The terms “flexible route” or “demand-responsive” transit or “Dial-a-Ride” refer to services that can vary their routing in real time according to the demands of customers. These routes are the subject of a vast area of research and practice, summarized here.
“Flexible” is a tricky word, because it may appear in transit contexts with several meanings. For example, you may hear that “buses are flexible, compared to rail” because it’s easier to change a bus line than a rail line. That’s a different meaning. Flexible route means a routing that changes in real time based on what a customer wants right then. On a flexible route, the 6:00 trip may go this way, while the 7:00 trip goes that way, simply because someone requested it to. Flexible service responds to some kind of customer communication, usually a request send by phone, text, or internet. In some cases, regular customers have “standing reservations” to travel at a particular day and time every week. In some cases, you can also board a flexible bus at an interchange (also called a transfer point) and state your destination to the driver, who plots a course spontaneously.
Those descriptions make flexible services sound like taxi services, and indeed, flexible routes are intermediate between taxi services and fixed routes. The difference from taxis is that flexible services may still require you to share the vehicle with others, and make intermediate stops based on their needs. (If you never ride transit you may still have encountered airport shuttle systems that work on a flexible-route principle.) This shared-ride feature is why flexible routes count as public transit in the developed world, while typical taxi services don’t. (Caution: taxi vehicles and drivers may sometimes be hired to provide flexible services, but in this comparision I’m referring to the typical taxi operation that serves one customer or party at a time.)
There are, broadly speaking, two reasons to run flexible rather than fixed service:
- Individual Needs. For reasons of disability, some customers need a service that pulls into the driveway and assists them in boarding and alighting. In the US, these services are called paratransit and must be funded by the transit agency. They are usually run with small vans and are inevitably flexible.
- Efficiently Serving Sparse Demand. There are a few places where a flexible service will serve demand more efficiently (in terms of service cost) than a fixed route.
The first reason will continue to drive the need for flexible services geared toward disability and other special needs. The second reason is more specialised than it looks.
The most important thing you need to know about flexible service is this: Cost-effectiveness of public transit lies in how many people can travel on the same vehicle with the same driver. The ratio of passengers to drivers is the most fundamental measure of effectiveness because transit operating costs are dominated by driver labor. In North America, for example, we talk about productivity as passengers carried per revenue hour — where “revenue hour” means “one vehicle operating with one driver for an hour.” If you care about transit that serves lots of people at a reasonable cost, you must be obsessed with that ratio.
Taxis are obviously dreadful on that score, intentionally carrying only one or a few people at a time, which is why we don’t think of them as transit. At the opposite extreme, a fully loaded rapid transit train may carry over 1000 passengers an hour with a single driver. A bus on a highly performing line can hit 100 passengers per hour in a very dense market, while 30 passengers per hour is considered pretty good in most suburban contexts where demand is more sparse.
But if you do something different for every customer, as a fully flexible route does, you’re not going to carry more than 10 passengers per hour. Physically, you just can’t, even if the demand is there. Ten passengers per hour would mean that each passenger requires only six minutes of the driver’s time, counting the time to reach their location and (in some cases) assist them in boarding or alighting. It also assumes that exactly ten people per hour would want such a service, when demand is obviously lower at some times. That’s why most flexible service performs in single digits.
Flexible services can be reasonable transit investments because they can carry multiple passengers or parties at once. However, flexible services are usually less effective than fixed routes, because they reach their capacity limits at such a low level, usually before 10 boardings per hour.
Flexible service can sometimes be the most productive option for a low-demand market, but in general, we make services more cost-effective by making them less flexible, and vice versa.
Paratransit services focused on the disabled have much lower performance (rarely even 3 passengers/hour) because they need the most flexiblity. The customer may need an unpredictable amount of time to board and alight, for example, and pulling into a driveway invokes more schedule uncertainty than stopping on the street.
