in D.C. long ago outlived their usefulness. Unfortunately, ripping them out probably isn't in the cards.

Compare Dupont Circle to Connecticut & K st. Down at K & Conn. they need to use at least 5 traffic officers to direct the flow during peak times.

Meanwhile Dupont circle handles P street, 19th street, Connecticut avenue, Massachusetts avenue, and New Hampshire avenue. Plus there is the pedestrian traffic from the park and the metro stop at Dupont.

Can you imagine doing that with only traffic lights? How many officers would be required to massage that mess?

Unless you propose building a spaghetti mixer, I can't imagine a better system for drivers or pedestrians.

New York and Chicago manage to handle streets just as busy with regular 4-way intersections. The problem with any K street intersection is the bizarre access roads on each side that confuse drivers to ill effect.

I've...never seen a roundabout with lights. Granted, the state I live in doesn't do them much, but I've visited places (like MA) that do them a lot, and they're all very much like European roundabouts, just with inferior signage.

We have a few roundabouts in my town in SoCal, small ones, and they all have stop signs.

That's kind of stupid...takes the point away from roundabouts. They're designed to keep the traffic flowing in low /medium conditions when a traffic light or stop sign would bring it to a halt otherwise. Saves on fuel etc.

You have no idea how frustrated they make me.

Above a flow of around 3,600 pcu/hr (passenger car units) per lane, the roundabouts does not balance each direction effectively. (No gaps).

Traffic lights (signals), can help improve flow further in these situations as well as giving pedestrians a safe way to cross.

Part-time signals (only on a peak) can be very effective.

These areas are way below 3,600 pcu/h, maybe down in the several hundred to a thousand area. We’re talking residential side streets.

Here's a rotary in Boston (Sullivan Square) with a traffic light - https://www.google.com/maps/@42.3840002,-71.0744129,3a,75y,9... .

Here's a roundabout in Cape Town (Adderley Street Fountain) with a traffic light - https://www.google.com/maps/@-33.9202591,18.4250697,3a,75y,4... .

Here's one in Cambridge, UK - https://www.google.com/maps/@52.2084812,0.136931,3a,75y,82h,... .

One in Milan https://www.google.it/maps/@45.480752,9.2227599,3a,75y,111.4...

there are a load there with lights, tram crossing it straight, pedestrian crossing that aren't on a road crossing etc. makes for some fun time crossing one at peak hour, it's not weird at that time to get stuck in one for 10 minutes or more.

The first 2 are lights near the roundabout (where streets cross, or at pedestrian crossings), not in at the roundabout.

Okay, next roundabout over in Cape Town, no crossing street, no pedestrian crossing - https://www.google.com/maps/@-33.9172315,18.4285489,3a,75y,1... .

You find that model in areas which in peaktime see too much traffic for a roundabout to work well. If traffic along one pair of roads is too heavy anyone on the minor roads can't get in. So the light serves to gate the heaviest traffic. During off-peak hours the light is off.

Every roundabout in D.C. has lights: https://www.google.com/maps/@38.9020548,-77.0505595,3a,75y,8....

Some have through streets with lights mixed in with the circle: https://ggwash.org/images/posts/201611-221233-1.png.

As an American, I detest roundabouts. I've never encountered one with a light before.

Why do you hate them?

No one uses turn signals on them, and you never know when, where, or if someone is going to turn. It's haphazard maneuvering. 4-way stops are far more predictable and functional.

Your first comment is accurate, no one uses turn signals in any of the ones I use on a daily basis either, but that doesn't make them inefficient. In fact I think it keeps everyone on the defensive, and makes them pay more attention before entering.

But as I said in an earlier comment, the reason they kind of suck in the US is we don't seem to do them right.

* A very wide circle (350ft/100m minimum) with 3 lanes

* Each entrance and exit has 2 lanes.

* The outside lane of the circle directly connects the outside entrance with the "first" exit.

* The middle lane of the circle and the inside lane of the entrance allows you to exit at ANY of the exits

* The far inside lane of the circle exits nowhere without a lane change, but allows you to bypass an exit that might be backed up due to traffic.

Huh? That's only feasible for intersections of large roads, but American four-way stops are usually on small(ish) residential roads, where single-lane roundabouts work just fine.

No one is saying that roundabouts should replace ALL intersections, and the few they have in small residential places operate just fine with a single lane, but I'm skeptical of their value over a proper 4 way stop or a 4 way protect turn light.

Roundabouts are simple to navigate. You simply go around them.

Stop signs are simple to navigate too. You stop, and then go when it's your turn.

Simple, but slow.

Properly sited roundabouts don't need lights. Anyone who "detests" them must not live or work near busy intersections governed by stop signs only. The town I'm working in now has two roundabouts and one 4-way stop. Morning and night, the 4-way stop backs up traffic for blocks, while traffic flows through the roundabouts without issue. The city council has set aside a million dollars to change the 4-way into a roundabout this coming spring.

There are lots of stoplights that should also be replaced by roundabouts. A solid line of traffic flows right through a roundabout, and most of it is delayed by a stoplight.

The only problem with roundabouts in USA is the 30% of drivers who don't know how to use them, and stop when they see the yield sign... don't be that driver.

As an American, I detest how other Americans use them. Some folks in the 'ring' slam on their brakes when they see another car approach. Others will randomly pull out in front of folks who are already in the 'ring', causing the first group to always slam on their brakes when they see someone approach. This causes the second group to always pull out into the 'ring' without yielding right of way.

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