I used to love topics like this, just as I vigorously defended the construction of roundabouts.
Now I have a roundabout right in front of my child's daycare. Due to the traffic rules surrounding that roundabout (Germany), it'll be impossible for him to cross that safely without me being very hands-on.
All these systems are completely made to get another 2 % of efficiency for cars, while massively sacrificing walkability and bikeability. I am not a fan anymore.
In London there's been a big move over the last decade to remove big urban roundabouts for precisely that reason. They make sense for vehicles, but they make mixed use areas much more dangerous. Roundabouts are slowly being converted to C shaped junctions with dedicated cycle routes and lanes in both direction. I imagine they're probably a lot less efficient for cars, but they also don't tend to kill people at the same rate.
I still think roundabouts are a great idea for more rural road junctions. They're much safer, and I suspect more efficient, than cross roads controlled by traffic lights. I was driving around Toronto last year and the amount of stopping and starting at traffic lights you have to do is infuriating.
I wonder if the distance of a pedestrian crossing from the centre of the roundabout relates to pedestrian safety. Subjectively, I feel like safety could be improved by moving the crossings farther from the centre.
Maybe I'm misinterpreting, but it's frustrating as a pedestrian if you ever are walking along a road and have to take a walk of even only a few metres in a perpendicular direction to find a crossing, cross, and then walk back to the original road.
It could be just five or ten meters in each direction. For example this crossing always felt unsafe to me. The original crossing has been obstructed to prevent crossings and moved about 5 meters out, and it feels much safer now: https://goo.gl/maps/ipe8RXkdSgpZz72r6
I wonder about this specific setup for cyclist. The yield sings when coming in are after the cross walk so here in Finland in those cases the cyclist would need to yield. And even more weirdly as car is turning when going out they would need to yield on that side to cyclists. Is that same or different in Czech.
In Czech all traffic must yield when entering a roundabout, including the tram. As far as I can tell the yield signs are there for foreigners might be used to different rules.
Regarding your second point, if you mean a car changing lanes from an inner lane to an outer lane, it does need to yield to everyone in that outer lane. This includes yielding to cyclists whose trajectory would intersect with the car. Again, the same applies to all traffic.
Do diverging diamonds get built anyplace a person would want to walk?
Around here (suburban DC,USA) they’re exclusively used where two massive highways intersect. Usually one 6+ lane interstate with a 4+ lane state highway. And the interchange itself is enormous - long, wide, noisy, stinky with fumes.
Any pedestrian traffic is carried by a physically separate walking path. Not “local” sidewalks.
It’s a very car-centric design. But where the highways already existed, it’s at least a traffic improvement with no reduction in pedestrian access (which was usually nil before the interchange, and improved by addition of separate walkway during construction).
I'm not an expert on this topic and while I have spent some time in the US, my experience is limited.
The particular case of two highways intersecting is one thing. Go ahead and have your diverging diamond interchange there. However, I don't believe Interstate-Interstate-Crossings are the most common use in the US? See for example https://wsdot.wa.gov/travel/traffic-safety-methods/diverging.... Three crossings, none are between two interstates? Or am I misreading this?
I actually believe it comes down to this: You only need the diverging diamond interchange if you build a highway in tight spaces, such as inside cities. In particular you want them on heavily frequented entries/exits that would otherwise let traffic back up into the cities. Thus, the interchange is at or close to a location with a lot of important destinations, otherwise you wouldn't build it there.
Not having that interchange then open to traffic other than cars then clearly makes it harder to cross the highway at these places. You'll have to go up or down each of the roads until you hit an over- or underpass or crossing. In my experience, in the US, this is generally very far - as of course everything is already bigger than in other places because of parking lots.
It handles the intersection of a busy interstate and a busy local highway/stroad.
Here's a new interchange just down the road from the first, same interstate, but even busier highway. Instead of a diverging diamond, it's a bowl of spaghetti.
