In normal operations, the suspension cable design requires that no more than one vehicle may be in any single cable-suspended segment (span; i.e., between two adjacent pylons). This provides maximum pylons-spacings for given headway and vehicle speeds, as follows:

Headway Velocity  Pylon Spacing
1.50 min 
1.00 min 
0.50 min
0.25 min
40 / 50 mph 
40 / 50 mph
40 / 50 mph
40 / 50 mph
5,280 / 6,600 ft
3,520 / 4,400 ft
1,760 / 2,200 ft
880 / 1,100 ft

It is seen that at normal line velocities and with headway of 0.25 minutes or above, this design constrain does not represent any practical limitation on pylon spacing, as considerably shorter spans are more than acceptable and practical. In case of emergencies there is no limitation on a second vehicle entering an occupied span for rescue purposes. The one vehicle per span rule is to satisfy ride quality in combination with limitations of the cable movement over the pylons.