GROTON — On roads that have no posted speed limit, Groton drivers are allowed to travel at speeds up to 30 mph. But the Board of Selectmen this week decided that’s simply too fast for one local byway that cuts across the Lawrence Academy campus.

The board voted unanimously to post a speed limit of 20 mph for Powder House Road on Dec. 7, after being advised by Police Chief Donald Palma that 30 mph isn’t safe for a road that frequently has about 300 students crossing for classes.

“I think 30 mph is a dangerous speed for there,” he said.

The issue was originally raised by Lawrence Academy, which recently contacted Palma with a request that the speed limit be dropped to 15 mph on Power House Road.

By law, a Police Department has to do a seven-day traffic study before any changes are made, with the goal of determining how fast — on average — motorists are traveling on the road.

Palma explained that the key number for setting a new speed limit is the so-called “85th percentile,” which is simply the top speed of 85 percent of motorists on the road. In the case of Powder House Road, that speed was 18 mph, which Palma said created a “jump ball” situation where the town would choose 15 or 20 mph.

However, Palma also noted that Mass Highway standards recommend a speed limit of 20 mph for school zones and he advised being consistent with that.

The sole dissenting voice came from Selectman Anna Eliot, who noted that fully 50 percent of motorists in the traffic study were traveling at 14 mph or less, and said 15 mph could be appropriate for a high foot traffic area that’s used almost exclusively by the school.

Conversely, Selectman Josh Degen said it was important to have consistent speed limits in local school zones, adding that fully 91 percent of motorists in the study were traveling 20 mph or slower.

At the other end of the spectrum, Selectman Stuart Schulman said he was surprised to see that one motorist was clocked at going over 80 mph on Powder House Road, saying it was almost impossible to image that. Palma’s response indicated that was likely somebody showing off with the traffic study equipment, which has a screen that can list the speed of passing cars, much like the miniature traffic control trailers that are often set up roadside across the state.

While Palma said that one high-end result likely didn’t skew the study, he said it did prompt them to turn off the display screen.