Most motorists speed occasionally, straying a few miles per hour over the posted limit. Then there are the more serious violators, who substantially exceed speed limits and maintain speeds that are way out of sync with nearby motorists.

Institute researchers measured speeds at 13 locations on northern Virginia roads, identifying vehicles going at least 15 mph faster than posted limits and at least 5 mph faster than 3 of 4 surrounding vehicles. These parameters helped to ensure that the identified motorists weren’t inadvertent or occasional speeders.

Researchers found 5 percent of drivers exceeding the limits by 15 mph or more. Three percent also were going faster than nearby cars. These proportions aren’t large, but they translate into millions of problem speeders on U.S. roads every day.

Analysis of photos of motorists in the group of problem speeders, snapped as they went by photoradar units, plus information obtained from authorities revealed characteristics of these drivers. They were more likely to be men younger than 30 and to drive newer vehicles and SUVs, compared with motorists who stayed within 5 mph of posted speed limits. As a group, the problem speeders averaged 60 percent more crashes per year and had twice as many violations on their records.

About half had gotten at least one previous speeding ticket, and 20 percent had gotten 2 or more. They also were more likely to have other moving violations on their records.

“This is why it’s important to slow down the fastest drivers. They’re high-risk in a number of ways,” says Allan Williams, lead author of the study.

For a copy of “Characteristics of excessive speeders” by A.F. Williams et al., write: Publications, Insurance Institute for Highway Safety, 1005 North Glebe Road, Arlington VA 22201, or email publications@iihs.org.