top of page
One-Way Split Intersection

by Michael Brown, PE, AICP
* Traffic Engineer, New Urbanist
* Founder of Urban Innovators

What is a "Split Intersection"?

When two Stroads meet, it creates a single huge intersection with an inefficient 4-phase traffic signal.  Instead, each Stroad can be split into two one-way streets just before they intersect.  The result is either two or four intersections that are each a lot smaller, safer, and more efficient to manage. Below is a very nice example in San Marcos, California.

Below, each intersection is very small and easy for pedestrians to cross.  Instead of just four premium corners with great access and visibility, now there are 16!

Walkable Facts About One-Ways, Relative to Stroads

  1. Pavement width is much less; thus easier to cross

  2. Signal cycles are shorter with fewer conflicts; thus faster and safer to cross

  3. Pedestrians only need gaps in one direction; thus safer when crossing without a signal.

  4. Narrow streets have better pedestrian enclosure.

  5. One-ways do not require a left-turn lane, which means more space for other uses.

  6. One-ways double the number of parcels with high visibility & good access, (i.e. Larger Activity Center).

  7. Easy to synchronize signals, which means it's easy to get drivers to obey a low speed limit. 

  8. Walkable development still needs auto accessibility or it won't get built. One-ways deliver that!

  9. Expands grid and connectivity, enabling larger Activity Centers.

Research Concepts from North Carolina

The North Carolina Department of Transportation hired Urban Innovators and NC State University to demonstrate how one-way designs could replace two-way Stroads to create walkable environments, and to evaluate some of the pros and cons of doing so.  Below are diagrams from that effort. 


In the above concept for Greenville, North Carolina (caution: unvetted and still needs alternatives analysis and a public involvement process), the new westbound street would go through a mall.  Obviously, this will be infeasible until the mall owners decide they must replace the mall, which could be very soon given the general status of malls.  The before and after graphics reveal impressive potential.


The Impressive Math of One-Ways

Notice in the bar charts that the Stroad intersection can only serve 3,600 vehicles per hour.  This makes it hard to create additional development, even if it is mixed use, because in the suburbs most trips will still be by car even if a greater share use alternative modes.  The new design can support 7,100 vehicles per hour - basically double.  This doubling, combined with improvements in alternative modes, will allow the area to support 5-times as much development as at present!  

Yes, it will be expensive to create these one-ways along with premium cycle tracks, on-street parking, uniform street trees, etc., but it will also offset about 1200 acres of Greenfield development, which eliminates the need for 34 miles of roadways and utilities.

Can't afford to build it?  You can't afford not to!

With One-Ways, it is Easy to Get Drivers to Obey a Slow Speed Limit

Many historic downtowns have been converting one-ways back into two-way streets.  They do this in part to help make the average speed lower (and thereby more compatible with pedestrians).  In cases where the resulting two-way streets will have only one lane each direction, such a conversion is likely to lower average speeds since it is impossible to pass slower drivers. 

But Stroads, by definition, have 2 or more lanes each direction, so whether one-way or two-way, maximum speeds will be about the same if both are designed without traffic calming features.  However, compared to Stroads, it is actually easier to get drivers to comply with a 25-30-mph speed limit using one-way streets.  Why?  Because it is easy to synchronize traffic signals on one-way streets to any speed you desire.  Want 25 mph?  Time the signals for 25-mph and drivers quickly discover that going faster just gets them to the next signal a little too early.  Two-way streets cannot be synchronized easily, so drivers tend to go about 10-mph faster than the limit since they can't perceive any synchronization. 

Using One-Ways to "De-Stroadify" Historic Main Streets

Walkability advocates may look at the situation below and advocate for a road diet, aiming for one-lane each direction and a center turn lane.  Nothing wrong with that if the traffic situation can tolerate it, but sometimes it really can't.  For cases where you really need to manage high traffic loads, say of 25,000 vehicles per day or higher, there are many advantages to either keeping existing one-ways, or converting to that if possible.  The diagrams below show how as a two-way Stroad, drivers regularly exceed the 35 mph speed limit.  But after conversion to two walkable one-ways synchronized to 25 mph, drivers rarely exceed this limit because they know they will reach the next signal a little too early.

Summary of Pros and Cons

Advantages of One-Way Couplets

  • Impressive vehicle capacity gains

  • Less pavement = Complete Street space

  • Shorter signal cycles

  • Safer for autos and pedestrians

  • Enhances and motivates TOD

  • Little additional cost if planned from start

  • Expands grid, enabling larger activity center

  • If synchronized, low speed limits observed

  • Narrow streets help pedestrian enclosure

  • Proven: common feature of thriving Centers


  • Initial confusion for drivers

  • Out of direction travel (offset by less delay)

  • Opposition if 2nd street has incompatible uses

  • Convenience stores may have initial loss.

  • Transit boarding stop different than exit stop

  • Risk of high-speed if designed poorly

  • Change affects many people (education effort)

  • Unwarranted stigma to overcome.

bottom of page