Summary of "Placemaking Alternative Intersection" research for the North Carolina Department of Transportation.
What is a One-Way Split Intersection?
Rather than a single huge stroad intersection, it is better for both livability and traffic management to split a Stroad into two one-ways, or two Stroads into four one-ways. Below is a four-path situation near San Diego. Notice how small and easily crossable these tiny intersections are for pedestrians. This idea is sometimes called a “Square-about” because it works a little like a roundabout, but in a larger “square” pattern. One-way streets usually come in pairs, and thus are often called “One-way Couplets.”
Note: The reason this is highly efficient (meaning lots of green time for the tortoise) is because left turns on one-way streets do not need a left turn arrow, since there is no oncoming traffic. Our research shows that even though this environment has small, pedestrian-friendly intersections, the overall system can handle up to 80% more traffic than had it been a single huge 4-phase intersection.
The fact that it can handle more traffic means it is easier to support more development. For example, say you have today’s auto-oriented environment where 90% of traffic comes by car. If you triple the density and improve transit, walking, and biking, maybe you’ll get to 75% traffic by car. That’s pretty good! But 75% x triple the density = 2.25, or more than twice as many cars as before. That is a PROBLEM if the road can’t handle more cars, because it will cause development to sprawl outward if it can't easily grow inward. Luckily, this design CAN handle more.
Note: If it is impossible to create one-way couplets in both directions (i.e., both NS and EW with four intersections), you still get much of the benefit by creating a couplet in just one direction, resulting in two intersections. In other words, two Complete Street one-ways are better for traffic and livability than a single huge two-way Stroad, and four Complete Street one-ways are better than two.
Before: Two Stroads and parking lot path that is "mostly" unobstructed
After: Capable of catalyzing walkable development.
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Left: Newly walkable, but prior to any new development
Right: Market excited by improved aesthetics, mobility, safety for all modes. Builds a significant Activity Center!
Caution: Both Greenville and Smithfield agreed to be "Guinea pigs" for our ideas, with a caveat that none of the ideas have yet been vetted within a formal planning process, and may never be. These are NOT recommendations for what each city should do, (though they eventually could be after a formal public involvement process). They are basically "free ideas," compliments of NCDOT and our research team, that each city is welcome to investigate further or reject. For research purposes, these are simply real-world models to learn from. This way NCDOT and other communities can determine if they'd like to pursue designs like this.
What are our ideas for Greenville?
The graphics below show how we applied some of these one-way ideas near Greenville Mall on Greenville Blvd, Arlington Blvd, and Red Banks Road. At the right is the study area. The red and orange lines show how to split today's two two-way Stroads into four one-way Streets. The lower left has opportunities for U-Turn designs, discussed here. The top left, just outside the view, is a great Quadrant opportunity, discussed here.
Note: The one-ways require two new streets, labeled "New SW" and "New NW". Both traverse mostly parking lots, but likely would hit the "blue buildings." The biggest hit is the southwest portion of the mall. This would only be implemented in conjunction with the mall owners - when of their own accord they decide they need to reinvent their property and want to explore this as an option.
3D Before / After Renderings, One-Way Area
The series of sliders below show Before / After for the crossing one-way couplet concept in Greenville.
Left side is a four-quadrant concept at Arlington and Evans. Right is crossing one-ways. White buildings are existing. Tan are new mixed-use buildings attracted by reconfigured traffic & placemaking investment. Able to hold 4-5x more development before congestion returns.
New streets would affect some small buildings and a large mall (only to be enacted after mall owners desire the reinvention). But benefits are astonishing! Double vehicle capacity, which enables 5x development potential before congestion returns. Expensive? Yes. But a LOT less so than the 34 miles of general infrastructure necessary to serve 1200 acres of sprawl that this offsets.
Today's enormous intersection, and how that space might be reallocated in a one-way scenario.
Same, but compares two-way bike lanes (on one-way streets) with one-way bike lanes.
