It is, the number of trips people can take that do not require the use of the service road network (e.g. because they are hauling work items, hockey moms, etc.) and therefore could be diverted onto mass passenger travel (vs. cargo, which also is important and bears directly on the question).
To give a sense of the numbers involved and why we care: if your service road network is not overloaded, then automobile transit, motorcycles, etc. clearly are economically effective to the transit purpose. To suggest by numbers: if you consider that the cost per one-way trip is $10, therefore a round-trip is $20, then if you took a roundtrip every day, your yearly cost would be $7300. While some occupations would require more spending, the rough number is clearly within the line of single-person transportation with a small amount of cargo, including depreciation of a used car. Those numbers sound large, until you realize that many megalopolis mixed-mode (including heavy rail) networks, such as those that include New York/Manhattan and London, can have per-boarding (effectively one-way cost) of ~$10 per trip (though their capacity is higher). Particularly, we require a service road network for many other purposes, and so while that network is subsidized through other means, the marginal cost to carry this primarily passenger traffic via personal automobile is not grossly higher than what would be provided through the average mass transit system. (22 wards of Tokyo and cattle car trains being a different story)
The question of “why would you buy/operate a billion dollar improved transportation network” therefore turns on the displacement of trips/passengers/light cargo, from the service road network, onto the proposed passenger transit system. That is to say:
- There must be a large number of passenger trips that clearly cannot be handled via the service road network (even when you e.g. mark off express streets vs. locals)
- Those passenger trips must be handled in a time-efficient way relative to a direct route (otherwise you would take e.g. a ring road/beltway – remembering that almost all cities/denser metropolitan areas worldwide are no more than 60 miles from one end to the other)
That is, the trips that meet those two criteria, are the potential ridership for any mass transit system. The trips such as:
- All contractor-type where a large amount of cargo/material/parts/tools has to be sent out with the technicians
- Service professionals routinely making individual house and business calls
- Trips that go to and from areas of the service road network that are not overloaded
- Deliveries, disposal, etc. where large trucks taking on or disgorging cargo are involved, especially when they make many stops
are not efficient to perform via mass transit, and hence we should not use them as the justification/planning for such networks.
Because we are talking about the economic upper bound, we also should consider factors such as:
- The ability to displace errand trips with home deliveries
- Remote work and the population that can perform it
- Changes in land use such as re-centering housing closer to office space
The first point is that not all of the population is employed: on the order of 60% of people in Western countries work. You will have children commuting to and from schools, day cares, etc. but it clearly is uneconomic for such travel to be across large distances; consequently, those trips are unlikely to be of the type that readily displaces onto a metropolitan transit network as a base of support (but in some cities you do have some use of those networks for school commuting).
The next step is to take out the office workers, contractors who use their own vehicles, etc. out from the 60%. Here, there has to be a specialization of the number that does not reflect population aggregates, since e.g. in rural areas, even farmers are routinely driving out for things. Moreover, even farmers that are relatively adjacent, are hauling cargo to markets, etc. So the population that accumulates eligible trips, typically (not always, e.g. in developing countries) is working in light industry, research labs e.g. with prototypes, and other such areas that have a place of business. Some number of construction workers can use the transportation network, but not all. Hence, it is difficult to make an accurate assessment, but if you sum up various numbers in something like a Bureau of Labor Statistics spreadsheet, what you come up with likely will be on the order of 50-60% of the workforce, that has to commute on a schedule, particularly one that involves peak times. In turn, that relates to 30-40% of the population that could be loaded onto mass transit, for occupational (not recreational) purposes.
Moreover, since we are talking about an economic upper bound, we have to consider, given the large costs of any transit mode beyond a lower-capital bus rapid transit (BRT), that we would move people close to their places of business, thereby eliminating or drastically shortening the work trip. That reduces many of the healthcare workers’ trips particularly, so that now we would be looking at something closer to 20-30% of the population for which we could get economic benefit out of mass transit.
To put numbers on it: in a city center with population of 1 million, the maximum number of commuting legs that could be moved onto mass transit is about 600,000; 300,000 in two peaks. Considering that it is reasonable that the rush hour can span 2 hours, that means a peak load of 150,000 riders per hour, plus other traffic, so likely something like 200,000 rider capacity that could be placed on the system per hour.
The next question is how that load gainfully could be placed onto the transit network. Not to be a transportation expert here, but:
- You can push roughly 2,000 cars an hour in a highway lane
- There is a huge amount of variation in buses and BRTs, but it could be anywhere from 1000 people per hour to upwards of 10,000, depending on how much priority and capital is given to the line
- Light rail would typically be in the 10,000 people per hour class
- Heavy rail would typically be in the 20,000-30,000 people per hour class, although there are some lines that when packed, can move more than 50,000 people
There are some practical limits to the efficiency of each segment; you can run a line from one end to the other, but the line will saturate early on, dump people near the middle, and then be underutilized for the remainder of the line. You can have an even distribution of people along the line, but that likely would mean you are not loading the line to capacity at any point.
Consequently, you cannot build one heavy rail line (at costs of ~$50 million/mile for something uncomplicated) and put the entire peak load onto it; fully to relieve load (which is not to say you would have to do this in every case to make the effort worthwhile), you would need to build 10-20 such segments. Because there would need to be some level of connectivity between the segments, you will have to build/operate cross-lines, so that your network converges into a hash pattern.
Here, we also must factor in the geographic density of people that have to access the network. To load 200,000 people, the network has to cover a large enough area and density so that this many people will be able to enter the network. However, since some of those people are living close to work, working remotely, school, etc. and hence will not be using the system at peak, following the above points about the percentages of people taking peak trips, you require more like 20-30 square miles of coverage in your network, even if the population density is 20,000 people per square mile. Continuing further that their destinations, light factories and such, to be linked via the network, force a further decrease in the effective density, the network has to grow more, maybe to 30-40 square miles. The exact span of the network is not critical; but if a region is 30 square miles, that means it has 5-6 miles of border on either end, in addition to internal connections. All of that border and internal connection represents service road capacity – which certainly is needed, if you are dealing with population densities that high. If we consider that the city blocks are 1/8 of a mile, this means that the service road network alone has at least 80 different cross city roads in the grid. If you consider that a BRT with theoretical capacity for more like 5000 people, might move 2500 people per hour, you could move those 200,000 people with just set asides on the service road network for the BRT. Hence the additional investments in e.g. heavy rail, would be to improve service speed; but such improvements would be marginal.
Consequently, the light and heavy rail concepts only are economically effective when:
- Dealing with a grossly mismatched commuter structure, especially one with limited telework
- Geographic or political boundary constraints, particularly rivers
- The leisure system e.g. entertainment districts, has sufficiently built up to generate said number of trips
and therefore the economic correctness of the resource allocation decision typically turns on the willingness of the potential transit riders to pay for their services; that is, the rail line should be an essentially profitable enterprise, aside from the need for eminent domain and similar facilitation.