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