A “flex route” for the general public will be more productive precisely by being less flexible than that. Rather than pulling into your driveway, it may stop only on the street. Rather than deviating anywhere in an area, it may go only on pre-approved optional routings, so you’ll have to walk to one of those. Every time we reduce flexiblity in this way we increase a driver’s chances of serving more people per hour.
Having said that, there are some very specialized cases where a flexible service will be more effective than a fixed route in a particular area. Here’s a classic example:
The east-west magenta line is a straight fixed route, but there are two deep “pocket” neighborhoods whose street patterns are cul-de-sacs emerging only at this point. In this case, we can cover both by saying that we’ll deviate into one or the other, but not both, based on customer requests. Whoever requests the deviation first gets it. As a result, the line ends up covering more of an area than the bus can actually cover on any single trip, which could result in slightly more passengers.
But only slightly. Most commonly, what we achieve with flexibility is not better productivity (passengers per hour) but simply more coverage. Remember, most transit agencies have a standard that says something like:
___% of our residents/jobs will be within __ distance of transit service.
This is called a coverage standard, and it is usually in tension with the goal of maximum ridership per hour, because it requires us to run into many areas despite relatively low ridership potential. Chapter 10 is all about this conflict and the real questions that must be asked.
Often, then, we may deploy flexible routes because they cover more area than we can actually cover on any one trip, and therefore help us satisfy a coverage standard. This may have nothing to do with increasing ridership per hour.
Assumptions and Distractions
The above argument is all geometry. If you encounter data about a flexible service that seems to contradict these generalizations, check for the following:
- Differences in operating cost per hour of service. Overall subsidy figures for flexible service may seem to tell a different story in cases where flexible service is cheaper to operate than fixed routes. This can happen if flexible service falls outside of the purview of labor contracts, while fixed routes are firmly inside it. If you can get much lower costs per hour to run flexible service (say, by contracting them out to a taxi company that has no union) you’ll get a better cost-effectiveness measured in dollars. You can also get some operating cost savings simply from the smaller vehicles. Neither of these issues, however, is an intrinsic feature of flexible routing. Labor costs result from local labor conditions and union agreements, not routing style. Smaller buses can be either fixed or flexible. So these are different distinctions.
- Differences in fare. If viewing through the lens of profitability or subsidy, you can obviously charge more for flexible service and thus make it perform better. That’s a feature of fare that’s compensating for the intrinsic geometric limits of flexible service. It doesn’t contradict the basic geometry.
- Parties travelling together. Often, an entire flexible route will show a better productivity (passengers/revenue hour) because of groups of people travelling together, essentially consuming the same amount of the driver’s time as a single passenger. The real test of flexible service, as of any transit, is how well it deals with customers travelling separately.
- Non-flexible elements. Many flexible routes are partly fixed. For example, a bus may run along a fixed route for a while and then begin deviating in response to demand, as in the “two cul-de-sac” example above. In these cases, of course, the fixed portion of the route may achieve higher productivity, so the flexible portion must be isolated to be assessed.
You can spare yourself a lot of confusion about flexible service by keeping in mind the physical facts of the matter: Driving a special routing to respond to a customer request takes more of a driver’s time than picking up a customer along a fixed route. Since we pay for service mostly in hours of labor, we have to care about how many passengers we’ll serve with each labor hour, so flexible service is intrinsically limited on that important score. That’s why when flexible routes near their (very low) capacity limits, we usually try to turn them back into fixed routes.
So when it comes to the challenge of serving a low-density area like Sparseville, flexible routes may have a role, but they are simply a different way of serving a very low-ridership area, such as low-density with labyrinth street patterns. In a few of these areas, a flexible route may deliver 6 boardings per hour while the fixed route is only carrying 5. But meanwhile, Denseville is probably delivering 20-100 boardings per hour in its services — levels of ridership that are physically impossible for flex routes. So flexible routes do not change the reality that low-density Sparseville will deliver fewer passengers/hour than Denseville. Sparseville’s service must be justifed based on Coverage policies, not just ridership.