Transportation · Uncategorized

Onramp Stoplights: Coming Soon?

Imagine entering the I77 onramp from the DDI and encountering…a stop light.  Sounds far fetched?

Last week a key organization called the Technical Coordinating Committee (TCC) reviewed a proposal for “metering ramps”–basically stoplights at the end of a freeway onramp. The TCC reports to the organization that priorotizes transportation projects, the Charlotte Regional Transportation Planning Organization (CRTPO).

If the preceding sounds a bit wonkish, remember this: the TCC is where the quad left turn at NC73 & US21, the DDI and – yes- I77 toll lanes were first given life. The TCC is composed of transportation experts who develop transportation projects & cost estimates. CRTPO votes on these projects, but given the TCC is the Subject Matter Expert, TCC recommendations hold tremendous sway.

So why on earth would anyone recommend putting a stoplight at the end of an onramp?


Believe it or, metering ramps work… in some cases. (More about that qualifier in a moment.) The theory for this is a well-established phenomenon in traffic congestion called the “backward bending curve.”  It works like this:

At high speeds (65mph+), traffic lanes move comparatively few vehicles per hour because the following distance between cars is so great. As traffic thickens, more vehicles per hour pass through the lane because, even though vehicles are travelling slower, they are closer together.

You’ve probably sensed this when you notice traffic is “heavy but moving.”

This throughput increases until about 45mph. At that point, slower vehicle speed takes over and throughput decreases (the curve bends “backward”). There’s no happy ending once traffic has rolled off the back of the curve… slower traffic means less throughput, which means even slower traffic, which means it takes forever for traffic speeds to recover.

You can actually feel this happening…once you slow below ~45mph, traffic changes from “heavy but moving” to “stop and go.”

Often a glob of cars enter the interstate all at once, slowing traffic below that magic 45mph threshold. Once that happens congestion starts building. The objective of metering ramps is to introduce vehicles in a controlled stream so traffic keeps moving above the “backward” threshold, thereby reducing congestion.  If they work properly, metering ramps should reduce or (hopefully) eliminate the amount of time the road is congested.

Below is an before-and-after measurement of a highway in Washington.  As you can see, after installation the amount of time traffic was spent in gridlock (above Cr, the dotted line) has been reduced.
Bottom line: metering ramps work.  But

… once traffic is congested (i.e. below 45mph), metering ramps are useless. All they do is hold up vehicles on the onramps. (You probably know where I’m going with this.)

Coming to an Onramp Near You?

So here is a picture of “feasible” onramps the TCC is poised to recommend. Notice that every LKN intersection- 23, 25, 28, 30 made the cut.
For whatever reason, all of the interchanges in Iredell were considered infeasible (a different chart I’m not showing here), and several interchanges throughout Mecklenburg are “for further study.”

Here’s what I don’t like about metering ramps:

If you’re on an interstate that is congested more often than not (like I77), these things won’t do much to help. They will, however, back traffic up on the onramps. Imagine what that could do to the DDI. Or Sam Furr.

But let’s say I’m wrong, and they do work- they relieve congestion to some extent. In that case, they probably violate the non-compete in the toll lane agreement, and Cintra would be seeking compensation.

So metering ramps through LKN are a lose-lose proposition.

While I’m sure the TCC does great work in most instances, this is one where they should meter somewhere else.

TCC Aug 3rd Agenda
FHWA “Metering Ramps: A Proven, Cost-Effective Operational Strategy”
World Road Association “”Congestion Charging”


One thought on “Onramp Stoplights: Coming Soon?

  1. Is this coming from the same people saying that I-77 will provide better travels on it, so if cars are bumper to bumper how in the world would this work . Ahhh the dizziness of NCDOT’s world . Not for nothing NYC tried to do this and it did not work !!!


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