In Austin they are putting in diverging diamonds where regular roads intersect the interstate going through town. One of those roads in particular (Parmer Lane), they've been building out its sidewalk on the east side. So it is meant to for local pedestrian traffic except at the diverging diamond.
I was curious and looked it up on Google Maps[0] and that's definitely not a road; it's a six-lane stroad[1] optimized solely for traffic flow. Those are really good at killing pedestrians because people perceive them as being wide open, all but inviting drivers to speed up.
I'm guessing the diverging diamond replaced a bridge with two intersections? In which case, the oddity of the diverging diamond might at least serve as a traffic calming measure. Even when you get used to it, switching travel directions is uncomfortable for drivers which can force them to be a bit more cautious.
That said, the new sidewalks in the satellite photos are probably a big improvement over the old ones (if there were any). They at least have some small curbs and ~2-4 feet of separation from the cars traveling at high speeds along the stroad. If you look at the sidewalks to the west, where there's no separation from the stroad, they're...well, at least not that? But for the most part, sidewalks alongside stroads are just an afterthought.
Oh wow, what a mess. That whole area is a pedestrian (and car and bike and ...) nightmare. Yeah, there's a sidewalk, but it's like a square mile of car-centric asphalt wasteland. Even if you don't get Froggered on the frontage road sidewalk or Palmer at-grade crossings, you still have 1000 yards of distracted drivers to dodge in all those parking lots.
It could be a means to an end. Finding safe bike routes to get from one side of a freeway/river/mountain to the other can be challenging. A few miles on a state highway shoulder aren't an uncommon requirement to make a longer trip feasible. If the diverging diamond isn't paired with a dedicated pedestrian crossing over the freeway then it significantly hampers bike traffic.
Understood. At least near me, when I cycle, I'll pick a local overpass/underpass instead of using the diverging diamond (when possible). There are usually enough of them to make that work, but there are definitely places where that means riding 5 miles out the way. On the plus side, VA and Fairfax County are doing a better job building bike/ped infrastructure on most new projects. Sometimes what the build makes me scratch my head, but usually it's a welcome addition.
I usually see some not-terrible walking infrastructure around these.
Though, having lived in Utah and having dined on I-5 up and down the west coast, the idea of finding oneself walking right over one, as if a car, isn't as far out there as some may believe.
Not sure the roundabout is the problem? What alternative would be better?
I mean adding zebra-crossing would be better, but you can combine them with a roundabout, can't you?
I do agree that cars are a big problem. I wonder if we should tax them higher to properly price in their negative externalities. (And Germany already has a lot higher taxes on driving than eg the US.)
>Not sure the roundabout is the problem? What alternative would be better?
A simple cross intersection controlled by traffic signals.
Yeah, I know I'm gonna get an imperial shit-ton of downvotes for daring to suggest traffic signals, but they are simple and involve traffic coming to a complete stop at the intersection.
Simplicity is a virtue, and it's why traffic signals still reign supreme at least here in the USA.
Roundabouts with "Raised Pedestrian Crossing" (or wombat-crossings as they're called in downunder) are pretty safe without forcing drives to wait for nothing at 3 AM.
In Australia, there is speed and light cameras everywhere.
Crossing a red light, sometimes even to make way for emergency cars, will earn you 3 demerit points, which gets you over 23% to 13 points and losing your license.
Now it might not seem so dumb to wait 30 seconds, even if it is 3 AM and the road --seems empty.
When one lives under perpetual surveillance, one’s pragmatic judgement reweighs the situation accordingly.
And for the record, “dumb” is your word not mine. People make choices based on their own situational perception. I strive never to mute a person’s lived experience.
Signalled intersections are likely a lot worse for pedestrians and cyclists, as non-compliant drivers can navigate them at much higher speeds. On account of its shape, it's impossible to blast through a roundabout at, for example, 100 km/h.
Combine with raised pedestrian crossings and you have natural speed bumps reducing car speeds even further at the potential point of conflict.
