The 1.2 metre sideways clearance for roads with speed limits of 50 miles per hour and over has proved to be a practical and effective minimum requirement for many years, at least since 1965. The Department has no record of how the dimension was originally decided. Although the Traffic Signs Manual and the Safety Code of Practice have been reviewed, this distance has not been challenged by traffic management experts.
For roads with a lower speed limit, the size of the safety zone is reduced. The safety zone is only required when operatives are on site and it can be reduced when a site is unoccupied.