Lesson #3: Reaction times

The answer is to use today's high speed computers, sensing systems and control systems to yield automatic and much faster reaction times. Humans typically are expected to use up about 3/4 of a second to react to an emergency and start hitting the brakes. The computers used in modern sensing and control systems can easily cut that decision/reaction time by a factor of 10 to .075 second.

Remember your last Department of Motor Vehicles written test? They usually have one multiple question on stopping distances. They gave you a speed and you had to pick which of the four distances shown would represent the actual automobile stopping distance. I never bothered to memorize those answers because a long time ago I just worked it out backwards mathematically. Whenever I encountered this question again, I just cranked out the simple math. The California DMV formula for stopping distances used to assume it would take the average driver 3/4 of a second at speed just to react to an impending emergency and get one's foot stomping on the brake pedal (reaction time). Then hitting the brakes hard, the driver will be able to get stopped (decelerate) at a steady maximum of 1/2 g. Simple as that. For some reason the DMV has increased the reaction time to 2.5 seconds and the same 1/2 braking deceleration. They may be worried that people are paying attention less than ever. This yields the following results:

Initial Speed	Reaction Distance	Deceleration Distance	Total Stopping Distance
35 mph	128.3 feet	81.7 feet	210 feet*
45 mph	165.0 feet	135.2 feet	300 feet
55 mph	201.6 feet	201.9 feet	403 feet*
65 mph	238.3 feet	282.1 feet	520 feet
75 mph	274.9 feet	375.6 feet	650 feet
80 mph	293.3 feet	427.3 feet	720 feet
90 mph	329.9 feet	540.8 feet	870 feet
100 mph	366.6 feet	667.7 feet.	1034 feet
*1999 officially recognized DMV reaction plus deceleration distances are given at: DMV

Automobile traffic is not that tightly packed because humans need more reaction times than computers do. Most, not all, drivers follow other cars with a spacing that they are comfortable with. Normal drivers do not relish focusing their entire concentration to follow right on someone's tail for minutes or hours on end. Without any mathematical formulas, whatsoever, we all figure out how close we can follow and still get safely stopped if something weird happens just in front of us. We know the vehicle in front of us CANNOT instantly STOP! We adjust our spacing mentally by simply including a factor for the distance the other driver would take to stop once we see his brake lights appear. (You may have read about a section of a San Diego, California freeway that is currently being used to test automatic computer driven automobiles. The cars travel at 65 MPH autonomously - while spaced just feet apart from each other. The benefit in tightly packing traffic is more people per lane per hour, which means less congestion, and the elimination of building more $24 million per mile freeways to carry the ever growing traffic.)

Public Transportation Braking Laws
Public transportation, such as trains, on the other hand, have to follow unreasonable braking laws which have nothing to do with the reality of physics. Trains cannot legally follow each other any closer than the distance required to make a complete stop. This is analogous to you having to be completely stopped by the point along the freeway where the driver in front of you tapped his brake lights. We all know this point has nothing to do with where you could possibly ever contact his car. His car cannot stop instantaneously! We all have learned to sense from assorted little cues whether or not the driver in front of us is paying reasonable attention or off in la la land (tough to check by looking directly into their eyes). Years of driving experience allow us to trust that the person in front will hit the brakes and gradually slow down and not just go piling stupidly into a stalled truck or whatever.

What do these train laws have to do with SkyTran?
Mainly, it got us thinking about just what would it take to still follow that train braking law and keep our high density 1/2 second spacing! If you are traveling at 100 MPH, you are covering 146.7 feet each second. Another SkyTran could be 73.3 feet in front of you (one half second ahead). Guess what, we can take great advantage of our SkyTran MagLev drive unit being trapped inside the hollow, roll formed, monorail track. What if we include a hydraulic brake (for emergency use only) that could squeeze against a rib that we also roll formed into the track (it runs the entire length of the track)? Then, no longer is emergency braking deceleration limited by the traction capabilities of rubber tires on asphalt or concrete!!!