ajGoes

In locations where heavy traffic from the highway would often interfere with traffic entering the intersection, a light can be added for the highway traffic.

It reduces the number of phases in the signaling system from six to two, thereby reducing the average wait time. Long waits to cross a freeway during rush hour should be significantly reduced.

It greatly reduces the risk of a serious accident caused by a left turn against traffic (in drive-on-the-right countries).

Palal

Depends on the volume of traffic. Roundabouts are at their best when the volume of traffic is balanced, which is usually not the case at freeway intersections.

The 2-roundabout intersections also exist in the US. e.g. here though in that case, the roundabouts are poorly striped.

dknn

Small world, I drive that exact diamond interchange everyday and it has helped with the traffic tremendously. There was a learning curve, but it seems like everyone has gotten used to them.

teddybear99

I know the costs are higher but wouldn't a three level bridge resolve this in highly traveled area? Traffic flowing in one direction would be above traffic flowing in the other direction with the Interstate Highway at Ground Level. See Interstate 95 and the SR-84 exit in Fort Lauderdale, FL as an example. Eastbound traffic flows on the upper bridge with no turn signal onto I-95 North, however traffic exiting from I-95 South has a stop light as the left lanes are segregated from the flow through lanes on the right and share the lanes of the people entering the interstate going north. The same thing happens with the westbound traffic on a lower bridge. This allows through traffic to continue over the bridges without stopping.

https://www.google.co.uk/maps/place/...!3m1!1s0x0:0x0

pogopossum

Those of you in Minneapolis should recognize this setup at the 494-34th Ave interchange at the Humphrey, excuse me, Terminal 2 exit. (The light rail is a nice addition.) It's odd but easy to get used to, unlike a traffic circle, which to me, along with "cloverleafs" are among the worst highway designs ever.

PP

Cloudship

Maybe it would help if we knew what it was like before, and why a cloverleaf didn't work.

TheBOSman

Too much space most likely. The cloverleaf is out of favor most places; you'll be very happy once they replace most of that terrible cloverleaf interchange that MA 128/I-95 has with I-93 in Woburn with a few stack ramps and possibly below ground ramps as well.

WillCAD

Holy poly oly... that thing looks more like a crop circle than a road network.

pogopossum

The basic problem with the cloverleaf is that the outside lanes are asking drivers to accelerate and decelerate in the same lane. There have been some modifications where it kind of works, (again, MN: 35E and 77 in Apple Valley) but it is still a stupid design (MN: 494 and 35W)

PP

DenverBrian

And most cloverleafs aren't nearly large enough to handle large trucks turning, so the ramp speed has to be 35 mph or less.

The reason the diverging diamond should be safer is that the two crossing intersections with traffic lights are "no turns" intersections. No left turns; no right turns.

Notice the no-left-turn and no-right-turn signage at the intersections in the YouTube video. Although I think it might be a bit more elegant simply to post "NO TURNS" signs. (Probably a desire to go with the international pictographs instead of English signs.)

I suppose the ultimate diverging diamond would overpass/underpass the two intersections - but you'd need a lot more land to handle the grade separations.

WHBM

I thought that at university as well

Diverging Diamonds :

A recent US feature, although there are a couple in France as well.

https://www.google.com/maps/place/Al...e28851299149aa

I'll speak about left/right in US terms as we don't have one in the UK (and are unlikely to get one), so UK readers reverse the terms. The key advantage is that it reduces the number of Stages at the signals to two. Stages are each different step of the signals; a complete set of Stages back to starting again is a Cycle.

A simple crossroads intersection signal has 2 Stages. Those turning left have to wait for and judge a gap in the oncoming traffic. This is called Gap Acceptance, or an Unprotected Turn. It's the No 1 cause of serious accidents at signalled junctions due to misjudgements and also with heavy oncoming traffic there may be insufficient gaps anyway and the turning traffic backs up. So the next step is to put in an additional Stage for left turners, with a Green Arrow indicating it's a Protected Turn. Now you have 3 Stages, and (simplistically) you only get 33% green time instead of 50%. So that cuts down capacity itself, and also extends the time taken to get through the junction. If the cross street also needs a Protected Left Turn that's 4 Stages and you are down to 25% green time for anyone. Even if you then add additional lanes to maintain the capacity (expensive), progress along the road becomes slower.

