Innovative Intersections and Interchanges
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What Is A Contraflow Left?
- A grade-separated interchange where left-turn traffic on the arterial crosses opposing left-turn traffic via channelized lanes
- Left turns onto the freeway ramps in both directions are made during the same signal phase
- The roundabouts can have two to four legs
- Can be designed as an overpass or an underpass
When Should It Be Considered?
- With limited right of way to add opposing left-turn lanes
- With heavy left-turn traffic volumes onto the freeway ramps
- Where there is limited roadway width for left-turn lanes between the ramp intersections and limited right of way to expand or to construct loops
- Improved safety: Vehicles waiting to turn left onto freeway are less likely to create a backup, reducing the potential for rear-end crashes
- Increased efficiency: Operates with three traffic signal phases rather than four phases, reducing overall interchange delay
- Better traffic flow: Provides additional space for left-turn traffic waiting to enter freeway, reducing congestion
- Cost effective: Less space is required between ramps, so the interchange can have a shorter bridge span, potentially reducing costs
How to Navigate
Below shows how to navigate a contraflow left interchange. Click the image to view a larger version.
The number of conflict points (locations where vehicle travel paths intersect) is one metric that can be used to evaluate the safety of an innovative intersection or interchange.
There are three categories: crossing, merging or diverging.
In general, merging and diverging conflict points — where vehicles are moving in the same direction — are associated with less severe crash types than crossing conflict points where vehicles are moving in opposite directions.
The diagrams below compare possible vehicle travel movements and associated conflict points at a conventional diamond interchange to a contraflow left.
These diagrams represent a general case, with one travel lane in each direction, and do not take into account pedestrian or bicycle movements at an intersection or interchange.
When compared to a conventional diamond interchange, a contraflow left has two more crossing conflict points.
Conventional Diamond Interchange: Conflict Points
Contraflow Left: Conflict Points