Human Transit

What’s wrong with the word transfer as used in public transit?  See my article here.

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.

The last section of Chapter 1, “Personal Mobility: The Freedom to Move” should be understood as a humanistic argument, not a technical one.  My resistance to the word access, and my sense that we still need the word mobility, arise from their connotations, not just from technical meanings assigned to them. 

In any technical discourse — such as the language of an academic study, professional report, or legal document — you can define a word however you want as long as you state that definition.  If you then use the word consistently, you can communicate with others who (a) recognize the discourse as technical and (b) understand the rules of technical discourse, including your right to define a word in a specific way that excludes other connotations the word may have.   

Over time, certain meanings may come to be agreed upon within a technical discourse – such as academic conversations about sustainable transport or urbanism.  The definitions of mobility and access that I present in this section (mobility is how far you can go in a given time, access is what you can do in a given time) are pretty close to accepted at least among sustainability-oriented academics and theorists.  Again, Litman offers a good presentation of them.

Still:  A massive amount of the confusion in transit debates arises from technical discourse being heard by people who can’t be expected to know how the words are defined.  If the word in question has an emotive meaning, that will get in the way of understanding the technical meaning.  For example, as I explore in Chapter 3, the term captive rider is often used in technical discourse to mean a transit passenger who doesn’t have the option of driving.  Technically, you can define a word however you want for the purposes of an argument, but if you start saying “captive rider” to the public, well, it sounds like you’re thinking of these transit customers as prisoners, which is both false (many people without cars will forge other options if transit gets too awful) and risks stimulating lazy thinking even among some professionals (“those people are captives, we don’t have to care what they think”).

When you use a word with strong emotive connotations, like captive, inside a technical discourse, all may be well if you’re only talking to people who understand your technical meaning and know how to set aside emotive connotations.  But anyone hearing or reading your conversation is likely to hear the emotive connotations, and may well perceive them as louder and truer than the technical meaning.

I have this concern about the world mobility, which is the opposite of my concern about captive rider.  Mobility has intensely positive connotations because we also use the word to mean the body’s own degree of freedom – “ease of moving about” is one of the destinations I cited.  Access also has positive connotations but they are more abstract.  Because I want to connect with a broader public, I use the term personal mobility because it describes the sensation of being able to go places.  It refers, quite literally and obviously, to a freedom.  And we have to value freedom. 

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My specific resistance to the word access arose out of conversations with streetcar advocates in which they insisted that the redevelopment benefits of streetcars – which would cause housing and destinations to be built closer together and thus reduce total travel demand – constitute an access improvement that compensated for the limitations that the mixed-traffic streetcar has in providing mobility – getting you where you where you’re going.  Considerable urban planning theory is advocating discarding the goal of mobility and replacing it with access.

There is no question that good urban redevelopment improves access  (by putting destinations closer together) and thus reduces the need for travel of any kind.  And there’s no question that this is an ideal outcome for the purposes of sustainability.

But putting access in conflict with mobility – understood as a freedom – creates several problems.

Many technical uses of these terms, including Litman’s, are talking about mobility and access as outputs, results of human behavior, and yes, if access is understood as “getting to something useful” while mobility just means “travelling a distance”, then mobility can turn into useless measures of sheer movement.  If I take the bus to a grocery store three miles from my house, and suddenly another store of the same chain opens a block from me, the transit system will lose a rider, less travel will occur, and yet everything is better from any sustainability perspective. 

But to most ears, mobility isn’t movement as an output.   It’s the sensation of feeling able to move.  To the ordinary ear the opposite of mobility isn’t access, it’s immobility, which means being trapped or locked in place.  Defining mobility as sheer movement is thus a very technical definition that’s likely to be misunderstood outside its technical context.  In particular, to people with an everyday concept of mobility, disparaging mobility can sound like disparaging freedom.

Second, of course, we have a long-term vs. short-term problem.  Redevelopment outcomes of a transit project are longer-term and proceed through several unreliable steps.   Mobility outcomes are immediate.  If you actually reduce mobility in order to build something that you think improves access, you’re impeding the ability of today’s customers (and voters) to get where they’re going.  This can happen, for example, in cases where a functioning transit network (perhaps run by buses) is made less functional by the demands of a new rail project.  Not all rail projects do this, but it happens.  It’s not wrong if everyone involved is aware of this impact, but that’s rarely the case.

