It is not illegal for Sanral traffic control vehicles, which are assisting on the Gauteng highways, to have blue lights, the road agency said on Tuesday.

"These vehicles need to have blue lights," SA National Roads Agency Limited spokesman Vusi Mona said in a statement.

"If they don't, they operate illegally."

He said the agency had obtained legal advice through every step of the implementation of the e-tolling system in the province.

Those who wanted to challenge Sanral on the legality of the blue lights were welcome to, Mona said.

"It is not their opinion that matters but what the law says and we are confident we have done everything by the book.

"What is more important here is the effort to do proper enforcement on our roads, thereby addressing issues such as non-roadworthy vehicles, vehicles without number plates or false number plates, and other road traffic offences," Mona said.

He said people who were complaining about Sanral's vehicles were trying to undermine attempts by the agency to assist law enforcers on Gauteng's highways.