Your message here! Some believe dynamic message signs distract drivers. Others feel they provide useful information. (Mark Gail/The Washington Post)

As a daily highway driver, I can’t help but notice that overhead signs seem to be the cause of slowing traffic. I think drivers slow down to read the signs, especially when there is an accident warning or Amber Alert — even the routine signs asking people to notify authorities if they see something suspicious. This slowing traffic causes backups and delays. Once traffic is past the sign, speed resumes. Are Washington-area drivers slow readers or just infatuated with the signage?

Bernie Brill, Potomac, Md.

The Washington area is home to some of the most well-read people in the country. When Amazon released a list last spring of the 20 biggest book-buying cities in the United States, Washington placed third, after Seattle and Portland, Ore. Answer Man thinks reading a simple overhead message such as “Click It or Ticket” should be fairly easy for us.

Of course, what we’re really wondering here is whether the mere act of looking at and digesting a message that appears above a highway is enough to cause an infinitesimal slowdown in each driver such that the entire corpus of traffic in that area slows noticeably, or enters what researchers call a “breakdown” state.

“There’s been a lot of work to determine what causes breakdown,” said Randy Machemehl, a professor of civil engineering at the University of Texas at Austin’s Center for Transportation Research. “It turns out that almost any kind of interruption, such as a driver hitting his brake lights, can cause what’s called a shock wave that moves back upstream. . . . When [traffic] volume is approaching capacity, then the whole system becomes extremely unstable. It seems to me very possible that drivers pausing or taking their foot off the accelerator just a bit because they’re reading a changeable message sign could in fact be a contributing factor to stop-and-go traffic conditions.”

How could we tell for sure? Some studies have had drivers fill out questionnaires — not while they’re driving, obviously — asking how they would respond to seeing what’s known as a dynamic message sign, or DMS. Would they heed the message? Would they slow down?

Another way is to stick someone in a driving simulator and introduce DMS to their ersatz experience. The problem is that most simulators have only one driver at a time. You wouldn’t be able to see how multiple drivers react.

Finally, you could study the effect in the field. That’s exactly what a team led by University of Maryland engineering professor Ali Haghani did in 2009 and again in 2011. Researchers placed sensors on either side of a dynamic message sign to capture the Bluetooth signals from passing vehicles. The time it took for a vehicle to pass from one sensor to another revealed speed. They compared results from when the sign was off to when it was on.

“There was no significant difference in speed that we could tell,” Haghani said.

In 83 percent of cases, speeds were unaffected or increased. Speeds decreased in 17 percent of cases. The average decrease in speed was a shade over 3 mph.

(Interestingly, a similar Norwegian study also found that drivers slowed by roughly 3 mph while reading a DMS, but termed that a “marked decrease in speed.”)

But haven’t we all seen it? Traffic slows on Interstate 95. You start inching along. Eventually you come to a message sign that reads, “If you see something, say something.” The instant you pass it, traffic opens up again.

Science does not trifle with anecdote. Haghani said he thinks one of the reasons the Maryland State Highway Administration commissioned his study of DMS and potential traffic slowdowns was “because traffic reporters were sort of talking about it without actually having solid evidence that this was causing [delays].”

In their paper, “Evaluation of Dynamic Message Signs and Their Potential Impact on Traffic Flow,” Haghani and his co-authors emphasized that not all messages are equal. Messages that warn of specific upcoming delays, crashes or lane closures — and even recommend alternate routes — are preferable to generic ones that exhort us to buckle up.

The Federal Highway Administration is pro-dynamic-message sign. It sponsored a survey of drivers, the majority of whom said safety and public service announcement messages were useful. Drivers also mentioned that the messages would affect their driving behavior.

But Answer Man finds it ironic that one of the recommended messages was “Don’t Be a Distracted Driver.”

Well, don’t distract him, then. He’s got someplace to be.

Helping Hand

It’s that time of year, time for The Washington Post’s Helping Hand fundraising drive. We’ve partnered with three great local charities that aid homeless families and youths: Homestretch, Sasha Bruce Youthwork and Community of Hope. For information on how to give, visit

Twitter: @johnkelly

For previous columns, visit