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Some Issues to Consider with Close Headway Operation

Some Issues to Consider with Close Headway Operation. Tom McGean APM Standards Meeting February 2008, Miami FL. Differences between Auto and PRT Collision Risks. ISSUE AUTO PRT Evasiveness Can swerve or change Can only stop lanes and/or stop to avoid collision

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Some Issues to Consider with Close Headway Operation

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  1. Some Issues to Consider with Close Headway Operation Tom McGean APM Standards Meeting February 2008, Miami FL

  2. Differences between Auto and PRT Collision Risks ISSUE AUTO PRT Evasiveness Can swerve or change Can only stop lanes and/or stop to avoid collision Liability Assumed by driver of car Assumed by transit agency Reaction time Driver reaction time plus Depends on detection, mechanical brake delays communication, and mechanical delays Crash protection Airbags, seat belts and car-seats ???? for young children + crashworthy design Information pro- Qualitative information on Quantitative inform- vided following location, speed and decel- ation on location of Car eration of both lead and both cars, speed of one following car or both cars, no decel- eration rate data. Braking Rate Driver adjusts rate for pavement Normally one fixed condition and situation ahead emergency rate

  3. What the Following Car Knows About itself About Lead Car Location U, K or KC K or KC Velocity K U or KC Deceleration U U K = known with a tolerance by the following car U = Unknown by the following car C = Requires communication from central or regional controller Lead Vehicle Loc/Vel/Decel K,U,U You must assume brick wall stop (traditional block) KC, U, U You must assume brick wall stop (moving block) KC, KC, U You may assume non-brickwall stop with lead car at max. stopping rate & following car at min. rate.

  4. Suggested Requirements to Permit Non Brick Wall Stop • Electronics and brakes meeting safety requirements of ASCE APM Standard including 10**8 MTBHE for CAS. • Passenger and child restraints equal to those required by Federal Regulations for autos (NHTSA) • Frontal crashworthy design similar to automotive (NHTSA) • Operator must be willing to accept residual liability • Must be shown safe by hazard analysis • Unless lead vehicle deceleration is always known, it must be assumed to stop at greatest rate possible from hazard analysis For low speeds such as Morgantown PRT docking speeds, requirements can be relaxed to permit “bumping” collisions.

  5. Failure Mode Accidental thrust reversal Lead car brakes locked Lead car wheel bearing fails Major suspension failure Loss of a wheel Propulsion motor locks or seizes Differential or rear end lockup Lead car derails Switch jams or fails Obstacle on Guideway CONCLUSION: Probably should assume a 1G worst case lead car failure decel rate. Note auto drivers can and do stop at this rate when necessary. Max G 1G 1G .25G 1G .25-5G 0.5G 0.5G 1-2G 0.5-10G 1-10G High deceleration Lead Car Failure Modes

  6. Brick Wall Stop is not the only Issue! Other Fail-Safe conditions are presently imposed on current train separation by APM Standard, IEEE CBTC Standard, and others. These include: Maximum attainable overspeed assuming runaway acceleration of following train combined with minimum emergency braking on following train including single worst case failure of brake system (typically loss of 25-50% of braking!) Combined worst case cumulative time delays, worst case grade and tailwind conditions, minimum adhesion/traction and worst case load condition (heaviest following train)

  7. If Runaway Motor, Worst Case Braking Failure, etc. Are Retained, Eliminating Brick Wall Stop Will not Make Much Diffference!

  8. Changes PRT Probably Requires to Current APM Standard • Eliminate non brickwall stop assumption • Eliminate worst case single failure of braking system assumption • Eliminate assumption of runaway motor on following car If all three changes are not made it is unlikely to improve PRT headway/capacity

  9. Chart assumes runaway acceleration & worst case brake failure may be ignored. Speed control tolerance is +-5%, deceleration is closed loop with +-10% accuracy, position tolerance is +-1 foot for each car, following car braking is limited by wet pavement traction number of 0.35

  10. Chart assumes runaway acceleration & worst case brake failure may be ignored. Speed control tolerance is +-5%, deceleration is closed loop with +-10% accuracy, position tolerance is +-.1 foot for each car, following car braking is limited by wet pavement traction number of 0.35

  11. Conclusions • Differences between auto and PRT situation must be carefully accounted for • More changes than just brick wall stop will probably be required • Changes to current standard could potentially reduce PRT headway and increase PRT capacity by about 20% • Whether this is worth liability risk needs careful consideration

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