Practical headway limitations for personalised automated-transit systems

Continued interest in short-headway personalised automatic-transit systems as one solution to urban congestion and pollution emphasises the need for a careful analysis of the constraints which limit minimum headway. This paper reviews the major factors to be considered in determining minimum headways and indicates the changes in hardware, or operating policy, necessary to reduce the allowable minimum. The paper suggests that fixed-block separation detection be replaced by a moving-block type if headways are to be reduced much below 10 s. Headways of the order of five seconds can be achieved by replacing conventional fixed-force brakes with constant-deceleration emergency brakes. Operation at still shorter headways will require assurance of limited maximum failure deceleration and, perhaps, acceptance of controlled collisions.