Finally, of course, transit deserves to be understood in its own terms, in light of its own purposes.  Treating transit as though its only purpose is to stimulate redevelopment is like treating plumbing as though its only purpose is to sell real estate.  Plumbing has a more immediate purpose, which is to transport a range of liquids safely through a building, making them available as required.  Likewise, transit has an immediate purpose, which is provide personal mobility within a city.   From that purpose follows many benefits – social, environmental, and economic – but we have to respect that these benefits arise from transit doing its own job well.

Transit that doesn’t work in the short term is a big risk for the long term.  And in the short term, we have a city full of people who need to get places now.  These people value their freedom, not just the outputs of their behavior.  They can appreciate the WalkScore travel time map because it is a map of freedom.  We need to be able to talk about that. 

Main points of this article:

• In wealthy countries, transit operating cost is mostly the cost of labor.  This is usually around 70% of the total cost of operations. So:
  • Smaller buses are not much cheaper to operate, unless you pay the driver less.
  • Faster service can cost less to operate, because drivers are paid by the hour rather than by distance.

If you’re going to form coherent views about transit, you have to understand what transit service costs to operate.  There are four parts to this cost:

• Time-based costs vary based on how many transit vehicles are operating and for how long.  The dominant time-based cost is the wages and benefits of the driver and any other on-board employees, which we pay for by the hour.
• Distance-based costs vary with the odometer reading of the transit vehicle.  As in cars, most of transit’s maintenance and fuel costs are distance-based.
• Fleet-based costs vary with the number of transit vehicles owned.  Fleet size is based on the number of vehicles needed to run the most intensive part of the service day, typically the commute period which transit planners call the peak.  Fleet size drives some maintenance cost, but it main impact is the cost of the vehicles themselves, and of the facilities needed to store and maintain them.
• Finally, there may be some administrative costs unrelated to any of these, though in fact most administration costs are roughly proportional to the other measures of size.

The mixture of these costs varies, but in the high-wage developed world, you can go far by focusing on just one element: the time-based costs.

It’s All About Labor

Driver labor, and related time-based costs, are the dominant element – often 70% or more — of transit operating budgets in the developed world.

Most transit vehicles require one transit employee on them to operate, whom I’ll call the driver.  This person’s job description varies quite a bit based on the technology and operations style of the service.  Obviously, driving a train is a different job from driving a bus, and you may or may not be interacting with passengers and dealing with fares.  If you’re the employee sitting at the front end of an automated San Francisco Bay Area BART train, you’re not really driving so much as watching for trouble.  Many North American transit agencies prefer to call the on-board employee the operator, but that term is too vague to be useful here.  So regardless of their exact job description, I’ll call a single on-board employee a driver.

For these one-employee-per-vehicle systems, then, we can understand the basics of operating cost by focusing on how drivers need to be working, and for how many hours.  You may sometimes may hear a transit planner say “if we do this, we’ll save a bus.”  What the planner really means, though, is that we’ll save a driver.  In the high-wage developed world, the driver, not the transit vehicle, is the basis of operating cost.

This is the main reason why transit agencies don’t save much money running small buses rather than large ones, as advocates of small buses often assume.  If an agency does talk about small buses as being much cheaper to operate, they’re probably referring to a difference in driver wages.  Often, some mixture of labor contracts and licensing requirements can allow small-bus drivers to be paid less than large-bus drivers.  In that case, the smaller vehicle is cheaper to operate only because of the pay scale, not because of any feature of the vehicle itself.

So how many service hours does it take to run a line?  This is the one equation that we can’t avoid.  Let’s ease into it.

Service Hours = Span x Vehicles (and drivers) Required.

If a line requires five vehicles (with drivers) in service to cycle the line, and it runs for twelve hours, that will be 60 service hours, 5 x 12.  Easy.  But how do know how many vehicles we need?  Here’s the crucial equation:

Vehicles (and drivers) Required = ROUNDUP (Cycle Time / Headway)

Suppose a line takes 20 minutes to run, in service, from one end to the other, including break time for the driver.  That’s 40 minutes round trip, so the cycle time is 40 minutes.  Every 40 minutes, a vehicle has completed a cycle of the line and is ready to start another.

