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Critique of Bicycle Transportation Systems "bicycle tunnel with tailwinds" proposal.

Bicycle freeways in which bicyclists are propelled by artificially created tailwinds have been proposed many times, for several decades. A company by the name of Bicycle Transportation Systems, of Denver, Colorado, is currently promoting this concept. A drawing from the Bicycle Transportation Systems Web site illustrates the concept. Bicycle Transportation Systems claims that this system is "the future of mass transportation."

Chalk drawing of bicycle tunnel (24585 bytes)

Drawing from Bicycle Transportation Systems Web site,
reproduced under fair use provisions of copyright law

Does the claim of "the future of mass transportation" for these tunnels make sense? In a word, no.

Let's first look at the issue of energy efficiency. Bicycle Transportation Systems further states that "[c]ycling inside the Transglide 2000™ is 90% more efficient than normal cycling outside the enclosed airflow enhanced system." This statement accounts only for the energy expended by cyclists and ignores the energy necessary to propel air through the tunnel. The great majority of power would be expended in friction between the moving air and the tunnel walls.

Prof. David Gordon Wilson has calculated power requirements as follows:

I simplified the tunnel to one of 4m diameter. I chose a breeze of 9 m/s, 20 mile/h, although a higher speed might be necessary. At that size the surface should be relatively smooth. With this, the pressure drop would be about 400 Pa per km, or 1.6" water. (It would be higher with the shape and size you indicated, and much higher if the speed were, say, 12 m/s.) If there were 100 bicyclists per km with a drag from going 3 m/s slower than the wind speed, the pressure drop would increase by 40 Pa. The power required per km, allowing for fan and motor inefficiencies and nothing else, would be about 65 kW/km. If one had a tunnel of 10 or 20 km long the pressure would begin to load the tunnel. I expected to find somewhat higher figures, and will try to check them later.

65 kW is approximately 100 hp. Note that this is a bare minimum power requirement, which assumes that there is only one entrance and one exit.

The efficiency would be reduced by air leakage at entrances and exits. The leakage would brake rather than assist the cyclists at exits upstream of the main fan and entrances downstream of it. Multiple fans (drawing additional power) could maintain constant air speed along a tunnel's length despite the leakage, but only airlocks or revolving doors could eliminate power loss due to leakage. If air were moved in a circuit, in opposite directions on the two sides of the tunnel, minimizing the power loss would require sealing all entrances and exits except at a single location. A single opening could serve both entry and exit if it were at the opposite end of the tunnel from the fan; otherwise, it could serve either entering or exiting traffic, but not both. If there were a different fan for each direction of the tunnel, all entrances and exits except those at the ends of the tunnel would have to be sealed, and as bicyclists could not pass through the fan, there would also have to be a seal where the tunnel passes the fan to eliminate power loss due to backflow. In a long tunnel, there would be a noticeable (ear-popping) difference in air pressure on entering and leaving.

Other issues:

  • The claims made for the carrying capacity of the tunnels assume that there would be little speed difference among cyclists, as is the case with motorists on a limited access highway. But cyclists must be strong to take much advantage of tailwinds. Only the more athletic and confident cyclists would travel at high speeds. The proposed 12 foot floor width of each side of the tunnel -- between walls -- would be inadequate to allow safe overtaking. The usable width would be about the same as that of a 7-foot path with grassy shoulders. Also, unless the tunnel were heavily patrolled to exclude them, pedestrians, and inline skaters (whose effective width is 6 feet with their flailing arms and legs), would use it, traveling much more slowly than cyclists. As is demonstrated during parkway closings on weekends, no increase in roadway width allows cyclists to travel safely at full speed, even without a tailwind, in the presence of pedestrians and inline skaters.

  • Additional power input is most useful when bicyclists are going uphill and is not needed when they are going downhill. A tailwind provides too little assistance uphill, but greatly increases the "terminal speed" (the speed at which drag balances propulsion) downhill. With a 25 mph (40 km/h) tailwind, the stronger cyclists would be able to travel around 40 mph (65 km/h) on level ground and at 50 mph (80 km/h) or more, with appropriate gearing, on moderate downgrades where slower, less confident cyclists were traveling at 20 mph (30 km/h) or less.

  • Not only operating costs, but also capital and maintenance costs, would be high. The drawings on the Bicycle Transportation Systems Web site all show elevated tunnel systems (very expensive), though only in one of the cases (a tunnel above a busy road) is the grade separation necessary to maintain traffic speed and flow. I can imagine that the proposed tunnels would be a graffiti magnet, as is the case with existing tunnels, increasing maintenance costs further.

  • If a tunnel were at ground level rather than elevated, it would be a barrier to cross traffic of all types, including bicycles, as a limited access highway is. The only exception would be where such a barrier already exists.

  • An extended tunnel isolates cyclists from trip origins and destinations except at predetermined entry and exit locations. On limited-access highways for motor vehicles, such isolation represents an acceptable tradeoff against greater cruising speed and the absence of conflicts with cross traffic. Due to the problems described above and the typical shorter bicycling trip distance, this tradeoff is not beneficial for nearly as large a proportion of bicycle trips as motor vehicle trips.

  • The risk of physical attack is high in an enclosed corridor. This problem already occurs in secluded parts of paths and trails, overpasses and underpasses.

  • The proposed tunnels would be of transparent plastic and/or glass material and would have a greenhouse effect. They would be more comfortable than riding in open air in cold weather, but in warmer weather and with the sun shining, the tunnels would become uncomfortably, even dangerously, hot. The temperature inside them would get hotter and hotter as a cyclist rode away from the place where the air enters.

  • Especially if the tunnels were elevated, how would emergency crews gain access to crash sites or crimes in progress?

Bottom line question: while I think that an argument can be made in favor of tailwind tunnels in terms of travel time (no waiting as with public transit, bicycle speeds made competitive with those of motor vehicles), would this system be competitive in popularity, safety and cost/benefit ratio with other options including bicycling in the open air, public transit (including bikes-on-transit) and private motor vehicles?

Bicycling is a superb collector mode for public transit. The proposed tunnels would compete with public transit for users and funding, rather than supporting public transit as bike parking at transit stations and bikes-on-transit can.

Even if it can be made as fast as public transit through artificial tailwinds or other means, bicycling can only partially substitute for public transit, because not everyone is able to ride a bicycle and because there may be the need to carry more baggage than can be carried on a bicycle.

The tunnel concept does have one significant conceptual advantage: it works as a speed amplifier (like the bicycle itself) rather than as a total replacement for the cyclist's power input. In this way, it promotes physical fitness, unlike motor vehicles. But the same advantage is available with self-contained, more efficient and more flexible power sources. It is also available through the cyclist's increasing fitness and through more efficient bicycle design.

I think that the tunnel concept might be useful under a very limited range of conditions (for example, where the route is flat, there is already a need for an enclosed right of way, and especially if natural winds can be directed into it). I think that as a rule, we had best leave tailwinds to Mother Nature and be thankful for them when we can enjoy them.

More drawings, and descriptions of the tunnel system, are at the Bicycle Transportation Systems Web site.

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Contents , 2001, John S. Allen
except drawing, fair use.
Last modified 5 December 2002