Traffic engineer brilliance

Mundane Pointless Stuff I Must Share (MPSIMS)
1

I’m not sure this is the right forum. but here goes.

The lightrail here is being expanded incrementally, I suspect as Federal funds become available. Its current terminus is on (literally) Main street about a half-mile to the west and a half-mile south of my house. The latest expansion is two miles east so it is crossing through the usual way I use to go north, a fairly major street.* Construction has been close to a year now and what with the ever-changing “You can’t turn/pass through here for a week” it’s been a PITA, but that’s not what this is about.

When the work was started on the intersection above one of the first things they did was claim the four corners way back from their former locations, like twenty feet and shrinking the parking lots of the businesses there a bit. I found this curious but a couple weeks ago the reason why became clear: The traffic lights are no longer there and instead, the intersection has become a roundabout. With a pair of tracks running through the middle.

Now, roundabouts have been springing up like toadstools around here and I’ve gotten used to navigating them but this one has me stumped. I’m thinking about approaching the intersection either to continue going south or leave it on Main Street going west, both of which necessitate crossing the tracks. I’m looking left to make sure there’s no one in the circle I have to yield to, entering the circle and Oh, my God! There’s a train coming from the right!

There’s a round RR Crossing sign before the intersection. Big whoop; it took me two days to notice it and it doesn’t indicate if there’s a train you need to worry about right then. Along the rest of the line wherever there’s a left turn lane crossing the tracks, there are Train Approaching signals that light up well in advance but there are none installed for this intersection and judging by a lack of any evidence, it doesn’t look like they’re planning on installing one.

No trains are passing through the intersection at the moment – there’s work to be done downstream. When they do I’m predicting recurrent disasters.

*It’s flat enough here the major streets follow the the range and township lines laid out generations ago. They’re referred to collectively as the “main milers” and generally have ramps on the freeways. At the halfway point are the “half-milers” and these usually cross over the freeways. “My” road is a half-miler.

2

Any traffic engineer who suggests roundabouts or rotaries should be taken out, covered in honey, and strapped to a red ant hill.
There is ample proof that driving straight is a challenge for too many people, let alone in a circle.

3

Granted, but short of replacing all street intersections with overpasses, then the alternatives to a roundabout are traditional intersections with stop signs or traffic signals. Guess what? People aren’t great with these, either.

A modern roundabout is much safer than a traditional intersection. See here.

For one thing, any collisions that occur in a roundabout are at low angles (i.e. side-swiping), whereas in traditional intersections, you have a far greater incidence of head-on or T-bone collisions.

4

Roundabouts with trams/light rail running through the middle of them destroy the whole concept of a roundabout. It’s an incredibly dumb idea. One of the worst black spots in my city used to be a big roundabout with a tram track through it just south of the university. They fixed it up by putting a bunch of traffic lights back in. It’s much better now.

In fact, now I come to think of it, the roundabout at the north end of the university was pretty much a disaster when they first put it in too. Until they fixed it by…putting in traffic lights at all the entrances

5

My guess is the train will blow its horn and go through the roundabout at crawl speed.

There are some famous roundabout s in Australia (at least famous to traffic engineers) with light rail or tram s through them, that handle a ridiculous number of cars per hour. The key is most drivers know how the roundabout has to operate, they all cooperate, and they know to listen for the train. You are going to find out what drivers in your neck of the woods can do.

If you are interested in further reading:
https://www.researchgate.net/publication/321572725_Exploring_the_Safety_Performance_of_Tram_Roundabouts

6

Never seen it in IRL (tho used em in Cities Skylines game), but I’d presume that light train also has to yield to circle traffic. Inbound and outbound. Or am I missing something?

7

I had a look, just to see if (as I suspected) the roundabout I was talking about was mentioned, and sure enough:

As you can see if you street view it, the way they made it safe was indeed to put traffic lights at every entrance, some exits, and every place where the tram track crosses the road. So … traffic flows perfectly well there now, (it’s a complicated intersection, any traffic flow solution is going to be complex) but it’s strayed pretty far from the ideal of a roundabout, and not a great poster child for the idea of replacing a functional set of traffic lights with a roundabout structure.

8

Here’s a roundabout in Santa Cruz CA with a rail line through it. It has crossing arms to prevent problems.

Here’s one in London, Ontario where they built a roundabout on an overpass over rail lines to avoid having to stop traffic for trains.

9

The circle in Santa Cruz has the rail line kind of nudging the perimeter as it makes a turn; crossing gates there would make sense. The one near me has the tracks bisecting the circle itself so I’m not sure where the gates would go. The Google image is too old to show the new work; even the corners of the original intersection have not been whittled back yet. On my monitor there’s a ghost circle in at least approximately the location and size. I’m not sure what that’s all about.

What really puzzles me is why this one intersection has been changed. The system is over 25 miles long with innumerable intersections. Many were cut by the line, leaving behind a pair of T-intersections. Others, mainly ones that had traffic lights already, have protected left turns and approaching train warnings in place, intersections people are used to. Mine is the only one where they uprooted the traffic lights and plunked down a roundabout in their place.

Aspidistra, doesn’t having traffic lights in a roundabout kind of negate the whole purpose in having one in the first place? You’re probably right, the idea is to have the train go slow through the circle making a lot of noise. Given the mad skillz drivers have here regarding roundabouts, I am dubious.

yo han go, the acceleration speed of a light rail train is slow enough to make the yield-to-traffic in the circle unfeasible. Besides, most trains are two cars long. If the train has to stop at the far side of a circle, the back end will still foul the near side forcing cars in the circle that want to cross the tracks stop anyway.