Yeah slowing down cars "naturally" by using narrower roads and roundabouts, and then built lots of non-signalled pedestrian crossings (sometimes with bumps) have been a trend for some time. The data seems to support it works.
I think the complaint here is about larger roundabouts that tend to admit higher speeds and have a large radius, not low-speed tight circles in dense mixed-use areas.
For those kind of roundabouts you add signals to the pedestrian crossing. Although above a certain size tunnels or bridges becomes justifiable, but then we are pretty far from urban environments.
>Roundabouts are much simpler, aren't they? They don't even have any moving parts.
With traffic signals, much of the cognitive load is offloaded from the driver. The driver only needs to pay attention to the signal and whether there are any hazards in his planned direction of travel. A four-way stop signed intersection can produce similarly simpler results compared to roundabouts.
With roundabouts the driver needs to count or be thoroughly aware of his planned exit, maintain an appropriate speed and curving direction of travel throughout the roundabout, yield or not yield according to traffic at any given moment, and obviously whether any hazards are ahead or near him. That is not simple.
Just because a driver has less things to attend to does not mean he is less attentive. In fact, I would argue a person becomes less attentive to any given thing the more things there are they have to pay attention to.
Burdening a driver with more (arguably unnecessary) cognitive load is not a good idea.
I would assume that a 4-way stop sign would also be preferable, as there's a requirement to stop.
The issue is that roundabouts generally only have a yield sign, which means cars are rarely at a standstill. This complicates pedestrians crossing the street.
Thank you. Every time the roundabout discussion comes up here on HN I get dismissed when I tell people that, as a cyclist and pedestrian who does not own a car, I do everything in my power to avoid roundabouts. Please just let me cross at a junction where I can see that all cross traffic has come to a complete stop, and can make full eye contact with the drivers whose lanes I am crossing (implying nothing less than a full 90 degree intersection).
I have several roundabouts near me and most have prioritized pedestrian crosswalks on all exits (Zebrastreifen), some with extra room for cars to break when they notice pedestrians. The ones lacking pedestrian prioritization are outside the city or in industrial zones.
See my other comment. There are no Zebrastreifen on my roundabout. It's tight, heavily frequented by cars (and all sizes of trucks, due to being next to an industrial zone).
Zebrastreifen would be a nice thing. However, around here it's either canonical rules for all roundabouts or even yield signs for non-car traffic. (I suppose they're only really valid for bikes which share the sidewalk, though? No idea, I just yield anyhow.)
Zebrastreifen would certainly be safer than nothing. My experience, though, is, that a sufficient number of drivers is already confused or overwhelmed by basic roundabout rules and don't even see them. A traffic light simplifies everything at the cost of a bit less efficiency for cars.
We have something that has been dubbed a "divergeabout" which is a hybrid diverging diamond intersection with roundabouts chucked on both ends. I'm pretty sure these work based entirely on fear and confusion. Fortunately this particular intersection is not really in a pedestrian area. We have roundabouts in our neighborhood and I had the same concerns about crossing when my kids were younger and walking to school.
California's first Diverging Diamond opened a couple of years ago with a separated bicyclist/pedestrian path through the interchange[0]. It's a busy road that crosses over a busy freeway, so that path is only needed for the busy road (pedestrians and bicyclists are not supposed to use the freeway). In reality there are very few pedestrians in this area because things are _so_ spread out, but there are bicyclists.
I'm not sure what you mean by "traffic rules", I have a roundabout in front of my home in the middle of a major german city that's surrounded by crosswalks, with a daycare and two schools next to it.
It's pretty annoying: Cars have to yield to pedestrians while exiting the roundabout, they don't yield while entering. (Though luckily, many are nice enough to do anyhow.)
In order to have any chance of crossing the roundabout in rush-hour traffic, I have to accept cars yielding and cross onto the middle island between exit and entry. The alternative to not crossing for yielding cars would be to wait until the roundabout is empty, which essentially never happens.
