LILO

A LILO is a three-way junction where traffic entering the main road and traffic leaving it both use lanes on the same side of the carriageway. The term stands for left in, left out, describing the directional movements: vehicles turn left to join the main flow, and vehicles turn left to depart it. This arrangement contrasts with a standard T-junction where entry and exit may occur on opposite sides, or with a roundabout where movements distribute across multiple arms.

LILOs are common in urban and suburban settings where space or property constraints make a conventional intersection impractical. The configuration uses a central reservation or traffic island to separate the entry and exit lanes, creating a controlled merge zone. Vehicles entering from the side road must yield to through traffic; vehicles exiting from the main road have a dedicated left-turn lane that bypasses the through traffic stream. This geometry reduces conflict points compared to uncontrolled junctions but requires careful lane marking and signage.

Variants and traffic control

LILO junctions can operate under traffic signals or as priority-controlled intersections. Signal-controlled LILOs allow coordinated management of entry and exit movements, useful where traffic volumes are high or where the main road carries significant through traffic. Priority-controlled variants rely on give-way markings and road signs, with entering vehicles required to find gaps in the main flow. Some LILOs incorporate filter lanes that allow vehicles to turn left without waiting at a signal, improving flow for exiting traffic.

The main operational risk with LILOs is confusion among drivers unfamiliar with the layout. Unlike a roundabout, the direction of circulation is not immediately obvious from the road geometry alone. Poor visibility of the central reservation or unclear lane markings can lead to drivers entering the exit lane or attempting to cross the junction at angles. Sight lines at entry and exit points must comply with local standards to ensure drivers can detect approaching traffic and pedestrians.

LILO is primarily British terminology; comparable arrangements in other English-speaking regions may be called slip roads, parclos, or directional T-junctions. The acronym has become a working term among transport planners and highway engineers, though the underlying concept predates the modern acronym. LILOs occupy a middle ground in junction design, offering better control than simple T-junctions while occupying less land and requiring less complex signalling than full grade-separated interchanges or large roundabouts.