10

Crossing gates are being installed on both sides of the circle. It is not clear yet whether they will be parallel to the tracks or radial to the circle. I suspect the former as they would interfere less with the traffic on Main St., by far the busier of the two streets.

It is the only such intersection on the lightrail system; all else are simply a traffic light with a protected left turn in order to cross the tracks. Minor streets that crossed the main street with no traffic light are not allowed across any more, either to turn left or go straight through.

Perhaps it is a cost-saving experiment. A traffic-lighted intersection with protected left turn lanes for the major way has been eliminated. OTOH a pair of crossing gates with their attendant circuitry can’t be cheap, either.

11

Not long ago, two roundabouts were installed in the closest city to me. Although they were opposed by many drivers initially, the traffic engineers said, “you’re gonna love 'em,” and they may have been right.

At first, there were MORE accidents for the intersections (they used to have lights or stop signs), but no serious ones. Now that people have become accustomed to them, they seem to be working well.

Both roundabouts connect two-lane roads (one lane in each direction). I have less experience with others, but here is how these work.

First, the traffic engineer that designed these particular intersections told us – think of how you approach a “normal” intersection of the same size. If you plan to make a right turn, get in the right lane. If you plan to make a left turn, get in the left lane (all approaches widen into two lanes per side). If you plan to go straight, get in either lane.

Second, traffic already in the roundabout has priority. Once you can safely enter the roundabout, continue in the desired lane – right lane for right turn, left lane for left turn, either lane for straight. Once you exit the roundabout, merge back into the single lane.

IMHO, it works, most of the time. It fails when there is excessive traffic in one direction with few gaps. Without traffic lights to create gaps, it can be hard to merge into the main stream of traffic. This often happens just before weekends and holidays, when it’s bumper-to-bumper coming in to the county, and in the reverse direction after.

Basically, roundabouts convert crossing and turning actions into merging actions. You’re using your brain, not following a mindless signal. If you time your actions just right, everyone benefits. I like 'em.

12

but can your brain handle the famous Swindon roundabout?
(Five mini-roundabouts , all inside one bigger roundabout…And each rotates in a different direction.)

13

I try not to think about it.

14

There is a similar one in Hemel Hempstead (also known as the Magic Roundabout after a fondly-remembered children’s TV show). The design has been changed several times.

15

You’d think so, wouldn’t you?

In the case of the Elizabeth Street roundabout, there’s clearly been an evolution in traffic-engineers’ thinking - as it turns out, you can’t just use a roundabout as a vehicle for safely removing all the traffic lights, if the structure is sufficiently complex. I’m glad to see that your local traffic engineers have caught a clue here - possibly working from experience of other well-studied intersections.

I think there ought to be a distinction between “fast” roundabouts - where the aim is to get as much traffic through as possible without holding them up with lights - and “slow” roundabouts - where the aim is for people to not get stopped by tailbacks due to the crushing weight of cars inching through slowly in their different directions.

Swindon, in the link above, is clearly a “slow” roundabout - there’s a lot of cars sitting at choke points waiting to go in the linked video, and the design seems optimised to let everyone through slow-ish rather than to let one direction zoom through while the other waits till the sun cools (which happens a lot at badly designed intersections). Elizabeth Street is a “slow” roundabout as well - right in the middle of town. I wouldn’t like to go through either of them at highway speeds.

16

A contributing factor at Elizabeth Street may have been traffic numbers. I’ve not really paid attention to that roundabout, but in general there used to be a lot fewer cars in Melbourne, and it may have been the case that when that roundabout was put in, there were only a couple of cars in it at any time.

Supplementary story: I remember getting a parking ticket in Swanston Street (the busier neighbor of Elizabeth Street), and looking up the street, and down the street, and not seeing a single other car. That was on a Sunday, but it speaks to how many cars use the city centre then as compared to now.

17

The work is all done now and the extension is open to the east. It happened a couple weekends ago when I was in Las Vegas. Grade crossing gates were installed parallel to the tracks and despite the small diameter, the circle has two lanes around it. If you’re approaching on the side street, the gate drops well before the train gets there so there’s no surprise or temptation to try and beat it. Almost everyone stops before entering the circle, even if there are no autos approaching from the left. If you are on main street and want to turn left, the gate will stop you. With two lanes, those wanting to keep going straight can get around you until about three are waiting for the turn, in which case everything stops (mostly during rush hour, natch).

It seems safe enough, stopping any but the most inept drivers before they are in danger. Most of the problems lie with those who don’t know how to use a roundabout, but that would be the case whether it had tracks or not. More than once I personally have been following someone up Main street and they stop at the circle, even if no one is approaching for a hundred yards, hesitate waiting for the second coming, I guess, then pulling into the circle. I’m waiting for someone to drive into the path of some poor guy who innocently entered the circle while they were screwing up their courage.

18

My favorite show as a little kid. I think I had a crush on Florence.

I never knew that Dylan was named after Bob, or that (looking at it now) he’s pretty obviously stoned all the time.

https://the-magic-roundabout.fandom.com/wiki/Dylan
19

It may be necessary to install them if traffic is getting so heavy there are no gaps for side street traffic to enter. In general, no, you don’t have signals at a roundabout. You would still have the safety benefit, though, of avoiding the broadside and head-on crashes which tend to result in serious injuries/fatalities.

20

I agree; I hate the ddamned things.