Diverging Diamond

The Missouri DOT will be building the first diverging diamond interchange in the United States in 2007.

Currently, a typical diamond interchange exists where Front Street in Kansas City, MO passes under Interstate 435.

The existing configuration can be seen to the right and below. From a pdf of traffic volumes, I found out that the Average Annual Daily Traffic (ADT) is 80,400 vehicles/day and the Daily Hourl Volume (DHV) is 9299 vehicles/hour on Interstate 435.

One of the major selling points of the diverging diamond interchange is the elimination of the left turn conflict at the the ramp terminals. Without the conflict delay is dropped and the level of service of the intersection(s) will improve.

The reason that MODOT chose this type of interchange over a single point is that the existing bridges do not need to be replaced or widened. A singled point interchange would have cost approximately $11 million, while the diverging diamond is estimated to cost $6 million! Nearly half the price!


The downside to a diverging diamond is that the traveling public may find it confusing. Between the ramp terminals, cars travel on the left side of the road. Good signing and striping should show the traveling public how to drive a diverging diamond interchange. Americans are becoming accustomed to roundabouts and single point interchanges and concerns about confused drivers were washed away after proper signing and striping explained the situation.

To the right is an example of a diverging diamond interchange. This particular design was rejected in Ohio.





And to the left is a sketch I made of a diverging diamond interchange at Front St and I-435 in KC. This is just a rough (very, very rough) sketch. I have no idea how the one designed by MODOT will look, but this should provide a general shape and configuration.

I have found a blog that opposed this interchange concept: http://blogs.kansascity.com/.

I read with horror the Missouri Department of Transportation’s newest plan to maim and kill our inattentive, inexperienced and out-of-town motorists, along with anyone else who is in their way

Like I said, proper signing and striping will make this interchange work. The intersections are not all that different from a single point interchange intersection. This will make the Front Street interchange safer, because it will eliminate the left turn conflicts, decrease delay, increase level of service... overall providing better flow and eliminating traffic congestion.

The only thing gained by this “diverging diamond” interchange is the reduction of a handful of stop lights. This cost savings will be more than offset by the first liability lawsuit for wrongful death or personal injury due to this faulty design.

In your article about the proposed interchange at Front Street and Interstate 435, Susan McCubbins of the Missouri Department of Transportation said, “As long as they are paying attention, they should be OK.”

Ms. McCubbins, all I can say is, you are making a really large assumption about the driving habits of Kansas City drivers.

Saving $5 million dollars is a huge gain. With properly designed curves, supers, and sight distance and adequate signs and striping, a wrongful death suit will fall on deaf ears. Kansas City drivers have adapted to roundabouts, why cant't they adapt to a diverging diamond interchange? People are initially afraid of new/different road scenarios, but get over their fear once they realize the increase in effciency.

A large assumption about the driving habits of KC drivers? What, do most KC drivers ignore signs, signals, and stripes along/on the road? I doubt it.

I applaud the MODOT and their consultant, the KC office of Wilson and Company, for thinking outside the box to solve a problem at half the cost of a conventional solution.