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What is a slip road?

March 10, 2023 by

Wondering what a slip road is? Wonder no more

If you’re new to driving, or perhaps new to driving in the UK, there’s a chance you’ve never come across the phrase ‘slip road’ before; or perhaps you have heard it mentioned in passing previously, and are now curious what it means.

Well, if you’re after a quick answer, a slip road is a section of road that leads onto or off a larger road, allowing you to speed up as you come onto a motorway, for example, or slow down as you leave it.

But while it’s not much more complicated than this, there there’s a little more to slip roads than that brief explanation, and we’ll get into the nitty-gritty, including how to use them, below.

Slip road meaning

T-junctions and roundabouts are part of the everyday landscape for drivers, and these work fine in most instances when one or more roads meets another.

But many roads – mainly motorways and major dual carriageways – are all about smooth, fast progress, and interrupting the flow of traffic with junctions that require slowing significantly or stopping completely would either vastly slow overall traffic speeds down, or introduce serious danger.

Enter (and exit) the slip road.

Slip roads, also known as slip lanes or filter lanes, allow vehicles to join new roads without using a stopping-style junction. Slip roads are generally small sections of road that allow traffic to join major roads from lesser ones, giving arriving vehicles the chance to speed up in order to match the speed of those already on the road being joined. Slip roads can either merge with the larger road, with the slip road narrowing before it disappears altogether, or they can continue alongside the major road, becoming a new, additional lane.

Crown copyright. This motorway sign indicates a slip road becoming a new lane on the motorway it is joining,

Slip roads are also used when leaving a major road: rather than have to slow down to take a right-angled junction as you exit a motorway, for example, you can come off at, say, 65mph onto the slip road, which gives you time and space to slow down further before coming to a stop at the roundabout or junction that would typically rest at the end of a slip road, which would connect to the rest of the road network.

Slip roads are always one-way, running in the same direction as the road they join, so you will find one on each side of a motorway.

But while slip roads tend to be used for motorways and fast dual carriageways, but they can also feature on slower, more urban routes where roads might merge.

In some instances a slip road will be a small section of road that exists only to take you, say, from a roundabout to a motorway, while in others one relatively major road may actually turn into a slip road, before merging with a new motorway.

What should you do when joining a dual carriageway or motorway from a slip road?

There are two major kinds of slip road: ones that merge with the larger road, eventually disappearing as described above, and ones where the slip road actually becomes an additional lane of the larger road, which widens as the slip road joins it. With the first type of slip road, you would need to move across, changing lanes from the slip road onto the larger road, while the second type does not require this: you can remain on the slip road, which then becomes the inside lane of the larger road, with what was previously the inside lane of that road now becoming a middle lane.

Crown copyright. This sign indicates a two-lane slip road, with the left-hand lane becoming a new lane on the motorway, and the right-hand lane merging with it. The right-hand slip lane would require you to change lanes to join the motorway.

Using a slip road to join a major road

What type of slip road you are on will be indicated by signs, which will display an arrow pointing diagonally into a larger road if you need to merge with traffic, or a line coming in at an angle before running parallel with the larger road, which would indicate the slip road becomes a new lane rather than disappearing.

In either instance, when you are on a slip road you should speed up to match the speed (within the limit, of course) of traffic on the road you are joining. You should also use your mirrors and check your blind spots so you know what is going on on the road you are joining, and this is all the more important if you are joining a slip road that merges, as you will need to check that there is space for you to pull across, moving from the slip road onto the inside lane of the major road.

If you’re new to driving this may sound rather daunting, and while it may well be so the first few times you do it, coming off a slip road onto a major road is a skill like any other, meaning it gets easier the more you do it. It is also fair to say that in most instances traffic on the major road will recognise that there is a slip road feeding traffic onto the main road, with other vehicles either moving across one lane to the right so new vehicles can join the road, or perhaps slowing down to make room for traffic from the slip road as it joins the major road.

Observation is key here, as you will need to check your mirrors and blind spots for clear space to move across. Other road users may also flash their lights indicating that they will allow you to come off the slip road and join the major road in front of them, though we should stress that according to a strict interpretation of the Highway Code, flashing lights only mean “I am here”, so you must be confident in any interpretation you make of another driver’s flashing.

Using a slip road to leave a major road

We’ve focussed so far on using a slip road to join a major road, as using one to exit is a fair bit more straightforward: while you may want to slow down a little before the slip road starts, and some slip roads are shorter than others (particularly on dual carriageways as opposed to motorways), in many instances the slip road is designed as both an exit point for a major road, and as a deceleration lane, allowing you to slow on the slip road rather than holding up traffic as you would were you to start losing speed on a motorway. Don’t forget to indicate well in advance of leaving via a slip road; on motorways you should begin signalling at the 300-yard marker (three white diagonal stripes on a blue background).

What to do if you run out of slip road

The reality is that you shouldn’t, and in all likelihood you will not: as mentioned above, other drivers will often be more accommodating than you might fear when it comes to traffic joining from slip roads.

But, as this is a perfectly rational concern to have, know that instead of making a wild swerve into the major road and hoping everything turns out okay, you should STOP on the slip road before waiting for a safe gap that allows you to join.

Slip road FAQs

What colour are the reflective studs between a motorway and slip road?

Reflective studs (also known as cats’ eyes) that indicate a slip road are green, as opposed red studs that mark the left-hand edge of a motorway, white studes that separate lanes, and amber studs that mark the right-hand edge/central reservation.

Can you overtake or undertake on a slip road?

If there are two lanes, certainly – though bear in mind the slip road is designed for slowing down rather than speeding up.  The Highway Code says ““Do not overtake on the left”, and this applies to slip roads as much as any other road. While undertaking is not a specific offence, it could be considered careless driving, which is.

Can you stop on a slip road?

No you cannot – unless it is an emergency (in which case you should pull over as far to the left as possible), or you reach the junction at the end of the slip road, at which you clearly should stop.

What is the speed limit on a slip road?

The speed limits on motorway slip roads is 70mph unless otherwise indicated. For slip roads on other types of road there is no standard limit, and while often the national speed limit applies, in many instances signs may indicate the maximum speed.

Who has the right of way on a slip road?

When joining a major road from a slip road, traffic on the major road has priority, so you must identify/wait for a gap before moving across, rather than trying to force your way in.

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