New Jersey towns and cities that paint a blue line in their roadways to honor law enforcement are violating federal rules, according to the Federal Highway Administration.

In a letter sent to Somerset County in December, a top official said that the blue lines ran counter to the Manual on Uniform Traffic Control Devices for Streets and Highways.

The letter said that the two solid yellow lines in a road are supposed to be separated by a blank space. The color blue on pavement, it added, is reserved for parking spaces for people with disabilities.

For several months, towns across the Garden State have been filling in their yellow lines with a long streak of blue, which they said was a way to champion law enforcement officers amid a national conversation on policing.

"The reason for the blue line is a message to the police department that they're supported by the government," said Alan Schwager, the mayor of Woolwich Township.

Schwager said the township's blue line is in its municipal complex — not on a public road — so it did not violate federal rules.

"The police see it on their way into work and on their way out on patrols to protect the township," he said.

Bridget Palmer, the spokeswoman for Cherry Hill, said the township painted a blue line on the road outside town hall after Labor Day last year.

"We wanted to just show the men and women of the department that we stand behind them," said Palmer. "It wasn't meant to be a political message, but really just a genuine sign of support."

Palmer said Cherry Hill would "wait and see" what other towns do in response to the federal warning.

Neil Gaffney, a Federal Highway Administration spokesman, said the guidance was not meant as a slight against law enforcement but rather a clarification of existing federal rules.

"We appreciate and understand the efforts by local governments and others that convey support for law enforcement officers," said Gaffney. "However, the yellow lines down the center of a road are meant to control traffic, and modification of that marking could cause confusion, accidents and fatalities. Our No. 1 priority is the safety of all drivers."