In October, scores of New Jersey towns painted blue lines in the middle of bustling downtown roads to show support for law enforcement.

But this practice is unsafe and towns should find another way to honor their men and women in blue, according to the U.S. Department of Transportation Federal Highway Administration.

"There are many appropriate and fitting ways to recognize service to the public that do not involve the modification of a traffic control device, which can put the road user at risk due to misinterpretation of its meaning," reads a Dec. 8 letter from the FHA to the Somerset County Engineering Division.

"The use of blue lines as part of centerline markings does not comply with the provisions of the MUTCD (Manual on Uniform Traffic Control Devices for Streets and Highways)," the letter states.

The FHA says the space in between the double yellow lines should remain empty. In certain cases, black paint can be used "where a light-colored pavement does not provide sufficient contrast with the markings," the MUTCD states.

Blue paint should only be used on roads to mark handicapped parking spaces, according to the MUTCD.

The letter from the Federal Highway Administration is a response to an October 2016 letter written by Somerset County's chief engineer, Matthew D. Loper. Loper had requested clarification on the federal guidelines when it comes to painting the blue lines on roadways, the letter states.

Loper could not be reached Sunday for an interview.

The letter does not condemn support for law enforcement. It states: "We appreciate the impact of expressing support for law enforcement officers and value their contributions to society."

When towns first started painting the blue lines on county roads, local officials said they did so only after receiving permission from the county.

In Mantua in Gloucester County, police Chief Rodney Sawyer said in October that the town painted the blue line in a park rather than on the road to abide by the FHA's MUTCD.

It's unclear if a local municipality can be penalized for not removing the blue line. An FHA spokesperson could not be reached by phone on Sunday morning.

A spokesman for the state Department of Transportation, Stephen Schapiro, said in a statement to NJ 101.5, who first reported the FHA letter, that the NJDOT "follows the rules set forth by the federal Manual on Uniform Traffic Control Devices" when it comes to road markings.

In Union County, Department of Engineering Director Joseph Graziano sent an email to police chiefs requesting they have the blue lines removed, NJ 101.5 reported.

Howell Township police Chief Andrew Kudrick said he had not been informed by Monmouth County to remove the township's blue line.

"Although absurd, we would paint over the approximately 200-foot line if required," Kudrick said. "I'll just paint the entire parking lot blue at the police department."

Alex Napoliello may be reached at anapoliello@njadvancemedia.com. Follow him on Twitter @alexnapoNJ. Find NJ.com on Facebook.