Slow vehicles on short on-ramps: Who should make way?


Question: I often see cars enter highways at speeds well below the speed limit. The cars merge without getting up to speed. Other drivers routinely move left to make way for them. I think it’s dangerous for merging drivers to expect those on the highway to yield or slow down. Most often, the cars do reach the speed limit, just later than they should. Maybe those who drive slowly should put on their flasher lights. Wouldn’t it make sense for there to be a minimum speed posted on I-5?

Answer: Back when America built the interstate system, our roads rumbled with V-8 powered cars ready to launch onto any on-ramp and reach freeway speeds before merging onto the highway. At least that’s how I imagine it; the Washington stretch of I-5 was completed in 1969, before I was born. But my imagination doesn’t reflect reality. Yes, the 1970 Cadillac El Dorado had a 500 cubic inch engine (if that doesn’t mean anything to you, just know that it’s huge). But as a kid, I envied my neighbor’s 1970 Dodge Dart Swinger, a car with about as much power as a Honda Fit. The reality is, as long as we’ve had freeways, there have been cars (and drivers) that don’t reach freeway speeds before merging.

If the slow merge issue isn’t caused by a net reduction in vehicle horsepower, why is it becoming more of a problem? I have a good guess: Traffic volume. As you pointed out, other drivers often move left to make way for slow mergers. That’s not a big deal when there are just a few cars on the freeway. But the population of Whatcom County has nearly quadrupled since I-5 was completed and we still have the same two lanes. As more vehicles cram into the same space, lane changes become more dangerous. I couldn’t find data specific to freeway lane changes, but a study by the National Highway Traffic Safety Administration found that 21 percent of crashes occurred on the same traffic way, traveling in the same direction. Sounds kind of like driving on the freeway.

Slow vehicles merging onto the freeway can create a hazard, but a posted minimum speed is not the solution. For one, as you pointed out, most of those slow-merging vehicles actually do get close to the speed limit; it just takes them a while. Minimum freeway speeds have been tried in some states; I was recently driving through Missouri where there is a 40 mph minimum speed on interstates. And from my brief experience, those signs were pointless. There was no traffic even close to being that slow.

Brief history lesson: Most states that established minimum speeds on their interstates did so when our national speed limit was 55 mph. A 15 mph speed differential between the fastest and slowest legal speeds on the freeway seems reasonable. But when freeway maximum speeds increased, the minimum stayed the same.

Even if we set a minimum speed of 55 mph in our 70 mph zones, it probably wouldn’t do much. It might also be problematic, since the speed for trucks in the 70 mph zone is 60 mph, creating a 5 mph window of legal speed. I borrowed a radar gun and got onto a stretch of I-5 to see if I could find any drivers traveling less than 55 mph in a 70 mph zone. In 100 vehicles, the slowest car I measured was 56 mph.

Is there a better solution than minimum speeds? Over the past few years several of the shorter freeway on-ramps in the area have been lengthened to give drivers more time to get up to speed and find a gap to merge. They work great, but I can’t imagine we’ll see extended on-ramps showing up at a rapid pace; freeway building is expensive. And no matter the length of the on-ramp, drivers should use every inch of it if that’s what it takes to merge safely.

Your recommendation of hazard signals is appropriate for a slow-moving vehicle on the freeway. Washington law actually states hazard lights are for “warning other operators of a vehicular traffic hazard requiring the exercise of unusual care in approaching, overtaking, or passing.” However, while merging, vehicles are required to signal their intention, so I’d save the hazard lights until on the freeway.

I’d like to close by reframing a statement in the original question; “I think it’s dangerous for merging drivers to expect those on the highway to yield or slow down.” While true, there’s another perspective. It’s dangerous for drivers on the freeway to assume they’ll not have to make speed and lane adjustments to safely accommodate merging drivers. Like many other traffic issues, it takes all of us to get where we’re going safely.

Doug Dahl is a manager with the Washington Traffic Safety Commission, Region 11 and publishes


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