Through-routing means that a route designed to carry people to or from a downtown doesn’t end in the downtown; instead, it flows across downtown and out the other side as a different route.

In Alon Levy’s vision of New York area commuter rail, for example, the trains flowing into Manhattan from Connecticut (red on the map below) would flow through the city and out onto southern lines in New Jersey and vice versa. Through-routing means that downtown is in the middle of a line, not at the end. You can do the same thing with any kind of radial transit service, bus or rail. (Editor’s note: Paris’ RER networks, or London’s Thameslink or Crossrail, are similar examples.)

Alon Levy’s regional rail plan for New York, originally published and explained on The Transport Politic.

Through-routing has four colossal advantages:

Some people are actually going from Connecticut to New Jersey, for example, and through-routing lets them make this trip without changing trains. More commonly, a lot of people from the north (i.e. Connecticut) are going to southern parts of New York City, while a lot of other people from New Jersey are going to northern parts of the city: a through-routed system serves both groups, which are briefly on the train at the same time in the city.  

Because of the decentralised structure of Paris, lots of Parisians are riding across the city to the far side, in both directions, so the RER’s through-routed structure is absolutely essential to avoid forcing huge masses of people to change trains. 

  • Reduced need for terminus facilities on expensive downtown real estate, and thus potential for higher frequency.

Ending a line downtown means having facilities to store a bus or train for at least a few minutes, consuming expensive space.

Trains typically reverse direction at an end-of-line station, so the driver needs to close her cab, walk the length of the train, and get herself set up on the other cab; she may also be entitled to some break time. A train occupies one of a limited number of tail tracks while this is happening, so this function becomes the limiting factor on the frequency of the whole line. And downtown, there are lots of physical and cost constraints on station design, so you almost never have as many tail tracks as you’d like.

Buses need space to turn around and their drivers, too, are entitled to some break time at the end of a trip – so end-of-line stations on frequent services need space for a number of buses to pile up in a first-in-first-out queueing arrangement. All this takes a lot of space, and this space is a lot cheaper at the end of a suburban line than it is in the middle of downtown. 

  • Fewer line ends, for reduced operating and capital cost.

The time it takes to turn a bus or train around, and provide the driver break, is usually not related to the length of the line. Through-routing two routes eliminates two ends-of-lines, which reduces the cost, both operating and capital, of those inefficient turnaround movements. Often, through-routing two lines actually reduces, by one or two, the number of buses or trainsets required.    

  • Fewer vehicles downtown providing the same service.

Sometimes, too, the pre-through-routed lines overlap in downtown, so the through-routing eliminates that overlap. Instead of having a bus dropping off passengers interacting with another bus picking passengers up, you have one bus dropping off passengers and picking them up at the same time. For buses especially, downtown street capacity is a very limited resource in big cities, and through-routing helps economise on it.

So why isn’t there more of through-routing? Here are five reasons.

  • Unbalanced markets on the two sides of downtown.

There’s never an exact one-to-one match between routes approaching from one direction and those approaching from the opposite direction.

For example, San Francisco’s downtown is on the bay at the northeast corner of the city – so there are no routes extending north and east that could be paired to routes flowing south and west. To the extent that such through-routes have been created (e.g. San Francisco’s Muni Metro T line) the result is a circuitous approach to downtown for one of those lines.  

  • Excessive line length.

The probability that your train or bus is delayed is directly related to how long it’s been running since it last had an end-of-line break. (When a vehicle arrives late at the end-of-line, its break time is reduced, so that it can leave on time, or at least not as late.)

Through-routing makes lines longer, so it can compound this problem. This is obviously more of an issue in services that are exposed to more causes of delay, such as services in mixed-traffic and services with driver-administered fare collection.

  • Jurisdictional barriers.

There are plenty of cases where through-routing would be in order but the two sides of downtown are different jurisdictional turf. They may be different states, as in New York, or different transit agencies.

In the UK and Australia, they may be different private operating companies – and although both are government-subsidised, UK/Australia governments tend to defer to private companies about network design, so you often get designs that reflect company turf boundaries rather than efficient use of subsidies or meeting the needs of the customer. (London is an exception; some Australian states are also working on this problem.)

  • Infrastructure barriers.

Your great-grandparents’ jurisdictional barrier is often your infrastructure barrier, and New York City is the obvious example. Because each commuter rail line was designed by a separate entity, and each of these entities was thinking only about getting people into Manhattan, the terminal stations are not physically connected in the way you’d need in order for trains to flow through.

This is the cause of most of the capital expense in Alon Levy’s proposal for New York above.

  • They just haven’t thought of it.

If your transit agency has non-through-routed lines, and you can’t figure out why, send them a link to this post – and ask them.

Jarrett Walker is an international consultant in public transit network design and policy, based in Portland, Oregon. He is also the author of  “Human Transit: How clearer thinking about public transit can enrich our communities and our lives”, which you can buy here.

This article was originally written for his blog in 2009, and is reposted here with permission.