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Capacity

Throughput capacity of a transit system is how many people can be moved per hour or day. Throughput capacity depends upon the number of people per vehicle and time between vehicles (headway). Capacity of a transportation corridor can be increased with larger vehicles, shorter headways, or both. As you can see in the following diagrams, PRT (Personal Rapid Transit), GRT (Group Rapid Transit), Light Rail, and Heavy Rail all travel at roughly the same speed (20 – 40 mph), but potentially transport widely differing numbers ranging from a few thousand to more than 50,000 passengers/hour.

Confusion exists about PRT's capacity to move people. Many people jump to the conclusion that small vehicles cannot move as many people as large vehicles. Perhaps the analogy of cutting a log with an ax vs. chainsaw will help. While ax heads are large with a big cutting edge, chainsaw cutting teeth are many, small and frequent. Professionals prefer those many small chainsaw teeth to the ax for removing wood chips. Here are 3 ways to view capacity:

Estimating Passenger Throughput Capacity of PRT Technology

As with heavy freeway traffic, computer-controlled vehicles can be spaced 1 second apart. Even if each vehicle contained only 1 passenger, each PRT guideway could deliver 3600 people per hour  (60 vehicles per hour X 60 minutes per hour) – or 86,400 (3600 p/h x 24 hr) passenger per day (p/d).

1 podcar with 1 passenger/second = 3600 passengers/hour = 86,400 p/d for each guideway direction

In the interest of accuracy, maximum possible throughput numbers should be reduced about 50% as noted in the October 17, 2012 memo (page 5) from San Jose Director of Transportation Hans F. Larsen to the TRANSPORTATION AND ENVIRONMENT COMMITTEE regarding the AUTOMATED TRANSIT NETWORK FEASIBILITY STUDY: Realistically, throughput for an ATN is likely to be much less – perhaps even less than half – of what would be indicated from an idealized capacity calculation derived from simple line-haul formulas.

Let's apply a realistic throughput factor of 50%. That yields 43,200 p/d per guideway, or 86,400 p/d for both directions.

(86,400 p/d for each guideway X 50%) X 2 directions = 86,400 p/d

86,400 p/d compares well with the 55,000 p/d that VTA expects will use the BART Extension Phase 2 (BART Burrow) in the year 2040.

For a deeper examination of the Patronage vs. Mathematical Capacity, go to Passengers per hour: how to calculate transit capacity.

The PRT company MISTER/Metrino has published a capacity throughput analysis. It appears that using a 0.7s headway at 50 km/h (31 mph) yields 5,000 vehicles/cabs per hour on a guideway. Applying a 1.3 occupancy factor yields 6,500 people per hour transported on a single guideway.

MISTER/Metrino guideway and station

Real-World Numbers

In the real world of Morgantown, WV, the PRT/GRT system built in the 70's provides about 15,000 rides a day (and double that when WVU has a home game). (A single trip on the Morgantown PRT costs just 50 cents, and is free for students.) According to a University report, to be able to provide that level of service with buses would require anywhere from 30 to 60 buses in loops, running continuously, to transport their students.

Comparing PRT speed & capacity to other technologies.

The Advanced Transit Association (ATRA) presents another way to see capacity as outlined in this ATRA article which includes this graphic.

 

capacity

As shown, commuter rail like BART can handle 35,000 to 60,000 people per hour (one direction). However, VTA estimates that 55,000 people per day (p/d) will use the BART Burrow in 2040. Clearly, BART is oversized for the demand. PRT, with a capacity of 3,000 to 5,000 people per hour, is scaled appropriately for the BART Burrow route.


Here is a similar capacity graph done by the Modutram people in Guadalajara, Mexico, who are developing the Autotrén PRT system.


