23

Slip roads are used to allow vehicles to merge in a road whose speed is higher or, conversely, let them leave it safely. This term appears to be British English usage. Here is an example of usage: Joining the motorway (rule 259). Here's an example image of a slip road:

motorway with road curving to the right and another road branching and going straight forward

Credits to Wikimedia

But why slip? I find it very unnatural to call them that way. In Italy, for example, they're simply called acceleration/deceleration lanes because that's what they actually are.

One of the meanings of slip is:

to move or cause to move smoothly and easily

Which may fit in a figurative way; but it seems quite forced. Is there a better reason, perhaps historical, to call those roads "slippy"?

  • In Australia there are three interrelated concepts: slip lanes at intersections, on/off ramps (what the OP is asking about) and access roads running parallel to a main road (also usually called slip lanes). – CJ Dennis Jul 6 '16 at 3:21
  • 1
    probably not the etymology but to slip in/out of a road easily by continuing in more-or-less the same direction, doesn't sound very forced to me. – barlop Jul 6 '16 at 9:47
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    Slip road is NOT acceleration lane. Slip road is the branch that merges into highway, acceleration lane begins where slip road ends. – Agent_L Jul 6 '16 at 16:14
22

The term was borrowed from railways. In railways, from what I could gather, a slip road is a short rail track connecting a main railway line to a siding or another railway line, which parallels the function of slip roads in motorways. This is clearest in Sessional Papers, Volume 89, Great Britain, House of Commons, 1902 (my emphasis):

The train engine, with five wagons attached in rear, was standing on the down main line, which runs approximately north to south. Parallel to this line, and on the west side of it, is the “mileage” siding, which is connected to the down line by a slip road, which enters the siding at a point about 60 yards from the buffer stops at the north end.

At 4.50 p.m., while engine No. 957 was standing on the slip road leading from the loco-sidings waiting for the outlet signal to be pulled “off” for the main line […]

The earliest mention I found of slip road is in Accounts and Papers: Railways; Turnpikes; Highways; Harbours, and Piers. Session 8 February ― 10 August 1870:

Clifton junction is the point at which the main lines from Manchester to Bolton and Bury diverge. There is a station close to the junction in the fork between the two lines, and there is extensive sidings on the Bolton side of the station, connected with both the Bolton main lines. The junction signalman’s view of these sidings is impeded by an overbridge, 270 yards from his box, on the Bolton side of it. There are two parallel sidings, each about 300 yards long, at the east side of the line; these unite and join the up line at the overbridge and the down line by means of a through crossing (in which there is a slip road for connection with the up line) 400 yards north of the same bridge.

The earliest mentions I found of slip road in a motorway context are from 1958. There’s one here and another one in The Road Way, 1958, which I reproduce:

[…] access point, you will approach it from the slip road on the left. Watch for a safe gap between vehicles in the near side traffic lane on the motorway and increase your speed in the acceleration lane to the speed of traffic in the near side lane before joining it.

[…]

Leaving the Motorway
32. If you are not travelling to the end of the motorway, watch for signs warning you of your point of exit; the first sign will be one mile in advance of the slip road to your left, the second half a mile in advance and a third at the exit.

  • 5
    Good research and sounds like the correct origin of slip road as applied to roads... but we're still none the wiser as to why slip? – nekomatic Jul 6 '16 at 9:05
  • @nekomatic That's true. But could you say a train slips off the main line onto the slip road? Using slip as in M-W 1a or 4a? – Jacinto Jul 6 '16 at 9:49
  • A derivative of slipstream, perhaps? – Kenneth K. Jul 6 '16 at 14:25
  • @KennethK. First known use of slipstream is from 1913 according to M-W – Jacinto Jul 6 '16 at 14:36
  • Slip road in railway is similar function to changing lanes, not roads. – Agent_L Jul 6 '16 at 16:11
17

It might be derived from the word "slipway", which is a ramp or sloping bit of ground which you use to get a boat in and out of the water.

https://en.wikipedia.org/wiki/Slipway

The act of getting a boat into or out of the water via a sloping bit of road seems similar to the act of getting your car on or off the motorway via a (often sloping) bit of road, and so it seems plausible to me that "slip road" could have evolved from the concept of a road which is "like a slipway, but for cars".

From Wikipedia: Slipways in the harbour of South Shields, Tyne and Wear, England:

I can't find any evidence for this theory, however (or for any etymology of "slip road" as it happens).

  • ships and boats are older forms of transport, the UK/GB is an island hence they would be very familiar with slipways. Makes sense to me. – Mari-Lou A Jul 5 '16 at 14:41
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    I have heard this explanation before as well in a port in Scotland (Fort William). There was some hilarity as there was a slipway close to a slip-road and someone (non-native speaker) got really confused about them. – Tonny Jul 5 '16 at 20:09
  • Is it possible that 'slipway' itself comes from 'sloop way'? – S. G. Jul 5 '16 at 22:16
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    @S.G. the consensus seems to be that it is from the fact that you slip the boats into and out of the water, although sloop does come from the Dutch sloep (same pronunciation), which is etymologically related to "slip." Slip has come to mean a place where a boat can be moored, without being suited to launching boats, but I suspect this came from "slipway" as a place where a boat can be launched. – phoog Jul 5 '16 at 23:53
  • Eventually I accepted Jacinto's answer, but yours was equivalently good. They just put a bit more research and references in the answer which made it more complete overall. – edmz Jul 7 '16 at 12:26
8

Slip coaches on the railway are one or more coaches that are detached from the train while it is in motion. Having no locomotive they gradually slow to a halt, helped by a staff member operating the brake. An express train would 'slip' coaches at intermediate stations without having to stop there. With careful operation of points (switches) it is possible to divert the slip coach a short distance onto another line once the express has passed, to reach a platform/station not on the main line or couple to a waiting locomotive. They were last used in the UK in 1960.

A slip road is the slip coach idea applied to a motorway.

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