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Advanced Transit Policy for the Seattle region       


PRT: Mass Transit Without the "Mass"

Imagine instead of a single huge bus, all these empty seats could run around separately (or in sets of 2, 3 or 4), picking up people on-demand and transporting them non-stop to their destinations. The result would still be MASS transit, except now it would be individualized — or PERSONAL.

We're all familiar with conventional transit—large vehicles that run on timetables, stopping at designated locations. Show up at the appointed place, hope the system is running on schedule, and you and a lot of other people ride the route together. The primary problem with this type of mass transit is that the "mass" is mostly vehicle mass, not people mass.

PRT turns the conventional formula on its head. PRT begins from the observation that automobiles don't operate on timetables, so why should transit? By running when YOU want to travel (demand-responsive, or "on-demand"), PRT takes the large groups of bus or train riders and spreads them out over time, like this:

Mode Time: 1 hour Total Riders
Buses 20 min apart, showing # riders per bus 0:05 23      0:2525      0:4517 65
PRT, on-demand operation, showing # riders per trip (each vehicle represents 1 trip) 0:002    2    2    1
   3    1    1    1
   2    2    2    1
   1    3    1    1
   1    2    2    2
   1    1    3    1
   1    1    2    2
   2    1    3    1
   1    1    1    1
   1    1    1    1
1   →| 1:00

Same numbers of people, but in PRT they get to travel at their convenience, and the ride is private or with others of their choice. Is this efficient? Yes—with PRT, vehicles only move when someone needs one. Buses and trains have to keep to their schedules even when partially full—and even when empty. Is PRT cost effective? Yes—three buses might cost $500,000 each. In the above table of 41 PRT trips, the 65 people could be served by only 10 PRT vehicles costing $10-20,000 each.

Only 10!? Because PRT doesn't force everyone to travel together, each of the 10 vehicles can pick up riders, drop them off, then pick up more. This sharing of vehicles by riders in succession is the key to PRT's total carrying capacity, and it's possible because service is on-demand.

One PRT vehicle can make 5, 6, or even more separate journeys per hour, depending upon average trip length. But because PRT travels nonstop at 20-45 mph the entire way, each trip will usually be short. Thus, if one vehicle makes 6 trips per hour...

# vehicles Trips per hour
20 120

If we assume an average of 1.2 persons per trip (about the same as cars), the 5,000 vehicle fleet can have a maximum hourly ridership level of 36,000 people, or 864,000 per day. For comparison, consider that (at this writing) total weekday bus ridership for all of King County averages about 330,000.


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