The problem of wide corridors

One longstanding characteristic of public transit, is its linear nature.  Transit systems are generally (and ideally) organized in lines, on which some vehicle travels progressively from point A to B, and then in the reverse direction.  Not all such lines are geographically straight, of course, and many systems also feature loops with no discernible endpoints--but even in those cases, the vehicle makes progress towards some goal or direction, stopping along the way to let folks on or off.  Vehicles cannot be in more than one place at a time, nor do they (Amtrak's Empire Builder notwithstanding), split into two to serve different sets of destinations simultaneously.  Lines can branch, but you lose frequency on the branches--individual busses or trains running on the line can only serve one branch or the other, not both. (see Jarrett's excellent presentation for more on branching, and numerous other topics). 

But what happens when you have a wide corridor--a geographic area which is linear in shape, but where the key destinations don't lie in a convenient linear arrangement?

Examples

67 Map, courtesy of TriMet

Consider TriMet's 67 Jenkins/158th route, driven by our good buddy Al (who wants it known that he doesn't speak for TriMet, or his union, or anyone else but himself).   This route run between PCC Rock Creek and Beaverton Transit Center.  The primary routing is kind of zig-zag (the as-the-crow-flies direction is diagonal, but the streets for the most part are not), but that's close enough to linear for our goals.  However, the route contains two backtracking deviations, or as I prefer to call them, "cherries"--places where the bus leaves the main street, goes some distance to reach an important destination, turns around and backtracks, and then rejoins the primary service corridor.  One of these cherries, serving the Cornell Oaks business park, will be eliminated in September (a move praised by Jarrett).  The other, a shorter-stemmed cherry serving the Merlo/158th MAX station, remains in place. 

Or consider the proposed Barbur Boulevard corridor for a new MAX or BRT line.  Between downtown Portland and Tigard, there are several highly-important destinations (Tigard TC, Washington Square mall, PCC Sylvania, Barbur TC, Hillsdale, OHSU-Marquam Hill)--all of which, except for Barbur TC, aren't on Barbur.  A simple line parallel to the existing boulevard misses most of the action in the corridor--which is why various proposals for tunnels get floated about.  (I have no objection in principle to the proposal, but I seriously doubt that TriMet can afford it).

Or consider the Washington Square WES station.  Of course--there is no such thing; the nearest station to the mall is the Hall/Nimbus station (which isn't even located on Nimbus Avenue).  The mall is located about 2000' to the east, across a busy freeway and a major thoroughfare.  Connecting bus service is actually pretty good (between the 76 and 78, and the occasional 43, up to six busses per hour connect the Nimbus Avenue stop with the mall--that's more frequent than WES itself), but the transfer to the bus lines plying Hall Boulevard is a bit inconvenient, especially in bad weather.  WES was constrained to operate on the existing freight tracks, which lie on the opposite side of the freeway--but the mall is one of the most important destinations in the corridor. 


Wide corridors--sets of destinations which are clustered around some linear corridor but not sufficiently close enough to it (or across some barrier)--cause lots of problems for transit.  Serving them adequately requires diluting available service hours over greater numbers of route-miles--resulting in a net decrease in service quality.  In some cases, rapid transit lines may not be practical due to dispersed demand--and in the worst cases, the only sorts of services which are practical are inefficient "social service" routes.

What to do, what to do

Some possible solutions to wide corridors are discussed below.  These aren't solutions in the sense that they solve the fundamental problem; however they are different ways to provide service in a difficult situation.

Courtesy of thetransportpolitic.com

• Longer routes.  One way to serve the major destinations in a wide corridor is to lengthen the route.  The "cherry" mentioned above is one case; serpentine routes such as a proposal for Maryland's "corridor cities" are another example.  Either way, lengthening the route to cover the greater distances imposed by dispersed destinations effectively slows down the line, making it less attractive to riders.
• Splitting and branching.  If most of the trips are between destinations along the corridor and destinations elsewhere, branching may be an option.  Branching reduces frequency in the branches, of course, so doing so may also make the service less attractive; it makes travel between branches more difficult.   If the line requires fixed infrastructure, such infrastructure needs to be built on both branches.  One useful option here, though, is "open BRT"--the trunk can often be built to high busway standards, with the branches running in mixed traffic--if the branches are short, this arrangement often results in service of reasonable quality.
• Transferring.  One reason "grid" or "fishbone" arrangements are useful is that intersecting lines can be used to provide service to destinations which are slightly off-track, while keeping the trunk line straight and fast.  This depends a lot on the quality of the transfers, obviously--if getting the last mile requires a 20-minute wait on the side of a road in the rain, this is not an attractive option.  If, on the other hand, transfers can be timed and waiting occurs in a protected space, this is an excellent option.

Mid-levels escalator (HK), image courtesy Wikipedia

• Dedicated connecting service.  This is a subset of transferring, but the previous paragraph assumes a generic line that happens to pass close to a point of interest, but serves much else beyond.  A dedicated connecting service, on the other hand, provides point-to-point service between a station on the trunk line, and a given destination.  Such services may take the form of shuttle busses (such as the shuttles running between PCC-Sylvania and Barbur TC), a dedicated people mover (the Portland Aerial Tram functions in this fashion; even though its primary purpose is not connecting the Marquam Hill campus to the Portland Streetcar), or even a special-purpose transit line (such as the "Disneyland Subway" in Hong Kong).  With a dedicated service, there's ample room for creativity--the mode used might be outside the norm of services typically provided by the transit authority--and might not be funded by the transit authority anyways.  The dedicated service might even be passive--a skybridge or tunnel where people walk, for instance, but one which lets users avoid weather, obstacles, traffic, or difficult terrain. Generally, a destination has to be fairly major to justify this sort of infrastructure, but things like gondolas, shuttles, elevated walkways, public elevators and escalators, moving sidewalks, and such are all pressed into service to connect a transit line to the places people ultimately want to go.  (Many of these things additionally benefit pedestrians who aren't using the transit line as part of their trip).

Of course, as numerous writers cited above have pointed out, the best way to deal with this problem is to avoid it.  Good land-use planning can avoid widely-dispersed destinations--and result in land-use outcomes which are easier to provide service to.  The benefits accrue to well beyond transit--many other public goods, from police and fire protection to utilities, are cheaper and easier to provide if confined within a smaller space (It ought to be noted that automobiles, on the other hand, frequently benefit from dispersion--mainly due to their need for parking when not in use).  But there are many existing wide corridors which need service; and many popular corridors can be victimized by their own success--attracting development which widens the corridor as the closest lots become scarce and/or expensive.