Number of limited access roads
Number of surface roads
Estimated number in UK
First built in UK
So many designs for motorway interchanges are complex, with layers of sliproads or complex patterns of merging and diverging traffic. What if you don't need all those elaborate things? What if you just need to merge two routes together? Well, if so, you need the simplest motorway interchange of them all: the fork.
The fork is so straightforward that not only does it not take much explanation, it also barely needs designing. It has a single bridge, and two routes merge into one in a perfectly obvious fashion. You can make it big and spacious, with wide, graceful curves that can be taken at full speed, or you can make it small and compact, with sharper corners and a steeper ramp. You can build it cheaply with a straight bridge and a full turn in the space beyond, or a skew bridge so you can start your curve before you've crossed it.
The first fork interchange on the motorway network appeared in 1959, at M1 J7 Beechtrees Interchange (then the terminus of the M10). But the UK's first fork junction is much older than that: the railways have plenty of examples of this sort of layout (often referred to in railway parlance as a flying junction) that had been around for more than half a century before the M1 started getting big ideas.
Why build one?
If your requirement is for one route to branch into two, with no requirement for turning movements between two arms of your junction, then the fork isn't just the best choice, it's effectively the only choice. What simpler or more complex way could there be to do the job?
- Adaptable layout can be made to fit surroundings and available space.
- Low construction cost as there is only one bridge.
- Simple to navigate.
- Difficult to correct navigational errors: turning movements are restricted and there is no way to u-turn.
There are three ways to build a fork. The first is the most traditional way, illustrated at the top of the page: one route takes priority through the junction, and the other is secondary, forming an exit sliproad and an entry sliproad. This is the form used where there is a clear main route through the interchange and is the most common.
The second is used where neither branch of the fork is more important than the other. Instead of crossing both carriageways, the bridge only crosses one of them. A1(M) J65 Birtley Interchange is laid out in this way.
The final option is that the sliproad that crosses the bridge goes around a 270-degree loop instead of turning 90 degrees to join the main route. That can be useful where space is limited and the sliproad can't turn in the direction you would expect, but it may also be done — as it was at M9 J1A for a long time — for no discernible reason when the sliproad could have gone the normal way if its designers had wanted it to.