News

RTA members and partners,

This weekend, the reconstruction of the Airport Boulevard interchange with I-40 will achieve an important milestone, when NCDOT reroutes Airport Boulevard traffic to a “diverging diamond” interchange pattern, or “DDI”.

Diamond, diverging, and crossover elements

A normal “diamond” interchange looks almost exactly like a diamond shape when the off-ramps and on-ramps are viewed as a whole from the sky or a map. A “diverging” diamond looks very similar when viewed as a whole, although the intersecting roadway itself looks a little different. My understanding is that the “diverging” part of the interchange name refers to the various diverging and merging movements impacting the through and turning movements across the configuration of the non-freeway roadway.

The key feature of the design is that the two directions of the non-freeway movement transpose or intersect with themselves on both sides of the freeway. To that point, I believe the federal government (USDOT) refers to this as a “double crossover diamond” (DCD) since the non-freeway through movement crosses over itself twice. FWIW:  I sometimes think of this interchange design as a “diamond crossover” interchange.

Layout and benefit of the design for travelers

At the Airport Boulevard interchange — similarly to the existing interchanges at I-440 and Western Boulevard, and I-40 and NC 42 — the two directions of travel along Airport will cross over themselves at-grade on either side of the freeway.

The crossover design seems unusual at first glance:  one might reasonably ask what the point is of making the non-freeway through movement cross over itself and back again. The answer is that doing so creates a series of “two phase” signals (i.e., you go, then I go, such that movements take turns). The two directional crossover two-phase intersections and the resulting two direct access left turn on-ramps create an overall configuration that seeks to minimize overall delay, maximize capacity, slow traffic for safety, eliminate left turn oncoming movements, etc.

If you are on Airport Boulevard and want to turn right onto the I-40 freeway you will do that as normal, by simply turning right before reaching a directional crossover intersection. If you want to turn left onto the I-40 freeway you will go through one directional crossover signal, find yourself on the “wrong” side of the road, but then realize you can immediately turn left, without any opposing traffic to contend with. Here is the NCDOT webpage on this type of interchange design.

A “hidden” benefit for travelers heading to I-540

The crossover interchange design also facilitates U-turn movements from the freeway to the opposite direction of freeway travel. This creates an important, “hidden” benefit for us with the new interchange configuration at Airport:  eastbound I-40 travelers that are looking to get onto I-540 eastbound, who might have trouble accessing the I-540 flyover ramp immediately after the three RTP entrance ramps (Davis, Miami, Page), particularly during high volume conditions, now have another way to go. They can pass the three RTP exits as well as the I-540 interchange (exit 283), then take the Airport Boulevard ramp (exit 284), and simply stay left the entire time, including heading towards RDU and then immediately using the I-40 westbound on-ramp. They will then enter the I-40 westbound roadway on the exit lane to  I-540 eastbound.

Of course, this is obviously a longer distance (around 2 miles further) then simply taking the I-540 flyover exit directly from I-40 eastbound. Still, it may end up being a much more comfortable way to travel during a good portion of the day, particularly given the number and close spacing of the preceding on-ramps on I-40 eastbound, as well as the volumes in the RTP/I-540 flyover area, which are the highest on any roadway in the state.

Final thoughts

A “diverging diamond” or diamond crossover interchange design is very promising, and has broad applicability for locations where left turning movements onto the freeway are high.

Of course, it is not “perfect” and it will not necessarily be the best in all locations, as each situation must be looked at individually as well as a system. The crossover interchange designs do create one slight challenge:  they are less helpful for travelers that accidentally exit the freeway, since they cannot easily return to the freeway in the same direction of freeway travel. So, if you were to take the off-ramp and wish to return to the freeway, you would need to turn right (or left) onto the intersecting street, travel just beyond the immediate interchange area through a directional crossover signal, and then execute a U-turn at a safe location before returning to the Interstate.

My sense is that the crossover design is very promising for Airport Boulevard, particularly given the overall traffic volumes and patterns in the RDU Airport area, as well as area growth, along with the bonus “hidden” benefit for I-540.

Kudos to NCDOT for keeping us moving, through and across the metropolitan Triangle.

Let’s get moving,

Joe Milazzo II, PE
RTA executive director

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