Number of limited access roads
Number of surface roads
Estimated number in UK
First built in UK
It's so simple it might not even be an interchange at all, but it is a type of junction that is extensively used on limited-access roads and expressways. It's a LILO — not an inflatable pool toy, but an acronym for Left In, Left Out. Its name describes the only turning manoeuvres that are possible at this type of junction. Traffic on the major road can go straight ahead, but if you are coming from or going to the minor road, you can only turn left into it and left out of it.
You could argue the case that this is really just a type of T-junction, and it has no place on a listing of interchanges like this. But a LILO is a specific type of T-junction, and it's used where very minor roads encounter major limited-access roads as an alternative to just closing them off altogether. A full T-junction or crossroads at a location like this would require a gap in the central reservation, enabling dangerous right-turns across oncoming traffic. If you close the gap, but there's no justification for the expense of a bridge, you've got a LILO.
Why build one?
The home of the LILO is on dual carriageway A-roads that have been upgraded on the line of an existing road. Where an ordinary road once had a junction with every minor side turning and track, its upgraded form might have a grade-separated interchange or a roundabout every few miles. The remaining minor roads, not served by a flyover or underpass, can often be stopped up, forming a dead end, but if that's not acceptable then the only remaining option is to give access to one side of the dual carriageway with a LILO.
Often a LILO might seem either useless or at least unhelpful: it might give access from a village to a major dual carriageway, but not allow the return journey to be made without travelling an entirely different route. But in some situations, LILOs are now being employed as part of a strategy to make routes safer without making access to minor side roads significantly worse.
On the A1 between Peterborough and Doncaster, for example, there were once countless gaps in the central reservation, where vehicles were invited to play chicken with oncoming traffic and dash across the busy dual carriageway to turn right. Over time, these dangerous junctions are being eliminated, with the gap closed and a pair of LILOs left behind. Rather than provide a flyover at every one of these junctions — which would be prohibitively expensive and wouldn't be justified by the amount of traffic using them — the policy is to provide full junctions with flyovers or underpasses at regular intervals, and LILOs everywhere in between, so traffic may have to turn left to join the A1 but can turn around and head the other way without too much of a detour.
The LILO may have a silly name but it's not such a silly idea; in fact, as part of a policy like that, it can be perfectly useful.
- Preserves some access to a major road where the alternative may be no access at all.
- Extremely low cost.
- Reasonably safe, as there are no conflicts or cross-traffic.
- Provides turns in one direction without corresponding turns in the opposite direction.
- Can be disorientating or lead traffic away from its intended route.
- Can result in a former through route being severed and traffic on the minor road being forced to make lengthy detours.
The LILO is so simple and unadorned that it's hard to think that there could be many variations on it. In some places, a side road may have access only on to or off the main road, making it effectively a single isolated sliproad. Presumably we would have to refer to such things as either a LI or a LO.