157> Got Bridgework?

Let’s start with a bit more about minimal systems.  In my recent post, “For Example,” I experimented with the limits of capacity within the confines of a minimum of track (4 miles) and (4) stations.  The reason such exploration is necessary is because for PRT to be adopted by a city as a viable mode of transportation, it must first pass the litmus test of being the cheapest solution to an immediate problem.  Bureaucracies aren’t known for being far-sighted or visionary, and individual decision makers need to cover their butts every step of the way.  They are, after all, custodians of taxpayer money. Therefore the fact that PRT should be set up as a distributed system with many small stations really doesn’t matter.  To these people, at this stage, any small station is one that should be eliminated to save a few bucks.  Their mindset is essentially reactionary - “If it ain’t broke, don’t fix it!”  Widening roads is their first instinct, not making a pedestrian-friendly city.  The exercise in “For Example” imagined stations with mythic levels of pedestrian traffic, just to examine the logistics.  What is not so imaginary, though, is that only moving great numbers of people will get the attention or funding that a project needs to get off the ground.  ULTra’s Amritsar project, I think, is case in point.  With up to 100,000 visitors per day and only 7 huge stations, it is not at all what most PRT pioneers envisioned.  Yet that was what got enough traction in the real world to actually get funding. 

Even without the timid bureaucracy, there is a basic problem with a small, starter PRT network - there are liable to be too few destinations available to attract a steady stream of walk-up customers to any given station.  Or put the opposite way, there are too few stations feeding any given destination.  Either way you look at it, it is the network effect in reverse.  It’s the internet with only a dozen web sites.  In addition there is a matter of distance.  For track to pay for itself it needs to be used.  If the stations are far apart, there will need to be more vehicles on route to pay those bills.  In the “For Example” post there were vehicles leaving four stations at one per two seconds at each station to keep the track at full capacity.  Suppose we had twenty stations over a similar four mile loop.  That is still one vehicle leaving each station every ten seconds if you expect to have the track at full capacity. Now where can you find that kind of pedestrian density for all twenty stations?  Luckily, with proper engineering, both track and stations can be made cheap enough to break even at far less than full capacity, and vehicles ought to be mainly amortized by mileage, so there is still hope.  But the problem of small networks is still real, nevertheless. 

So PRT badly needs a strategy to “get a foot in the door” so at least one network can grow to reach critical mass.  Otherwise it is very hard to see how this “chicken or the egg” dilemma can be broken.  Trying to ensure that every possible situational roadblock has been addressed, I have designed ultra-small open-air stations, ultra-high capacity stations, track that can turn any corner, vehicles that can even drop down vertically … you name it.  But the best strategy is to have some kind of a way for a transit authority to ease into PRT - to “dip their toes in,” so to speak.  This must involve some kind of modest starter network that doubles as being clearly the best, most cost effective way to solve some big, immediate mobility problem.

One very insightful reader commented that a good place for PRT to start might be a city (or cities) straddling rivers or other bodies of water.  I must say that this is the best suggestion that I have heard in quite some time. Let’s examine it.

In such cases a good percentage of the mobility problems might be for lack of bridges or good approaches to them.  Bridges, like roads in general, must be engineered to handle bumper-to-bumper cement trucks, so they are vastly more sturdy and expensive than they need to be to accommodate a steady stream of puny humans in evenly spaced, featherweight pods.   A bridge for PRT vehicles would be absolutely tiny - in both stature and cost - even while being capable of delivering people at a prodigious rate.  But the bridge itself is just the beginning.  On either side of the bridge there must be approaches to it.

These days to justify a new bridge, you pretty much need to be building it as part of a freeway.  When is the last time you saw a city inaugurate a brand new two lane bridge?  Consider the devastating effect that steering that much traffic towards a new crossing point would do to the riverfront real estate on each side.  Also, it’s not just on the banks.  Unless there are already riverfront highways on both sides, such roads will cut neighborhoods in half, bring noise, and, of course, will be terribly expensive and disruptive to build.  

Adding a PRT bridge would require no new roads, no demolition.  Transit planners would have a totally unique opportunity to create a painless shortcut between the most important destinations on each side.  Also, cities often make riverfront property into parks because of the possibility of flooding. In many cases these are important tourist destinations, with nearby businesses catering to pedestrians as well as drive-up customers.  Joining such car-optional areas on either side of a river would be a boon for such destinations.  Car-free zones are highly symbiotic with PRT, since every additional loop extends that “cars not required” status further.  Add a zoo, theaters, museums or a shopping mall, and the system gets real gravity. A group of destinations becomes the destination itself, once it is all walkable. And let’s not forget, apartments and condos on “The Riverwalk” would then follow, and those folks could live quite well without a car. Real estate developers love this kind of stuff. One deal leads to another as the whole area goes “upscale.”

One twist I would like to mention in passing is the possibility of using old bridges.  I recently heard of a case in Louisville, Kentucky where they saved the old “Big Four” railroad bridge across the Ohio River, and converted it to pedestrian use. This reminds me of the “Rails to Trails” movement, where they convert unused railroad lines to “hike & bike” trails. I’ve often wondered if they would consider sharing their trails (bridges and all) with a nice, quiet and green PRT system!                        

Anyway, in a nutshell, the premise is this;  Building a shortcut between important destinations on either side of a body of water may be too expensive and disruptive to contemplate for ordinary road or rail.   PRT, however, being lightweight and elevated, can make the overland portion with the least disruption, and requires a bridge costing only a fraction as much.  To top it off, bodies of water typically have parks along their banks that could use PRT service as well.  These factors might combine to create the potential for an inexpensive starter PRT system that can still attract sufficient ridership to turn a profit.   

Could this be the “foot in the door” that we’ve been looking for? It certainly seems to me that PRT developers would do themselves a big favor by having the resources in place to accomplish such a project.