Summary of "Placemaking Alternative Intersection" research underway for the North Carolina Department of Transportation.
What is a One-Way Split Intersection?
The basic idea is that rather than having a single huge stroad intersection, it is better for both livability and traffic management to split two stroads into four one-way streets, creating four small intersections, then come back together on the other side. Below is such a situation near San Diego, and you can see 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.
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.
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.
Concepts in Smithfield: Converting Brightleaf and Market Street Stroads into Four One-Way
Below is a slider showing today's 4-lane, 78-ft cross section on Market Street. It has a lot of nice "Street" elements: many historic buildings adjacent to sidewalks, some street trees, on-street parking, and very narrow traffic lanes. But it is also incapable of supporting much traffic due to shared thru-right, thru-left, and side friction with parking. A "road diet" with just one lane each direction and a two-way left-turn lane would help walkability and might even help a little with today's traffic, but it cannot accommodate growing traffic associated with the fact that this street connects with a freeway interchange AND a rare river crossing. Something will give, and that something is almost always a "bigger stroad," - in this case a likely loss of on-street parking ad a minimum.
The one-way cross section shows how you can get a better pedestrian environment, with a "slow lane" of about 10-15 mph for bikes and vehicles turning left or accessing parking. It handles more traffic, creates a wider pedestrian "furniture zone," accommodates bikes and transit, has safer parking, and encourages mixed-use development not only here, but also on a similar eastbound street. Given that all things must be compared NOT to the ideal, but to the likely alternative, what is not to love about this?
Below is today vs what NCDOT may be likely to do as traffic continues to grow. Notice today's single left lanes become double lefts. Today's shared thru+right becomes a dedicated right. A few of the auto oriented buildings on the bottom and left give way to "slightly larger" auto-oriented buildings.
Below compares the NCDOT "double left" default (from above), to how it could be if some directions of flow are relocated to other corridors. The much more human-scale streets easily have room for premium street trees, "slow lanes" for bikes and other low-speed vehicles, and impressive on-street reverse angle parking. Combine this infrastructure with form-based zoning codes to allow mixed uses up to 3-5 stories, and developers will salivate to construct "missing middle" projects not only here, but in the nearby areas also.
Below is the same angle, but zoomed out to see what we envisioned for the rest of the area, along with all four one-way intersections. Peach colored buildings are new, while white/brown and white/cream buildings are existing.
This is a good view of the pedestrian environment made possible by these one-ways.
Top View of Downtown Potential
White is existing buildings, and tan shows what could be catalyzed by the reinvention from Stroads to Streets.
Corridor Views and Benefits of One-Way
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 Road Diet. In some cases, traffic keeps growing to where DOTs will eliminate on-street parking to make 6-7-lane cross sections.
The slider below shows Market Street today vs as it might become someday if traffic keeps growing without an effective strategy to divert it to somewhere else (Parking replaced by lanes, and drivers continue to break speed limit).
If eastbound traffic can be moved to a parallel street, then Market Street can be "destroadified" and become a "Complete Street." Some people criticize one-ways as "too fast," but it is easy to get drivers to comply with a 25 mph speed limit. How? One-ways have perfect signal synchronization, so drivers learn to obey the limit since driving too fast just gets you to the next signal a little too early. This is much better than the Stroad!
Below is the same location, but viewed from the top. This shows one row of reverse angle parking, but could also do two rows of parallel parking.