Q: Can you explain the rules about driving the posted speed limit versus keeping up with the flow of traffic? I have a teenage driver, and we don’t agree on this issue.
A: It’s amazing how wrong a study can become when it’s reduced to one sentence in a Facebook post. Actually, I can’t blame social media entirely for this misinformation. The “keep up with the flow of traffic” error has been around long before the internet. Now it just gets passed around faster.
The premise is based on something called the Solomon Curve. Back in the 1950s, researcher David Solomon collected crash data from about 10,000 collisions and graphed them based on vehicle speed at the time of the crash and the average speed of traffic on the road where the crash occurred. The results were a U-shape curve, with the bottom of the curve (where the smallest frequency of crashes were) roughly aligning with the average speed of traffic, and the U extending up on either side as speeds increased or decreased.
People who haven’t read the study share things like, “driving 10 mph slower than traffic makes you six times more likely to crash (not true)” and “slower drivers pose a greater risk than speeders (also not true).” Other people believe it because they haven’t read the study either. Then people apply that error to their freeway driving and think they’re safer by going faster. They’re not. Also, speeding laws don’t have an exception for “keeping up with traffic.”
Of the multiple reasons to reject the Solomon Curve when it comes to freeway driving, the big one is this: of the 35 road segments in the study, only one was what we’d consider a freeway. Solomon collected data from 1955 to 1958. President Eisenhower signed the Federal-Aid Highway Act (the basis for the interstate system) in 1956. Freeways were barely a thing when this study was done.
The rest of the roads were mainly rural highways with intersections, driveways, businesses, and all the other things that cause drivers to slow down in the roadway. No doubt that if you’re on a 50-mph highway and you slow down to 25 mph to pull into a driveway you’re at a greater risk of getting rear-ended. But those crashes aren’t because of drivers cruising along the highway at slower speeds; they’re the result of intersection conflicts. Nearly a quarter of traffic fatalities in Washington occur in and around intersections.
Another key point in Solomon’s study that gets overlooked is crash severity. Even though there were more crashes involving slower vehicles, the injury and fatality crashes were on the fast side of the curve. Drivers crashing at the fastest speeds had a fatality rate 22 times that of the slowest drivers.
Also, Solomon’s research is almost 70 years old. A lot has changed since his study, and data collection has become more of a science. More recent studies have concluded that “no relationship between slower speeds and increased crash involvement was found,” and “no evidence was found of a U-shaped risk curve whereby slower vehicles were also at greater risk.”
We like to believe information that supports our behaviors, so people who want to speed misinterpret the research. But crash data and physics both confirm that as speeds increase so does both crash frequency and severity. Solomon’s study showed that speed differential is a risk factor for crashes, but speeding up to eliminate it is not a real solution. Drivers who respect the speed limit are not the problem; it’s the other drivers speeding past the people who follow the law.