December 4


Imagine this new configuration for minor arterial corridors that carry 30,000 to 50,000 vehicles per day –

  • Build single lane roundabouts every quarter mile at collector or minor-arterial junctions
  • Build a shallow tunnel that provides a through lane in each direction for passenger vehicles only that goes underneath the roundabout on the minor arterial corridor we’re discussing
  • Provide four through lanes between the roundabouts, with the middle two lanes being passenger vehicle only lanes – striping for these middle two through lanes would be similar to HOT lanes on freeways to allow for weaving lane changes between the roundabouts for those who want to enter/exit the passenger vehicle only through lanes

The tunnel for each lane would be a modular box culvert that provides 14 feet of width and 6 feet of height inside for the through lane.  If they came in 10 foot lengths, you’d need about 15 of them to get under the typical single lane roundabout.  Two box culverts would be placed side by side, one lane for each direction.  About 30 of these box culverts would be needed to build the two through lanes under a 150 foot outside diameter single lane roundabout.

I assume the top of the box culvert would be about a foot thick and there’d be another foot for the pavement.  This means there’d be 8 feet of depth to transition down.  Doing this at a 2% grade results in the transition taking 400 feet to get down to the tunnel and then 400 feet of transition up out of the tunnel.  This leaves about 500 feet between the quarter mile spaced roundabouts where motorists could weave in and out of the tunnel.  I’m talking round numbers here that would be tweaked during design, but these transitions would be shorter than the 480 feet a standard left turn lane takes (300 feet of storage + 180 foot long taper to transition over 12 feet of width).

These dedicated passenger vehicle lanes could free flow at 30 mph with a capacity of about 1,900 vehicles per hour each.

Based on the Highway Capacity Manual, we assume 3% of the vehicles on a corridor per day are heavy vehicles.  This means at least 90% of the vehicles on most corridors would be short enough to use the tunnels.  Of course some motorists would be claustrophobic or skeptical and avoid the tunnels.  But moving 50 to 70% of the through traffic out of the intersections would provide a massive improvement.

This configuration could replace the typical four lane divided corridor that provides dual left turn lanes and free right turn lanes at signalized intersections.  It might even replace some six lane wide corridors.  I don’t do final design, so I wouldn’t know where to start on costing this concept out, but my gut tells me it’s not significantly more than the standard big intersection design.

The shallow tunnel configuration/roundabout concept would also be more environmentally friendly.  It would better serve bicyclists/pedestrians and need less porous pavement than our standard corridor designs.

These shallow tunnels could be stacked at minor-arterial/minor-arterial junctions.  The tunnel going on the bottom would need 800 feet of transition in and then out, which probably would move your intersection spacing from quarter mile to half mile.

The weaving aspects would need to be studied in full detail to make sure the configuration would work, but I think this concept at a steady 30 mph would improve delays over the standard 50 mph dual left turn lane/signalized design we typically have on these corridors.

A key design aspect would also be placing gate arms and other measures along the passenger vehicle only lanes to keep out heavy vehicles, over-sized passenger vehicles, and mini-vans with kayaks on top (or anything else on top).

The concept could be tweaked in final design to use signals instead of roundabouts.  Or maybe put the shallow tunnel lanes on the outside of the corridor instead of down the middle.

Thoughts?  Is there a master’s thesis buried in this idea?

(Hat tip to Colin MacGillivray and Bob Poole who inspired this thought process.)


  • This configuration is already in place at several major traffic circles in the District of Columbia, including DuPont, Washington and Scott. The major street thoroughfares at these locations go underneath a traffic circle that has connections to cross-streets. I do not know if there’s anything like this anywhere else in the US.

  • First, did you really mean 6′ box culvert height with 8′ change in grade? or 16/18?

    This seems like it could be an effective de-stroading component during sprawl repair. But I don’t think it would work without a coordinate change in adjacent land use.

    I know one of the study options for Cedar Ave (Dakota CSAH 23) was a sort of tunnel design through Apple Valley. But it was deemed to be too expensive and land consuming since it would have been a lengthy ~3 mi trench with compressed diamonds or something similar at 140th, 150th, and possibly 160th streets. Thus we have the BRT superstroad design that opened last year.

    The general concept you describe seems like what has worked great in many dense urban environments around the world. Here in the US, there are plenty of traffic circles in Washington D.C. with through tunnels: Connecticut Ave/Dupont Circle, 16th St NW/Scott Circle, Mass Ave/Thomas Circle, K Street NW/Washington Circle.

    The latter is my favorite because it incorporates a multiway boulevard configuration on K Street (though I hear that config may be under assault by planners). The solution you propose seems like it would couple very nicely with a multiway boulevard in a situation where there are adjoining land uses up to and including dense mixed use such as in Downtown Washington. Or it could work in lower density residential areas such as the Esplenade in Chico, CA.

    The worst part about the modern suburban stroad is that it is not deemed suitable for access to adjacent land uses. Furthermore there’s often large landscaped berms or even sound walls to screen adjacent uses. But this reinforces the hierarchical regime of vehicular transportation which is highly inefficient and causes frustration to motorists and inaccessibility to non-motorized users.

    Your idea seems like it would couple wonderfully with complex intersections where multiway boulevards would create stronger places while maintaining reasonable traffic volumes. I hope to hear more.

  • Matt – Yes, I did mean a six foot tall box culvert. The goal here is to make them just deep enough so a passenger vehicle can use them. The “safety” factors – added height – engineers would like to layer onto the concept would kill it. The trenches you discuss are rarely going to be cost effective and do not fit into a collector or minor arterial corridor. Mike

  • Any photos or schematics you can share? I know of many tunnels in the U.S., but none that are built six or eight feet deep. All of the one’s I know of are deep enough to accommodate full semi-trailers (which I believe would kill the concept 99% of the time).

  • Ah, so this post is even more revolutionary than I first realized. I like the idea. The vast majority of traffic is automobile traffic, so this could work well. Though my general view is we ought to change our land use to one that minimizes the number of routine and necessary automobile trips for things like job/school commuting and basic daily tasks.

  • As a casual reader,

    I wonder if these type of 6 foot tunnels would be a problem for emergency responders to access in case of accidents.

  • Sam – The tunnels themselves would be less than 200 feet long, so while clearing them out to get to the crash would take some time, I don’t know that it would be a deal breaker. Mike

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    Mike Spack

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