Guides/Theory test
What Is an Auxiliary Lane on a Motorway?
Updated 22 September 2026 · 3 min read
An auxiliary lane is an extra lane on the left of the main motorway lanes that links the on-ramp of one junction with the off-ramp of the next. It lets traffic joining at one junction and leaving at the next stay out of the main lanes altogether. It is marked off from lane 1 by a broken white line on its right, with dashes that are shorter, closer together and wider than the ordinary lane lines.
Where you will meet one
Auxiliary lanes appear where two motorway junctions sit close together, which in Ireland mostly means the M50 around Dublin and the widened stretch of the M7 near Naas. Between the junctions the motorway looks one lane wider than usual. The lane ends at the next off-ramp: it either becomes the exit slip or is closed off with hatching.
When to move into it
The rule the theory test wants is simple. If you are in lane 1 and you intend to take the next exit, move into the auxiliary lane as soon as it opens, then follow it off. If you are not leaving at the next junction, stay in the main lanes and do not use the auxiliary lane to pass slower traffic.
- Check your mirrors and signal left before you cross the broken white line, the same as any lane change.
- Traffic joining from the on-ramp is already in the auxiliary lane; let it settle before you move in.
- Do not drift into it by accident when lane 1 widens. Read the overhead signs: the lane is signed for the next exit only.
- Never treat it as a hard shoulder. Stopping in an auxiliary lane is stopping on a live motorway lane.
Auxiliary lane, hard shoulder and slip road: the difference
| Lane | What it is for | Can you drive in it? |
|---|---|---|
| Auxiliary lane | Linking one junction to the next, on the left of lane 1 | Yes, if you are leaving at the next exit |
| Hard shoulder | Emergencies and broken-down vehicles | No, except in an emergency |
| Slip road (on-ramp or off-ramp) | Joining or leaving the motorway | Yes, to join or leave |
How the theory test asks about it
One question, worded the same in the car and truck banks: "When should a driver move into an auxiliary lane on a motorway?" The correct answer is "to leave the motorway". The wrong options are about overtaking slower traffic or letting others overtake, which is what the lane discipline questions cover.
See the question itself
The official question, with all four options and the explanation, for the car test and the truck test.
Auxiliary lane question (car)Motorway questions are only one topic of 40
If this term caught you out, sit the full car mock exam: 40 questions, 45 minutes, pass mark 35, same format as the RSA test. Truck and bus drivers get the 100-question version.
Sit a free mock theory testFrequently asked questions
+Is an auxiliary lane the same as a hard shoulder?
No. The hard shoulder is for emergencies only and you must not drive on it on a motorway. An auxiliary lane is a normal running lane for traffic leaving at the next exit, marked off by a broken white line.
+Can I overtake using an auxiliary lane?
No. It is for traffic leaving at the next junction. Passing slower traffic on the left in the auxiliary lane is undertaking and is a fault on the driving test.
+What does the broken white line beside the auxiliary lane mean?
A broken line means you may cross it when safe. It separates the auxiliary lane from lane 1 so drivers can move across to leave at the next exit.