Diverging Diamond Interchanges (DDI) and European Roundabouts

(Satellite)

When they added a lane to IL-59 in west-suburbia Chicago, they rebuilt the interchange for I-88. I did not understand why this is better.

Satellite

But a comment on this post explained why a DDI is better.

Highway Engineering Discoveries posted

Multiple Diverging Diamond Interchanges (DDI) are coming to Interstate 4 (I-4) as part of I-4 Beyond the Ultimate. These innovative interchanges allow about twice the traffic flow in the same amount of time. DDIs also reduce the number of traffic signal phases and allow motorists to enter the freeway without crossing oncoming traffic.

Will McBurney: The point is these intersections work specifically when two conditions are met:

- More traffic is coming from the highways than from the top and bottom.

- Traffic coming from the tup and bottom tends the flow one direction at a time (ie people driving towards town during the day and driving home at night)

In those two conditions, you get dramatically improved traffic flow

Those conditions are true for the I-88 vs. IL-59 intersection. The main reason for IL-59 in this area is to get people to and from I-88. In fact, a fourth lane of I-88 terminates here, and a second exit-only lane was added years before this interchange was rebuilt. And the two exit lanes are long because traffic trying to get off I-88 during the evening rush hour would back up so far.

Satellite

Several comments said a roundabout was better. I could not image how they could be better for multiple lanes of traffic until a comment showed how the Europeans do it. That is a much more elaborate roundabout than I'm used to. As pointed out by some other comments, through traffic is not hindered by this type of interchange. I'm also learning that what we call ramps are called slip roads in other countries. Another comment observed that an advantage of DDIs is that they use about the same footprint of land that a standard intersection used.

Andrew Virnuls commented on the above post
It's the standard way of doing it here.

8:30 video