The reality and fantasy of induced demand.

One of the tales that has always bothered me is that building more freeways will only induce more traffic. It is only partially true. There needs to be considered the notion of latent demand when the notion of capacity is considered. When we talk about the size of a pipe and flow, we rarely consider the notion of the lake that is not being emptied. If you increase the size of the pipe, more trips will be made, but how many of these are trips that were worthy but frustrated, and how many trips are those that were never even considered prior to the capacity being there.

wired.com Traffic induced demand.

The note in the story that is important in my own mind, is that there are limits. However, the other point that needs to be appreciated, is that building freeways induces demand in not just the primary manner noted, but in another way, which is at least as important. However, the story does not deal well with the issue, of people who should travel choosing not to. We lose economic activity that would benefit all, because of people whose real demand remains latent.

The building of freeways changes the structure of development. The post war primacy of the automobile changed the way we built cities in general. The other issue with building freeway, as discovered in the US, is that it allows people to travel further, thus encouraging development further out, meaning not only more cars but traveling further.

People sometimes discuss the same notion for transit, however, there is a very real limit to the concept of induced demand. Yes people will avoid trips or reschedule them to periods that have lower ridership, but there are many of their trips they cannot avoid, and they will not travel simply to do so.

The limits, however, are a less of a reality with expressways in cities like Toronto, just because the region is adding 100,000 people per year. If this means an additional 30,000 commuters, that is something on the order of an additional 15 lane/hours per annum in each direction of travel. Clearly this sort of demand will not be met with expressways, as development will follow availability, swamping it very quickly. The reality is that development does not wait for the availability to be present once it is fully committed and under construction. Thus it is not surprising that in the US large expressway projects did not resolve congestion in rapidly growing cities. The “induced” demand in this case, is development that located, based on new expressway construction. The largest part of this demand would be there anyway. Also if there is a shortage of road capacity, and ample transit to the destination required, people will choose to use transit.

The situation for rapid transit is slightly different. Transit has tended to come to areas that are already built up, and typically does not come in the small increments of highway. The nature of our commuting has meant that the vast majority of commuters who drive do so alone. The typical car thus has only 1 person in it, and lane capacity is only about 2000 vehicles per our, and that requires people to be tailgating. Three lanes in each direction (6 lane expressway) therefore only means a realistic capacity of 6,000 vehicles per hour. The LRTs for Eglinton, applied on a 2 minute headway would comfortably carry 13,500 passengers in each direction every hour, or very nearly 7 lanes in one direction. These can be expanded to 4 cars, which would mean 18,000 or 9 lanes. A single GO train is like a lane/hour of commuters, run them every 10 minutes, and you have 12,000 – 6 lanes – running in a single direction.  LRT – even running in the median of a street and restricted to say a train every 4 minutes – would still have a capacity to a very comfortable 8,000-9,000 passengers – the equivalent of 3 standard buses per minute.

Subway, the capacity King, can carry enormous loads, to something like 40,000 passengers per hour in each direction. The question is how and why would we want to focus that extreme a load in a city with as low a density as Toronto. Transit oriented development would also increase density, however, this requires the development to be built around the presence of transit, where transit will be the primary mode for the area. Toronto looks for subway to solve too many of its troubles, and has in essence create a problem for its busiest line, which will require significant redevelopment in order to achieve capacity like 40,000 passengers per hour, and even then if we continue to use it the way we have, it will be overloaded.

Extending subway into areas that will generate lower ridership, is very expensive, and creates problems down the line, as it increases load in areas that are already at, near or even beyond capacity. However, BRT and LRT create a similar speed of transit, and thus desire to be close to the line. Right now Toronto needs one subway project, in order to divert riders going to the core from the Yonge line. This single project would enable substantial load growth for the Danforth line, substantial ridership for the Crosstown to core and even a Don Mills LRT to permit several busy bus lines to bypass Yonge.

Toronto needs to be realistic in terms of what its underlying demand is, to what degree its choices have and will form development, and the need to use transit as a means to form that development. The positive latent demand needs to be met, but not by creating very high capacity for miles where it is not required, or encouraging development where it will be hard and expensive to serve.  These are the real lessons from freeway development in the U.S.   Toronto, needs to reserve space for dedicated ROW for transit, that will permit transit to run at higher speeds and reliability, ideally so that they could also support LRT in a grade separated  application, where it would be possible to expand capacity to meet almost any reasonable demand.  Lets be realistic, anywhere away from the core where a service is not a primary link to the core, 9000 passenger per hour per direction on a single route is huge.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s