The National Roads Agency says an increasing number of motorists are driving illegally on blocked-off sections of the highway.

A construction worker broke his leg on Friday after he was hit by a car in a closed lane of the N1 south between the New and Allandale Road off ramps.

The driver was arrested and his car impounded ? he will face charges of reckless and negligent driving when he appears in court next week.

Sanral's Alex van Niekerk said the incident pointed to a worrying trend endangering workers and the offending motorists.

"We do get assistance from the provincial and metropolitan traffic police and it would benefit us even more if people obeyed the rules of the road."