Speed questions rarely end when one number is recalled. Build a stack from the road, vehicle, current situation and driving conditions, then use the lowest applicable limit and slow further whenever that result is unsafe.
Build the limit from four layers
Start with the road. On an open road with no different sign, the default open-road limit is 100 km/h. For any route, begin with the limit displayed for that location: a default fills a gap but does not override a corridor's sign. Compare that road limit with any lower vehicle or temporary control. Next check the vehicle. A light vehicle towing a trailer is limited to 90 km/h on the open road, even when the ordinary road setting would allow more. Then check the situation. When a school bus is stopped to pick up or drop off children, approach at 20 km/h or less from either direction and remain at that speed until well past the bus.
| Layer | Question to resolve |
|---|---|
| Road | What limit does the posted sign set, or what default applies when no different sign is present? |
| Vehicle | Is there a lower ceiling, such as the 90 km/h trailer limit? |
| Situation | Is a stopped school bus or an active variable sign creating a lower limit? |
| Conditions | Would rain, visibility, traffic, the road surface or the vehicle make the maximum unsafe? |
School variable-speed signs are controlled by their state, not by one memorised national school figure. An illuminated or otherwise active school variable-speed sign is enforceable at the displayed value. When the sign is off, the ordinary posted limit resumes. Finally assess conditions. A speed-limit sign states a maximum, not a target. Weather, poor visibility, traffic, road condition or vehicle condition can make it necessary to slow below whatever maximum applies; keep reducing the speed until travel is safe. This final safety step can reduce an already lowered limit; it never raises one.
The traps
- Applying the ordinary open-road default to a light vehicle towing a trailer. The vehicle-specific open-road ceiling is 90 km/h.
- Borrowing 110 km/h from a higher signed corridor for a light vehicle towing a trailer. Its open-road vehicle ceiling remains 90 km/h.
- Memorising one permanent school-zone figure. An active school variable-speed sign is enforceable; when it is off, the ordinary posted limit resumes.
- Treating an active school variable-speed display as advice. When the sign is illuminated or otherwise active, its displayed limit is enforceable.
Try it
An ordinary light car that is not towing enters a motorway corridor. No lower temporary or vehicle-specific limit applies. What should determine the legal maximum?
- Assume 100 km/h on every motorway
- Use the open-road default even when a sign differs
- Choose the highest speed shown on the route map
- Follow the posted speed-limit sign
Heavy rain makes a road unsafe at its posted maximum. What does the national rule require?
- Maintain the posted maximum unless Police direct otherwise
- Slow below the posted maximum until the speed is safe
- Stop in the traffic lane until the rain ends
- Increase speed to spend less time in the rain