by Michael Brown, PE, AICP
* Traffic Engineer, New Urbanist
* Founder of Urban Innovators
The Quadrant has huge 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.
Read this to discover huge untapped potential!
Example from Research in North Carolina
The North Carolina Department of Transportation hired Urban Innovators and NC State University to demonstrate how Quadrant designs could be applied to walkable settings, and evaluate some of the pros and cons of doing so. The idea is to reroute either two or four left turns behind development. Double-left turn space at the main intersection can then be repurposed: create a narrow, planted median with pedestrian refuge, and on the sides create cycle tracks or premium pedestrian space. With lefts relocated, the system has less delay, which makes it easy to reduce the speed limit without making overall drive trips take longer.
As shown below, there are thousands of intersections in America where candidate Quadrant backways already exist or can easily be created with minimal impact to existing development. In this case, today's system supports 3,700 vehicles per hour, but the new system can support 5,700. This 1.5x (50%) vehicle capacity increase, combined with improvements to walking, biking, and transit, can support a 4x increase in overall development!
The ability to handle 4x development means this system can offset 350 acres of suburban development, and eliminate the need for about 10-miles of roadway and utilities. Thus, while it will be expensive to create the quadrant pathways and install the walkable features, it will still be far less than a "no frills" Stroad + ten miles of additional roadway!
Can't afford to build it? You can't afford not to!
Below is a birdseye view of the main intersection showing planted medians, pedestrian refuge, and space for alternative modes. Even though the speed limit is reduced to 35 mph, vehicles still get through the overall area in just 80 seconds as opposed to 100 seconds before (stopped time + in-motion time). Drive Slower, Travel Faster!
See below, when left turn lanes 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. There are fewer pedestrian conflicts, making it safer. Vehicles can access parking from the back-way, 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 back-way visibility that can activate more parcels, catalyzing 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 reduce delay by converting the main intersection from a 4-phase signal into just 3-phases or even 2-phases. They can also be operated as a “mini-cloverleaf,” where 3-rights make a left (not shown). Or they can be operated similar to a CFI as shown on the red paths above, where people use a mid-block intersection, but instead go behind existing development.
Quadrants vs Continuous Flow Intersections (CFIs)
Perhaps by now you’ve seen a Continuous Flow Intersection, which state DOTs are rapidly installing. Quadrants and CFI’s have a lot in common – except that Quadrants can be far better for walkable development. In the three diagrams below, the top diagram shows the basic pattern of a CFI. 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.
CFI: Not Great for Placemaking
The CFI is good at congestion-relief, but bike, pedestrian, and transit access is very intimidating. Businesses suffer from restricted access.
Quadrant: Similar to CFI, but Great for Placemaking
Traffic waits in a mid-block left pocket just like at a CFI, but instead of crossing over the oncoming lane then in front of development (creating a huge mess of spaghetti and access challenges for businesses), it crosses behind development with a “backage road.” Let’s explore what that can do for walkability and 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
Impressive vehicle-capacity gains
Center-transit, pedestrian refuge, etc.
Safer for both autos and pedestrians
Short signal cycles
Expands activity center grid connectivity
Catalyzes Economic Development
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
Initial confusion for drivers
Potential out of direction travel
May add signals to corridor
May be hard to create back-side streets
May affect some parcels negatively
Resistance if back-way is single-family
QuadrantIntersections.org has a collection of existing locations (not comprehensive), along with videos, articles, and other links.