A plain freeway interchange is a Diamond, from its shape viewed from above. Where the ramps (UK: Slip Roads) meets the surface street on each side there may be a signal, you only need 3 stages but still are losing capacity compared to there being just 2. So the Diverging Diamond, with just 2 stages, seems to offer a capacity advantage.

Downsides:

The through traffic on the cross street has to cross itself twice. If it is the dominant flow, which it often is, this is wasteful of capacity in itself.

The waiting area across the centre of the intersection, where the traffic is reversed, probably needs to be several lanes wide, including the left turn provision, which needs its own lanes from some way back to avoid being delayed by through cross street traffic waiting at the signal. This means the bridge over or under the freeway has to be significantly wide to accommodate all these lanes plus the crossing angle.

It's a nuisance to build because unlike many traffic enhancement projects you can't build it incrementally, you have to open it all in one go.

The angle at the intersecting points is very shallow, and indeed in the UK would contravene formal guidelines for minimum intersecting angle (90 degrees is best). There is a danger that less competent drivers would take the wrong side at either signal. This can be countered by swinging the carriageways to a greater angle, but that takes more land.

The waiting area in the centre is short and with heavy cross street flows would be liable to Lock Up (US: Gridlock).

The Weaving Area across the reverse direction bridge for traffic leaving the freeway and turning left, and traffic from the cross street turning left onto the freeway is too short; traffic leaving the freeway may find that their continuation is obstructed by through traffic blocking back from the second signal, in turn this will block those turning left onto the freeway. This is a feature of the worst design of Cloverleaf, with a too-short Weaving Area, but amplified because of the signal stopping the through traffic.

Provision for pedestrians is poor, it will take them for ever to get across. I'm aware that US solutions do not have the levels of pedestrians that we have in the UK, but you do need to provide for them. I cannot think why in the diagram above the pedestrians are put in the middle, it would be better down either side.

Most of all, it's not intuitive. Solutions need to b designed that every road user to understand and use. There is an unfortunate history of some ideas that a traffic engineer could negotiate but the general public have a lack of safety with it.

This is a further article about DDs:

http://www.divergingdiamond.com/history.html

And this explains a few traffic engineering terms:

http://www.traffic-signal-design.com...ology_main.htm

And here's a previous FT thread where we did Roundabouts For Americans

http://www.flyertalk.com/forum/u-k-i...l#post22603447

Cloudship

Now here is something i never got about traffic light programming. Sure it gives you only 33% versus 50%, but percentage of what exactly? A two stage light means that a good portion of traffic, particularly if much of the traffic is turning, ends up having to wait anyways. In a two stage system, just because you have a green light doesn't mean you can go. That extra time that the non-turning traffic has during a light cycle is time that is actually taken from turning traffic. If the time the turning traffic needs isn't as important, then the stages can be adjusted to give straight-flowing traffic more time.

Palal

33% of green time vs. 50% of green time.


Which is exactly why most protected turns (especially in US suburban areas) also have sensors on the, so that they actuate only for the time that's needed, up to a maximum.

In a diverging diamond, your capacity is limited by the capacity of the diamond. A microsim of a specific one would be cool to do.

If you put in all the sidewalks, I bet you the time it takes for peds to get across the entire structure (lengthwise) will be better than if you had the traditional interchange with all the protected turns. It would be interesting to simulate this in some microsim software to see. It probably wouldn't be much different than a SPUI

ajGoes

The developer of divergingdiamond.com sent me a link to an interesting US Federal Highway Administration document about DDIs that addresses many of the questions raised in this thread.