98> SPEED UP! SLOW DOWN!

Every now and then an alert reader asks a question so profound that I just can’t keep it buried in the comments section. This time the honor goes to cmfseattle for this question, posted under Control Issues Part II.

“Why would vehicles be going at different speeds, anyway? For any given segment of track, there is a maximum safe speed. Why not just have the vehicles go at that speed?”

The short answer is passenger comfort. A system like the one I envision is capable of much higher speed maneuvers than most passengers would be willing to endure. But let me put this in context. I do not believe that previous PRT designers have chosen centralized or zone-based control because it is better, but rather because they had to. Autonomy requires cheap, powerful computers, sensors and very fast and flexible communications. Since all of this is pretty standard stuff these days, these obstacles have been removed. Another factor is the nature of the network. Most PRT designers and vendors have envisioned systems where short trips are taken around a downtown area by hoards of passengers. Since they look at their systems primarily as a business venture, if a track segment wouldn’t be packed with vehicles, that segment would not be built. The strategy of only “picking the low hanging fruit” and then moving on to different city makes perfect business sense, and requires only relatively slow vehicles moving in unison. Let me give an example of how this philosophy creates limitations.

In most PRT systems, with the vehicle riding on top of a guideway or rail, the need for going fast around sharp curves is addressed by banking that curve. In the case of “y” interchanges, however, banking is impossible. This leaves these systems with two choices. One is to go slow, the other is to make the “Y” structure very gradual, creating the need for much more track. It has been pointed out that with frequent stations, a large percentage of the overhead track would, in fact, be double, creating extra cost and an unwanted canopy effect. The option of slowing for a curve is not needed because the vehicle is going slow in the first place. I do question just how smooth these interchanges can be made without slowing down, though. Going at a fixed, slow speed has other benefits. All curves can be sharper and banked at the optimum angle for line speed. The vehicles need not be particularly crash-worthy. Of course less expensive propulsion systems can be used as well. Finally, since they (PRT vendor/designers) see themselves having absolute control over all aspects of the system, there is no need to consolidate control. Some control could be with the stations, some with the vehicles, some with the track and some with a central system. This hairball approach is perfectly logical if nobody outside of your company will ever have to work on it. Anyway, within the context described above, a fixed speed which is controlled from outside of the vehicle makes perfect sense.

As soon as you move to higher speeds, however, the fixed speed approach gets much more cumbersome. Even with a hanging, self-banking design, there are limits to what kind of G-forces passengers will tolerate. Variable speed allows PRT vehicles to do what ordinary cars do for any particularly sharp turn, which is to slow down. But the faster they were going, the more time is spent in a transition speed. I would note that even with the fixed speed systems, there are always transition speeds anyway, such as entering or leaving a station or creating a slot for a merging vehicle. Longer, smoother transitions generally mean a more comfortable and energy efficient ride, something left out of the “line-speed” paradigm.

There is a trade-off that exists between speed and system efficiency and the nausea prone stomachs of a small but significant portion of the ridership. In a heavy traffic situation, there is only one choice. Slow everybody down to a point where the ride is acceptable to the vast majority and just let the rest be uncomfortable. With variable speed, however, if there are no motion sickness prone passengers on a track segment, everyone could go faster. The information for this could easily be gathered at the point of payment or stored in the form of account information.

Another advantage to variable speed has to do with inertia. Heavily loaded vehicles will tend to take longer to accelerate and decelerate, and should have longer headway distances. This might influence the how the vehicle behaves in merges and turns. These effects also become more profound with greater speeds. For example, why waste the energy quickly accelerating a 2 AM delivery? Even with regenerative braking, it would still be advantageous to coast to a stop if time and traffic allows. Everyone who drives knows that the acceleration/deceleration profile that is best for energy efficiency is not the same as for getting somewhere in a hurry.

Finally, it would be more efficient if the PRT behaviors included a bit of opportunism. Ever get on a freeway entrance ramp and realize that if you hurry you can slip into a spot without making anyone squeeze you in? In a PRT system, there could easily be very limited merging opportunities onto a busy track, and since relative positions would be known well in advance, the merging vehicle could “step on it” for a very long time to catch such a fleeting slot. This is essentially the “step forward” maneuver that Anderson envisioned but with many steps forward by a single vehicle rather than many vehicles all stepping back one step. This maneuver, again, involves straying from line-speed for an extended period of time.

In the end I think it will just be simpler, more comfortable, cheaper, easier to manage, and more energy efficient to program the PRT vehicles with behaviors that take into account the same sort of factors that influence drivers every day. How is traffic? Who (if anyone) is my passenger? Will I hold everyone up if I drive slowly? Will I benefit merging traffic if I speed up? Should I drive more conservatively because my vehicle is loaded down? Is this an emergency? Can I get through downtown before rush hour starts?

In the end, it all comes down to the PRT equivalent of intelligently using the gas and brake pedals. Any system that fails to address acceleration/deceleration properly is going to be uncomfortable to the rider and will constrain the options available to the track designer. The other option is to just go slow.