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.)