Bidirectional Diamond Lanes Are Here!
Many cities built diamond lanes in pairs, oriented in the same direction to favor the commute, instead of single lanes moving opposite directions. The San Francisco Bay Area should consider this option.
Have you ever been in the diamond lane with your extra passenger, moving as slow or slower than the other traffic, while watching the opposing lanes run with light traffic and the opposing diamond lane specifically running empty? Do you ever wonder why the non-commute direction diamond lanes are even in force in Santa Rosa? Perhaps you thought it might be possible to adjust lanes to favor the direction of the commute, such as they do on the Golden Gate Bridge with the “Road Zipper?”.
Actually, several jurisdictions have had the foresight to install reversible diamond lanes, which consist of 2 central diamond lanes separated from normal traffic by two different concrete k-rails. This allows these 2 lanes to favor the direction of commute, thereby doubling traffic capacity of the diamond lane system. Who gets to use diamond lanes is another matter and varies from State to State. The trend is towards fee-based customers (monitored by a Fast-Trak type transponder) mingled with the traditional multiple passenger motorists. At some point in the future, these lanes could be easily adaptable to driverless “auto-trains” that would traverse with short headways at high speeds (also fee-based) while leaving traditional autos and trucks in the standard lanes.
A substantial section of Interstate 395/95 in Washington DC/Virginia has used this barrier-separated reversible lane system since 2006. There are similar systems in place on I-5 near Seattle, I-15 near San Diego, and I-90/94 in Chicago to mention a few.
Reversible diamond lanes might be considered a type of “increased passenger density” utilizing existing facilities. In layman’s terms, it sort of answers a child’s questions such as “why are there so many cars over here, and so few over there, Mommy and Daddy?” Other ways to increase passenger density would include a) car pools b) buses c) smart cars (they can adjust more quickly to distance between cars, or “headways”, and thus they can pack cars onto a highway more densely). In fact, smart cars bring up the possibility of narrowing lanes (and even greater traffic density), but that is a topic for another day.
To be fair, a study done for the Georgia Department of Transportation by consultant HNTB points out the downsides of reversible diamond lanes, most of them involving safety and the difficulties of routing traffic at transfer points (intersections and the like). At present, there is always the possibility of rogue drivers going the wrong way in reversible lanes. At transfer points, most systems require drivers to exit the diamond lanes and merge to their exit point, which is less efficient than dedicated carpool entry/exit ramps, which don’t quite lend themselves to reversible diamond lanes. As smart cars, smart buses, and “auto-trains” become more common, these problems may be diminished, as “driver error” will be minimized.
Of course, long commutes to single exit points (such as most North Bay commuters, who enter from various points but are mostly headed to San Francisco or the Richmond San Rafael bridge) lend themselves to reversible lanes. Due to high traffic volumes, dedicated reversible diamond lane ramps could be built at 101/580, and perhaps 101/Hwy 12 Santa Rosa, 101/Rohnert Park Expressway Rohnert Park, 101/Washington Petaluma, 101/Rowland Novato, 101/3rd St San Rafael, with automated barriers to route traffic in the right directions.
Assuming safety and ramp efficiency issues can be solved, the issue becomes not one of whether reversible lanes work, but why there aren’t more of them. If the future calls for smart cars and buses, and computerized auto-trains for shorter trips, hyperloops and mag-lev trains for medium trips and large air travel conveyances for long trips, it would be prudent to invest in reversible carpool lanes and make them fully grade-separated (not subject to conflicts with other traffic). If today’s diamond lanes become tomorrow’s driverless lanes, computerization may enable narrower lanes and faster speeds.
Like this article? Read more in Vern Scott’s new book “Civil (Engineering) Disobedience”, available on Amazon.com
Also, enjoy these other Transportation-related articles by Vern Scott!
