On-ramp stop signs: Friend or foe?

  • Traffic engineers contend that stop signs at the end of highway on-ramps are effective safety measures under specified conditions. This is the ramp from Tilghman Street west to Route 309 south in South Whitehall Township.
Traffic engineers contend that stop signs at the end of highway on-ramps… (DAN HARTZELL, THE MORNING…)
December 01, 2013|Dan Hartzell | The Road Warrior

Q: Why are stop signs posted at the end of some highway on-ramps? Examples include the ramps from Tilghman Street east to Route 22 east, and from Tilghman west to Route 309 south. These signs seem dangerous and unnecessary. The ramp from Tilghman to 22 is short to begin with, giving the driver little time to gain speed to safely merge, but now I need to bring my car to a complete stop at the merge point. This must be an impossible feat for tractor-trailer drivers. A yield sign would be appropriate. Why are stop signs used here and at some other places in the Valley?

— Andrew Graver, Hanover Township, Lehigh County

A: I asked PennDOT Traffic Engineer Dennis Toomey whether the choice between yield signs and stop signs at highway on-ramps is based on highway specifications or engineering judgment, Andrew. "It's a little bit of both," he said.

PennDOT's Highway Design Manual provides more standards and specifications for every aspect of roadway design than anyone without a pocket protector would want to peruse. They pay me to read stuff like that, and still I find it a hard slog.

Fortunately, Toomey was willing to tutor me on the basics: Features such as ramp length, curvature, and slope or terrain come to bear, he said, with the length of the ramp taking the inside track. On the judgment side of the road are factors including accident frequency and severity, and sight distance.

At the Route 309 Tilghman Street interchange, the relevant features vary from ramp to ramp, so that one ramp gets the standard yield sign while three qualify for stop signs.

There, Toomey said, "We have an older design, from the '50s, maybe early '60s, and some of [the ramps] have virtually no acceleration lane. If we see that we are substantially short of what ought to be there, combined with some history of incidents — crashes — then we would put the stop control there rather than the yield," he said.

That's for the benefit of motorists on the on-ramp, to let them know it's a short acceleration lane, and for drivers on the highway, "who don't want someone pulling out in front of them that's going much slower than they are going," Toomey said. Poor decisions on highway entry can be dangerous to both parties.

Incidentally, the motorist entering the highway always has the duty to yield to traffic already on the highway, even if no yield sign is posted. As I've often mentioned, that seems to be poorly understood by many motorists, who behave as though their turn signals confer the right to blast from an on-ramp onto the highway regardless of conditions there.

It's true that highway motorists who can shift to the left lane or slow down (or even speed up) to clear the entry point for on-ramp traffic should do so, but that is a courtesy, not a mandate of law. Motorists clipping along in the middle of two jam-packed lanes of tightly spaced, high-speed traffic cannot be expected to halt on the highway to allow ramp traffic to enter.

Anyway, I understand Toomey's concept, and I suppose it's possible that conditions at some ramp ends are such that it's necessary to stop every time. But I can't cite an example offhand, and on balance I think engineers turn to stop signs too often. They seem to post stop signs on the assumption that it's better to err on the side of safety, but in some cases the stops ramp up the danger factor.

The sign for Tilghman west to 309 south is an example. The ramp itself is long, but most of it consists of a wide, soft curve, and by the time you near the entry point to 309, it's a "hard" sight angle to oncoming traffic, which is coming up a rise, both factors making it difficult to gauge speed and volume, and even which lane the oncoming traffic is in. Tough conditions all around.

Still, I can get a good idea of what's coming with three or four car-lengths' worth of ramp remaining ahead of me — no less so than when I stop at the stop sign (which has a stop line) at the very end of the ramp. With my approach, I have only a short ramp to work with, but it still beats accelerating with no ramp at all, like launching from the deck of an aircraft carrier.

Tilghman east to 309 north seems the most challenging on-ramp of the four, but even there a slight running start is possible. I feel the same way about Tilghman to 22 east, one of the worst on-ramps in the Valley (with stiff competition from Fullerton Avenue to 22 east, though that dinosaur is scheduled to be lengthened in 2015-17 as part of the planned interchange reconstruction).

Toomey allowed that our view on this matter may be valid when highway traffic is light, but if high-speed volume tends to be heavy at various times on the average day, and chances of having a sufficient gap are low, then stop signs are posted. With signs in place, "You need to wait for a gap that's sufficient" to enter the flow safely, he said.

From my driver's seat, any length of acceleration ramp, however short, is safer than none at all. Sometimes you're forced to stop on a ramp — those yield rules again — but you should do so early enough on the ramp to allow at least some acceleration space. Warriors who think Andrew and I are drifting over the center line here, please let me know, and cite examples of stops that make entry safer.


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