One-Way Cross-Sections Near Mall
Below is how Greenville Blvd might change from a two-way Stroad into a one-way Complete Street. Notice that private parking between existing buildings and the right-of-way is a factor that inhibits walkability. The new design inverts private parking into public, allowing sidewalks to be pushed to the far outer edge. The overall amount of parking is similar, but it is also much more efficient (i.e., likely to be used), when it is in the public realm.
Concepts in Smithfield: Crossing One-Ways
Consider the slider graphic below. Light blue segments are two-way Stroads today, and the red and orange represent opportunities where one-way couplets could be created with only minor impacts to existing development. In today's Stroad environment, Market Street currently has 4-lanes East-West through its historic downtown. It connects to I-95 on the east and is a rare crossing of Neuse River on the west. This means that as the city grows, there will be more traffic. It will be very hard to justify a “Road Diet” from 4 to 3-lanes. Instead, NCDOT will face pressure to remove on-street parking and have a 5 or 6-lane cross-section within the 78-ft ROW. Brightleaf is already a 5-lane Stroad. With traffic sure to grow, it will be very hard to manage the present system. Even if NCDOT widens these - makes them even bigger Stroads - they will not work very well for traffic, and any placemaking actions will amount to "lipstick on a pig".
These one-way couplets would create capacity for the future and reduce today’s congestion considerably. But "more traffic capacity" need not equate to less walkable. In fact we think the one-ways create a bridge to walkability that is impossible to get otherwise. The graphics below tell the story of how it is possible to create capacity for the future, and at the same time catalyze walkable development.
Both Greenville and Smithfield have a nearly an identical opportunity for replacing two 5-lane Stroads with four much friendlier one-ways. In the Greenville case, new one-way alignments must be developed through parking lots and by removing a significant building at the mall. In this Smithfield case, the needed parallel streets are already there, and would just need upgrading.
The graphics below focus on this area. Both Greenville and Smithfield have a nearly identical opportunity for replacing two 5-lane Stroads with four much friendlier one-ways. In the Greenville case, new one-way alignments must be developed through parking lots and by removing a significant building at the mall. In this Smithfield case, the needed parallel streets are already there, and would just need upgrading.
Top View of Downtown Potential
White is existing buildings, and tan shows what could be catalyzed by the reinvention from Stroads to Streets.
Four One-Ways are Better than Two Stroads!
Below is a system of four one-way streets. This system can handle significantly more traffic while also establishing a walkable environment, relative to two crossing two-way Stroads.
This focuses on the top intersection from the scene above. Notice before and after.
Below is what typically happens instead of one-ways. Two stroads are widened into even larger stroads. In this case, single lefts and shared thru-rights are converted to double left and dedicated right. Why not try one-ways instead? They handle even more traffic than this, but in a much friendlier way!
This is a good view of the pedestrian environment made possible by these one-ways.
Benefits of One-Ways for Historic Main Streets
Across America there are many historic Main Streets that operate as Stroads with 4-5 lane cross sections. They stay this way because there is too much traffic for a 3-Lane Road Diet. In some cases, traffic keeps growing to where DOTs will eliminate on-street parking to make even larger cross sections.
Left image has on-street parking, but lacks a center left turn median, creating significant delays.
Right image removes parking to create a median and slightly better sidewalks. It also has trees in the median where possible. In either case, because there is so much pavement, average speeds are well above the speed limit.
By moving half of the traffic to a parallel street, there is room left over for parking, bike lanes, transit, and much better sidewalks and street trees. You can lower the speed limit from 35 to 25, and drivers WILL obey it. Why? Because one-ways can be perfectly synchronized. Speeding only gets you to the next signal a bit to early, so why bother speeding?
The series below is the same location, but viewed from the top.
Left: 4-lane with parking. Right: 5-lane, no parking, but better sidewalks and planted median where possible
Left: Best-possible two-way. Right: One-way is still so much better!
Left: One-way with parallel parking and bike lane. Right = angle parking and "slow lane"