Why it matters: One-way streets, narrow single carriageways, separated carriageways and service roads

These rules protect against exactly the situations where a driver misjudges the road's direction of travel: reversing or making a U-turn on a one-way street puts you head-on against traffic that expects one direction; picking the wrong carriageway on a separated road with no sign does the same. And on narrow roads outside the built-up area, the verge is often lower than the road surface without warning — one brief moment of inattention, and the car is already tipping before you can correct.