So if we only wanted service once every 40 minutes, or longer, we’d just need one vehicle to drive the line.  That elapsed time between consecutive trips on a line is called the headway, and it’s the main measure of frequency.  (Remember that high frequency means a low headway.)

Now suppose we want service to come at a 10-minute headway – that is, service every 10 minutes.  We’d need four vehicles and drivers to drive the 40-minute cycle.  Do you want service every 5 minutes?   That will be eight vehicles, and drivers.  If you double the frequency (by halving the headway) you’ve doubled the operating cost.

Lumpiness

Now, suppose we want service on our 40-minute cycle to come every 30 minutes.  We’ll need two vehicles and drivers to do that: the cycle time is 40 minutes, the desired headway is 30 minutes and 40/30 rounded up is two.  But in this case, the cost of running a 30-minute frequency is the same as that of running a 20-minute frequency.

In short, transit’s operating cost is lumpy.  You cannot hire a fraction of  driver.  So at low frequencies, you often end up with inefficient patterns due to the relationship between headway and cycle time.  If we run a 20-minute headway on our route that cycles in 40 minutes, we’ll need two vehicles.  If we run a 30-minute headway, we’ll still need two vehicles, which means we’re really paying the drivers for a 60-minute cycle.  In that case, the drivers will have 20 minutes of extra break time every hour.  Drivers may like these shifts, but transit managers don’t.

Lumpiness has important consequences when designing lower-frequency networks, such as local bus routes in low-density suburbs.  In these cases, good planning designs routes to be of a certain length, so that they will run an efficient cycle. If our network of local routes is meant to all run every 30 minutes, for example, we try to design routes that cycle in 29 or 59 minutes, but not 31 or 61.

A small deterioration in speed can cause sudden big changes in operating cost.  If we’re running 30-minute frequencies on a route that cycles in 29 minutes, that will require one vehicle.  But if for some reason the line slows down just a little, so that it now cycles in 31 minutes, we have to add a whole additional vehicle and driver, doubling the cost of running the line.  A mere 7% increase in the cycle time has become a 100% increase in operating cost.  In that case, a planner may try to redesign the route to make it shorter.

Lumpiness is an important reason that route design and scheduling need to be done together – especially in low-frequency networks such as those of small cities, outer suburbs, or in the middle of the night in big cities.  Some transit agencies try to think in separate, rigid, non-repeated steps:  First, planners design the route.  Second, we drive it and establish the running times.  Third, schedulers write the schedules.  Thinking that way makes it impossible to optimize the efficiency of schedules and connections.  The three tasks have to work together, or at least in several cycles of revision so that planners can revise their structures in light of the apparent running times.

The Elements of Cycle Time

Finally, let’s take cycle time apart.  It has three parts:

• The length of the line, in km or miles, round trip.
• The average speed at which the service can operate, including passenger stops.  This speed, times the length of the line, is the running time.
• Added factors called layover and recovery.  Technically, layover means driver break time, which is usually specified in labor agreements, while recovery means time added to the schedule so that a late vehicle has a chance to catch up to the schedule.  In practice, these two kinds of time are usually added together as one factor.  For example, an agency policy or labor contract might require adding 10% to running time, for layover and recovery, to generate the cycle time.

The One Equation

So here’s how it all fits together.  Operating cost varies mostly with service hours (technically called revenue hours in North America), and these hours are figured like this (click to enlarge and sharpen):

Too hard?  Fine, just remember this:

• Every increase in frequency is an increase in service hours, and thus in operating cost.  If you want to increase service on a line from every 30 minutes to every 15 minutes, that will double the cost of running the line.  This is why most transit agencies would like their service to be more frequent, but have trouble affording that frequency.  We explore frequency in Chapter 7.
• Every increase in average speed is a savings in service hours, and thus in operating cost.  If we can cut the cycle time of a line by 25%, that cuts its operating cost by 25%.  This is why transit agencies are always trying to control delay (Chapter 8).
• At low frequencies, operating cost is lumpy.  Because you can’t run a fraction of a driver, small differences in speed or frequency can create large differences in operating cost, if the overall frequency is low.

Frequency and speed are both great things for the customer.  But for the transit operating company, frequency costs money, while speed saves money.   When discussing the hard choices surrounding frequency and speed – choices that really pervade every part of this book – it’s essential to keep that in mind.  Frequency costs; speed saves.