Diverging Diamond Interchange

What would happen if the lights at either end were replaced with roundabouts, with a Y shaped entry exit pattern (single 2 way entrance/exit, one 1 way entrance and one 1 way exit.)?

Living and driving in a roundabout-heavy town (we have about 10 with more on the way) would be much easier if clearly marked signage was followed more consistently, but I suppose that's true for all driving. IMO roundabouts are difficult because they demand a perfect balance between assertive and passive driving--you have to judge traffic in the circle very carefully to get through, but if you're too passive you'll sit there forever waiting to get in.

I wouldn't be thrilled about the DDI, although with a 97% satisfaction rate I guess I'd have to try it firsthand.

I don't think I've ever gone through a roundabout in heavy traffic without having the urge to yell at somebody....

Done right...

Funny wording:

is around in Europe since years....He developed the design ....

It either exists or it doesn't.

And then there is this

How much easier is that without traffic light!

Very safe, if you have the physical space to install one. Not always an option on "retrofits".

We have all three here in St. Louis the Round About and Diverging Diamond are pretty new the clover leaf IdeaSmith has in his comment is still the most used.

As far as the safety factor goes the Diverging D and Round About are new enough that even the most non-paying attention to their driving driver has to pay attention enough at these intersections to reduce accidents. Once these drivers get used to them they will do what they do now and ignore the rules.

The diverging D Still makes you cross traffic but now you do it twice. Though because you are more of an angle less likely to be a head on collision and therefor maybe less dangerous.

The only problem I've ever noticed with the old fashioned Cloverleaf is the accelerating/ merging lane distance is sometimes to short for some drivers to get up to traffic flow speed to merge without causing issues with the already fast moving traffic.

I hate Round Abouts. Like J. Fuller says some drivers are more passive than others and hesitate to pull into the traffic so they don't really work any better than a four way stop with lights or with just stop signs. But again less severe damage when an accident occurs.

Has anyone had to deal with an accident at a Diverging D or Round About yet?
How much more screwed up does the traffic flow get when there is even a minor fender bender at one. Around here if there's so much as a car parked on the side of the road the traffic gets messed up.

Here we go again! Roundabouts were discussed in length in CR4 (ad nauseam I might add) a few years ago.

DDIs may work, IF drivers RESPECT both traffic signals and not blow through them. Therefore, I see that they offer no distinct advantage over traditional signaled intersections

Most American drivers are not use to them and may become confused, generally because they are not accustomed to them.

The only advantage I can see with employing a DDI is that they use less Righ-Of-Way real estate than a traditional Roundabout and cloverleaf design.

But hey, whatta I know, I'm not a highway engineer any hoots......

I am curious about the lane width in relation to the s-curves. I see in the picture that 2 semi trucks are using this interchange. While the Trucker with the doubles combination could maneuver through the s-curves easily, I wonder if the Trucker with the 53ft. Trailer would have a more difficult time.

Years ago, when I worked for an outfit as a driver trainer, my students would encounter s-curves such as this. Drivers are trained to normally drive in the right hand lane on a street that has two lanes going west to east for example/ ( w-e, n-s, s-n ), the driver would have to know well in advance that an interchange such as this was coming up. The driver would need to move into the left lane before the interchange in order to " see " what was happening on the trailers "blind" side. The truck driver would need to position the left tire as close as possible to the inside of the left radius of the s-curve, the watch the right hand side mirror to see where the right side of the trailer was tracking, so as to not "run over " a vehicle in the right lane. The problem is when exiting the s-curve, once the articulation between the truck and trailer exceeds 15 degrees, the driver is blind to the off tracking of the right hand side of the trailer. This maneuver is difficult enough on a clear sunny day, but it takes nerves of steel when the weather is inclement.

Most trailers are 102" wide, on a 10 ft. wide lane, this gives slightly less than 1 ft. clearance on each side, when driving on a straight roadway, my guess is that the s-curve would need to be longer, or the lane would need to be at least 14 ft. wide to take into account for a 45 ft. trailer bridge length ( the " bridge" ) is the length from the center of the trailers kingpin to the center of the trailers rear axle, in order to provide a margin of safety to not only the truck driver, but to person in the fiat 500.

Here we call them traffic circles and I love them. We have two at the exit I take to get off the freeway. Traffic flows very well most of the time and very rarely do I need to come to a complete stop.

Scary things I've seen on the traffic circle:

1. Someone was driving the wrong way (clockwise). He/she did this in front of a CHP officer and was pulled over.

2. I saw a pretty bad accident. He/she didn't turn and went straight into the center. The car was pretty messed up and it really screwed up traffic.

3. Young kids come flying into the circle without slowing down or looking - they think the right lane is like a right turn lane without a stop sign.

Some frustrating things I've seen:

1. People in the circle stopping to let traffic in the circle.

2. People who are in the outside lane, yet they continue past a two lane turn off.

3. Our turn circles are two lanes. People tend to drift in and out of my lane.

4. Semi's don't stand a chance in the smaller turn circle. The radius is too tight and they take up both lanes.

5. Bus drivers are horrible at turn circles. Don't get stuck behind one!

The first thing that occurs to me, with regard to "scary things" numbers 2 and 3 is that the approaches have to provide good visibility and with some large signs straight ahead that make it clear that the straight-ahead path is blocked. The signs have to permit good visibility of oncoming traffic around the rotary. I have seen many rotaries that are too heavily landscaped for this. It's not a bad thing to have some trees in a rotary, as long as their foliage is high enough not to interfere, and some low bushes or ground cover.

Regarding the "frustrating things," I think everybody should get the word out that failing to use the right of way when you have it can be almost as bad as failing to yield it when you should yield. Here in my politically-correct city, there are lots of women who engage in a kind of self-congratulatory, ostentatious "courtesy" that involves stopping for jaywalkers and holding up traffic. Many of the jaywalkers here engage in a conscientious form of jaywalking that avoids holding up traffic, but these women (mostly women), in their grandiose gestures of magnanimity, embarrass the conscientious jaywalkers and annoy the drivers behind them. (The grandiosity is evident in their facial expressions.) Timidity is another problem at rotaries.

Rotaries should have a large enough radius to permit trucks and buses to turn comfortably, unless they are on "No-trucks-or-buses" parkways.

I recently came across an article that mentioned a CFI (Continuous Flow Intersection) being constructed in my home state of Florida in the city Ft. Meyers. I may take short trip down to have a look at it. I just checked google earth view and it doesn't show the new design (or at least not that I could tell).

Here is a link to what I saw on the subject.