Additional Notes on System Capacity

Can small cabs move large numbers of people like traditional mass transit? Yes. Uninterrupted flow is the key to capacity, not vehicle size. For example, 60-passenger buses arriving two minutes apart (a very high flow rate for an American bus system) can carry 1800 passengers per hour. PRT vehicles every two seconds can provide the same capacity.  PRT capactiy depends on headways:

  •    0.5 second = 120/min or 7200/hr
  •    0.6 second = 100/min or 6000/hr
  •    2.0 seconds = 30/min or 1800/hr

A commonly accepted safety margin on roadways is 2 seconds between cars. Although automatic control of PRT cabs is safer and more reliable than human drivers, let's assume the Milpitas PRT system starts with that comfortable 2 seconds of space between each cab, aka "headway". At that headway, 1800 cabs per hour can roll down the guideway. That's 1800 people per hour assuming sole-ridership will prevail (30 cabs/min * 60 mins/hour = 1800 cabs per hour). That approximates the maximum volume of a freeway lane of traffic (2200). After a few years of operation, we may have the confidence to reduce the headway times to only one half second. That would quadruple throughput to 7200 cabs/hour -- roughly the volume of three freeway lanes in less than the space of one physical lane.

Now, compare that volume to LRT and trains. Although LRT systems may be designed for high volume, the actual limit of any operating LRT system in the U.S. is 1200 riders per hour; peak  in Sacramento is about 1000 passengers/hr.  Likewise for trains where the theoretical limit is 20K riders/hour, actual loading often tops out near 7K riders/hour. An exception may be BART trans-Bay tube where reports indicate near-saturation at 20,000 riders/hour.

Another capacity comparison could be made with computer controlled cars as demonstrated in the late 90's near San Bernadino, CA.  Partners for Advanced Transit and Highways (PATH) ran Buick Le Sabres by computers on a dedicated strip of freeway with magnets embedded so the cars could be computer controlled. Platooned cars ran for thousands of miles at 60 mph with 0.25-second headways.  In 2014, Hundai demonstrated even more control of vehicles at speed. Today the technology for driverless cars is advancing quickly.

Speed is another factor in capacity. Here are critical ideas from PRT pioneer Ed Anderson:

Tires vs. maglev are not the most important considerations. Curve radii increase as the square of the speed and off-line guideway lengths increase in proportion to speed. These are the most important factors. Life-cycle-cost per passenger-mile is the annualized capital + operating cost divided by the annual ridership. Costs increase with speed regardless of the means of suspension and ridership will increase with speed to a point. After a certain speed, costs increase faster than ridership so the cost per passenger-mile increases. - JEA

LoopWorks is choosing a speed that ensures high ridership by offering 1) low capital cost per passenger-mile and 2) speeds that compete with the automobile.

What about crowds?

Can a bunch of small cabs handle large crowds?

Some people question the use of Personal Rapid Transit (PRT) to handle crowds of people. To them, it seems obvious that small PRT cabs simply cannot handle surge loads such as when a commuter train stops at a station or a stadium crowd leaves. In the case of commuter rail (Caltrain and BART in the San Jose, CA area), consider these factors:

  • Only a portion of people getting off the train at a station would want to use PRT service rather than other options (walk, bike, electric scooter, taxi, bus, Uber, friend, etc.).
  • Not all of those who do want to use PRT will transfer within the same 60-second timing window; rather they will be spread out over a few minutes.
  • Multiple PRT loading areas operated in parallel can handle stadium-sized crowds, so a crowd getting off a train is easy.

 

On the "world's best general-knowledge PRT website", the subject of capacity is well-covered. Additionally, the particular problem of demand bursts such as when a stadium empties ("But what about crowds?") is examined. Using their estimate of 15 seconds as the average time between PRT departures, 10 berths would be required to clear 120 people in 3 minutes (4 people/min/berth X 3 min X 10 berths = 120 people). It's likely that 2 stations (with 5-7 berths each) would serve BART and Caltrain stations. If more than 120 people want to exit the station via PRT (instead of the other available options) - or 3 minutes is deemed too long - additional stations could be built for less than $1M each.

This video shows how 2 PRT stations can clear as many people as 2 BRT stations.

This 2.5-minute video shows how 64 passengers can board at a single 4-berth station in just 2 minutes, or the equivalent of 1920 passengers per hour.