By Michael Brown, PE, AICP
This article features the Quadrant Intersection design, and it’s potential to tame high-speed suburban arterials, so that you can drive slower, but travel faster. Opportunities exist at a huge number of strip mall intersections, but very few have been built to date, and none have been built with a Complete Street, Place-making vision. Huge untapped potential!
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A Quadrant Intersection reroutes left-turning traffic on “backage roads,” which eliminates the need for left-turn arrows at the main intersection, and that reduces delay. There may still be left-turn arrows at secondary intersections, but they can usually be coordinated well-enough with the main signal, and the overall effect is reduced delay.
The Quadrant and Continuous Flow Intersection designs have a lot in common. In the three diagrams below, the top diagram shows the basic pattern of a CFI, which is increasingly popular with traffic engineers. If you haven’t seen it yet, you will. (Click here for a map of some existing CFI locations). With a CFI, left-turning traffic waits at a mid-block location, and then crosses over oncoming traffic during the east-west phase. When north-south turns green (shown in yellow), the red lefts can proceed at the same time as yellow.
The CFI is good at congestion-relief, but it usually won’t fit in with would-be mixed-use environments. Bike, pedestrian, and transit access is very intimidating. Businesses suffer from restricted access. Even if fixed guideway transit can be accommodated, the CFI may impede transit-based economic development.
The middle diagram overlays a quadrant’s path (orange) on the CFI diagram to show how quadrants are similar to CFIs in function, but different in important ways that benefit place-making. Traffic waits in a mid-block left pocket just like at a CFI, but instead of crossing over the oncoming lane and in front of development (creating a huge mess of spaghetti and access challenges for businesses), it crosses behind development with a “backage road.” What would that do for walkability and development?
The bottom diagram and the rest of this article address walkability. First, the main intersection can be much tighter than other designs, with far fewer pedestrian conflicts. Previous left-turn pockets are no longer needed at the main intersection, so the space is available for other uses such as landscape, pedestrian refuge or fixed-guideway transit. Next, vehicles access parking from the back-side, which allows the elimination of driveways on the main streets, replaced by shared-wall buildings. The design also expands the local grid, creating additional circulatory options, and creating high visibility from additional parcels to catalyze a “town center” or business district, where before the highest land values were concentrated at just the four corners of a super-sized intersection.
Quadrants may require mid-block signals for handling left turns. These mid-block signals can be synchronized with the main intersection to avoid impeding traffic. Such mid-blocks are also good for pedestrians, who simply cannot cross legally or safely without additional signals.
How they work
A Quadrant is extremely versatile. It can be operated as a “mini-cloverleaf,” where 3-rights make a left. Or it can be operated similar to a CFI, where people use a mid-block intersection, but instead go behind existing development.
Preserving the Option for Quadrants
At any Greenfield intersection that is likely to someday have a lot of traffic, consider creating “backage roads.” In the short term, there is no need to route left-turn traffic on these – just use regular left-turn arrows if that’s what is politically possible. Even if you don’t reroute lefts on the quadrant roadway, it will still provide better connectivity and opportunities for a larger Activity Center rather than a focus on a single intersection. And in the long run they are “get out of jail free” cards to be invoked at any time to reroute lefts, reducing congestion and freeing up former turn pockets in support of a stronger Activity Center.
|Advantages of Quadrants||Disadvantages|
|Impressive vehicle-capacity gains||Initial confusion for drivers|
|Center-transit, pedestrian refuge, etc.||Potential out of direction travel|
|Safer for both autos and pedestrians||May add signals to corridor|
|Short signal cycles||May be hard to create back-side streets|
|Expands activity center grid connectivity||May affect some parcels negatively|
|Enhances / Catalyzes Economic Development||Resistance if back-way is single-family|
|Adds value to many more parcels|
|Low cost (if backage roads already exist)|
|Political salve to obtain transit lanes|
|Walk front-door/drive back-door to retail|
|Compatible with traditional signals|
|QuadrantIntersections.org has a collection of existing locations (not comprehensive), along with videos, articles, and other links.|
Please Comment on Quadrants! We need to hear more from everyone who cares about urban streets if we’re to discover how these ideas can really help.