However, the island is tiny. My son's bike just about fits, mine does not even fit remotely. Thus I now have to teach him to stop exactly on that island (Which, admittedly, he does very well. I don't have the confidence, though, that he'll be able to 100 % of the time. Driving past once while a car also goes by has a significant chance of causing an accident, as there is no space to evade for anybody.)
This whole ordeal we have to do twice every day. It's not totally unmanagable, but a crossing with traffic lights is so much safer than these maneuvers, that include communicating and trusting cars that they know the rules as well and understand what you're about to do.
In Denmark, pedestrians can walk in the same direction as the main flow of the road, and people who need to turn / cross the path of pedestrians yield. The priority is pedestrian > bike > car.
We have these in Atlanta and while I get the concept I think they are often improperly sized and become their own form of congestion. Some of them create weaving due to lames becoming turn out on ramp lanes with little warning. It’s not uncommon to have a gridlock due to four lanes wanting to pour into two lanes. In examples where there are +1/+2 lanes where two are used for the respective on ramp/off ramp traffic there seems to be less gridlock.
I didn’t grow up here so it’s possible these were built to the existing road width and the improvement was worth it even though the narrower versions are not ideally sized.
They are getting pretty popular in central Texas where I live. I think they work best in a car only place where there isn’t neighborhood style traffic or pedestrians. This almost often means a location where a major road intersects a freeway.
The one by the IKEA in Round Rock at 1431/I35 has been a great improvement, for example. The one on Parmer and I35 has also worked quite well. There is a similar approach, called “displaced left” down 1431 towards cedar park, however (Parmer and 1431) that has only been ok in terms of perceived improvement for traffic in the area.
I will also note that they only started appearing here around 2015 onwards so maybe some design issues that you are talking about have been corrected in these newer iterations
I have the impression the one at Pleasant Hill just isn't long enough between the lights and needs the sequencing adjusted to accommodate the incredible quantity of Atlanta-area drivers who were never taught not to block the box or that it's not a mere suggestion to stop at a red light.
It has succeeded in reducing my trips to Microcenter tho.
Yeah, no help for me there. I used to live walking distance to the Marietta Microcenter and cam take surface streets to it now.
Driving standards here are all over the place. That's a whole other issue. Atlanta has done perhaps too good of a job of attracting people who spent a long time driving in small towns where traffic is simpler.
One of the very first DDIs in the country is in Springfield, MO. They’re trippy at first, but not nearly as confusing as you might think. They’re also much more efficient than I would have intuited.
We've a bunch of these being built in the Detroit suburbs and while I love them, the lanes on them aren't as intuitive as on a straight road, and when the roads are snow covered they can be a bit weird. I hope the road authorities keep up on line divider maintenance as well, otherwise we'll have the same problem of people drifting between lanes that snow causes.
These also seem to be a bit weird for pedestrian crossings, but at least there are lights/crosswalks at all crossings as opposed to the old cloverleaf style which had nothing more than striped crossings a few feet into the highway onramp portion.
I saw a proposal for an oval system, ie more like an elongated stadium. No left turns. If you are coming onto this system and want to turn left, you actually turn right and go to the end where you turn around.
Or if you want to leave by turning left, you first go to the end, turn around and then turn right on the way back.
There are more lane changes in this system, so that's a minus. But traffic lights might not be mandatory. It would be possible to have a continuous central barrier etc.
Where I live, they're proposing a Single Point Urban
Interchange (SPUI) so that all interstate traffic goes through one set of traffic lights. To make that work, they have to build two flyover ramps to send the northbound traffic over the southbound traffic. https://www.penndot.pa.gov/RegionalOffices/district-11/Publi...
Now I have a roundabout right in front of my child's daycare. Due to the traffic rules surrounding that roundabout (Germany), it'll be impossible for him to cross that safely without me being very hands-on.
All these systems are completely made to get another 2 % of efficiency for cars, while massively sacrificing walkability and bikeability. I am not a fan anymore.