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In road design, a slip lane is a road at a Road junction that allows road users to change roads without actually entering an intersection. Slip lanes are "helpful... for intersections designed for large buses or trucks to physically make a turn in the space allotted, or where the right turn is sharper than a 90 degree turn." Slip lanes may reduce congestion and "t-bone" motor vehicle collisions, but they increase the risk for pedestrians, cyclists and horse riders who cross the slip lane.
Some intersections that are controlled by traffic light offer a slip lane, which allows users to bypass the lights when they turn. That helps ease congestion and improves journey times, as people who are turning do not have to stop at the light but can continue at the same speed. There are two types of slip lanes at intersections: slip lanes that end and require traffic to merge to join the main road, and slip roads that continue onto the main road as another traffic lane.
The Federal Highway Administration (FHWA) in United States has indicated that "to accommodate safe pedestrian crossings," intersections should be designed "using tight curb radii, shorter crossing distances, and other tools.... While right-turn slip lanes are generally a negative facility from the pedestrian perspective due to the emphasis on easy and fast vehicle travel, they can be designed to be less problematic."
In right-turn countries, designers of right-turn slip lanes may try to increase pedestrian safety by including a range of features. These may include "a raised "island" for pedestrians with a curb and ramps for wheelchair users. To encourage drivers to slow down for crossing pedestrians, edge lines or painted cross-hatching may be used to "narrow the driver's perceived width of the lane." As well, pedestrian safety in the crosswalk may be improved with "high-visibility crosswalk striping, flashing beacons, and/or signage" and raised crosswalks. Flashing "warning beacons may be desirable in locations where there are high traffic volumes and vehicle speeds." Some states in the United States require a stop sign at slip lanes to control entry to the main roadway.
The organisation Strong Towns argues that slip lanes exist only to prioritise the speed of motor traffic, and it calls for the removal of slip lanes on local streets.
When poorly designed, slip lanes can be a dangerous design element. For reasons of urban design and pedestrian safety, many road-controlling authorities are actively removing them in urban and suburban settings. Slip lanes may need to be removed if considerations such as pedestrian safety grow to a point that they override the desire to facilitate free passage for drivers. "Slip lanes reduce drivers’ awareness of crossing pedestrians because they are led to focus on the traffic stream into which they are merging, and also impair visibility of the traffic stream because of the angle of approach." As well, "slip lanes also greatly increase crossing complexity for pedestrians and cyclists, by increasing total crossing distance, requiring judgement about crossing fast-moving traffic without the benefit of a traffic signal." To minimise risks of collision, slip lanes can be shaped to enter the traffic flow at an angle higher than the 45 degrees that is shown in the sketch. Such lanes are called high-entry angle slip lanes.
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