What is a central reservation? Rule 173 explained
Updated •5 min read
The central reservation is the strip in the middle of a dual carriageway or motorway that is there to separate the carriageways. When you cross or turn right, Highway Code rule 173 says first assess whether it is deep enough to protect the full length of your vehicle.
If it is, cross in two stages. If it is not, wait until you can cross both carriageways in one go. Every rule on this page is quoted from the Highway Code on gov.uk, read on 27 October 2026.
What the central reservation and the carriageway are
The Highway Code says a dual carriageway is a road which has a central reservation to separate the carriageways (gov.uk: rules 103 to 158). Each carriageway is the half of the road you drive on in one direction, and the central reservation sits between the two.
| Part of the road | What it is | Highway Code |
|---|---|---|
| Carriageway | The half of a dual carriageway or motorway that carries your direction of traffic | Rule 173 calls each half "a separate road" when you cross |
| Central reservation | The strip that separates the two carriageways | Definition before rule 137 |
| Amber studs | Mark the central reservation of a dual carriageway or motorway | Rule 132 |
On a dual carriageway at night, the amber studs show where the central reservation is. Our road markings guide covers the other stud colours.
Crossing a dual carriageway: rule 173
Rule 173 is short. Here it is word for word (gov.uk: rules 159 to 203):
Dual carriageways. When crossing or turning right, first assess whether the central reservation is deep enough to protect the full length of your vehicle.
- If it is, then you should treat each half of the carriageway as a separate road. Wait in the central reservation until there is a safe gap in the traffic on the second half of the road.
- If the central reservation is too shallow for the length of your vehicle, wait until you can cross both carriageways in one go.
The picture with the rule is captioned "Assess your vehicle's length and do not obstruct traffic". That is the reason behind the rule. If your vehicle is longer than the gap, the front or back sticks out into a lane of traffic while you wait.
Crossing in two stages, step by step
- At the junction, look at the gap in the central reservation and judge whether your whole vehicle fits in it.
- If it fits, wait for a safe gap in the traffic on the first half, then cross and stop inside the central reservation.
- Treat the second half as a new road. Wait there until there is a safe gap in the traffic on it, then go.
- If it does not fit, stay where you are until both carriageways are clear at once.
The theory test question
One phrase learners search for word for word is an answer option: "check that the central reservation is wide enough for your vehicle". A typical question asks what you should do before you turn right or cross a dual carriageway.
The right answer: check that the central reservation is wide enough (deep enough) for your vehicle.
Why: rule 173 makes that the first thing you do. The answer decides whether you may stop halfway, or must wait to cross both carriageways in one go. Options such as "wait in the central reservation" are only right when it is deep enough, so they are the trap.
The central reservation on a motorway
A motorway has a central reservation too, but you never cross it. Rule 263 says that, unless directed to by a police or traffic officer, you MUST NOT cross the central reservation. Rule 271 says you MUST NOT stop on the central reservation, or on any carriageway, slip road or verge, except in an emergency or when told to (gov.uk: rules 253 to 273).
Lane use, speed limits and the "dual carriageway ends" sign are in our dual carriageways guide. The rest of the motorway rules are in our motorway rules guide.
Test yourself
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Frequently asked questions
What is the central reservation?
The strip in the middle of a dual carriageway or motorway that separates the two carriageways. The Highway Code defines a dual carriageway as a road with a central reservation to separate the carriageways.
What is the difference between the central reservation and the carriageway?
The carriageway is the half of the road that carries your direction of traffic. The central reservation is the strip between the two carriageways. Amber studs mark the central reservation (rule 132).
How do you know if the central reservation is wide enough for your vehicle?
Judge whether it is deep enough to protect the full length of your vehicle. If any part would stick out into a lane, it is too shallow (rule 173).
What do you do if the central reservation is too narrow?
Wait at the junction until you can cross both carriageways in one go. Do not stop halfway with your vehicle sticking out (rule 173).
Can you cross the central reservation on a motorway?
No. Rule 263 says you MUST NOT cross the central reservation unless a police or traffic officer directs you to.