¿La demanda inducida es en realidad sobre capacidad de la calle?

Origen: Is induced demand really about road capacity?

 La demanda inducida es una advertencia comun de los urbanistas cuando se propone una nueva calle o la ampliacion de una existente. La idea basica es que añadir capacidad a la calle generara nuevo trafico que recreara la congestion que justifico la capacidad adicional a la calle en un principio. Algunos han concluido que cualquier calle o carretera adicional siempre inducira demanda.

He visto un caso de esto recientemente cuando Laval, un suburbio grande de Montreal (muy auto-centrico) propuso construir un bulevard a travez de una pequeña zona boscosa. Mucha gente argumento en contra, diciendo que esto era propuesto solamente por planificadores obsesionados por el coche y que resultaria en demanda inducida. Aqui hay una imagen de la reticula vial de Laval, con la seccion propuesta en rojo

The proposed new street in context

Pienso que esto es un error. La gente a tomado casos especificos domde incrementando la capacidad se  incrementa el trafico y la asumido como una verdad universal

Pienso que esencialmente, la demanda inducida no es sobre la capacidad per se, sino sobre  la velocidad.

Velocidad, ‘escurrimiento’ y comgestion

El punto es que la velocidad es tal vez el factor individual mas importante que determina cual modo de transporta domina. Puedes tener modo de transporte gratuito que extremadamente confortable y placentero, pero si se arrastra lentamente a 1 kmh,  nadie lo tomara. Es por esto que las carreteras son tan disruptoras en zonas urbanas, van a velocidades que ningún otro modo puede alcanzar. Si la carreteras son efectivamente el modo de transporte mas rápido, capaz de mantener velocidades de 70 a 100km sin parar durante kilometros?. La velocidad al Caminar es 5 kmh, en Bicicleta es 15 kmh, en Buses varia entre 10 y 20 kmh. Aun el transporte rapido promedia 30 a 40 kmh.


La gente tambien tiene una cierta tolerancia para las distancias, pero no medida en kilometros, sino en minutos. Asi, si la gente digamos tolera un viaje de 15 minutos de distancia para llegar a su destino. Dependiendo de la velocidad promedio de la calle por la que conduzca, desearan habitar una cierta area. Para hacer una analogia, las siguiente cifras son el ‘escurrimiento’ de las carreteras, similar al agua de un escurrimiento fluyendo hacia un rio por garvedad

Watersheds of an urban street, a suburban stroad and an highway
In the previous figure, you can see the “watershed” of different kinds of streets and roads, one an urban street, max speed 40-50 km/h (25-30 mph) with many stops or traffic lights, the other a suburban stroad (mix between street and road), max speed 70 km/h (45 mph) with few intersections spread apart, the last an highway (as in limited-access motorway/expressway) with a max speed of 100 km/h (60 mph). With the highway, people will be willing to live much, much farther away, in places where the only viable mode of transport is a car on the highway.
From experience, I can tell you that “induced demand” look a lot like “induced development” in lands once thought too “far”, but that are brought closer to points of interest (jobs, shopping malls, etc…) through a faster road opening.
Where road capacity enters the picture is when congestion rears its head. Look at the watershed of the highway versus the urban street, it’s ginormous. If the population density is the same in the watershed area, the highway could have 3-4 times the demand of the urban street, so it requires way more lanes. When it doesn’t, well, congestion occurs, which slows down traffic significantly. Roads that normally allow cars to zoom around at 100 km/h suddenly are full of cars in stop-and-go traffic (or at least, the entry points of the highways are). So the effective speed during peak hours crumbles down, reducing the watershed. People know this and farther areas along the highway become less desirable.
The presence of a chokepoint during peak hour congestion reduces the desirable lands around the highway
In this situation, developments farther than the chokepoint are slowed, if not downright stalled, because the speed of the highway collapses past it. This limits traffic by making developments less likely, or if they occur, they will be “edge city”-type developments, meant to create new points of attraction outside the zone surrounded by chokepoints.
In such a situation, adding a new highway to “relieve” the current highway, or widening the highway may eliminate the chokepoint (for a while) making lands that used to be undesirable desirable again, and restarting developments as lots of land come on the market at one time, making land cheap and thus, housing cheap.
So road capacity isn’t really what induces demand, speed is. It’s simply that when capacity is too low, congestion happens, reducing the effective speed of roadways. Increasing capacity only induces demand in that case when it restores speed to previous levels.

Why does it matter?

This distinction matters because it means that we don’t need to be afraid of capacity in itself or of building new streets with low speeds. Yes, highways are highly disruptive, but a street grid isn’t an issue. In fact, many cities with extremely high capacity street grids do not really sprawl far but are instead great multi-modal cities. Vancouver for instance has a very high-capacity street grid but without any highway within the limits of the city itself. Though streets still have unused capacity, recently car use has fallen and other modes of transport have increased. Why? Because on Vancouver’s tightly packed street grid, pedestrians, bikes and bus riders aren’t that much slower than cars.
The same can be said about Manhattan or Portland.

Return to Sapporo

I’ve spoken of Sapporo recently, a Japanese cities built on the same street grid of wide streets that Dallas, Austin or Houston were initially built on, yet that has a pretty good record in creating a multi-modal city, with only 45% of all trips in the city and its suburbs taken place in cars. When you look at Sapporo and see the grid of 4-lane streets everywhere, you can be amazed at the road capacity of it all:
Yet, despite this capacity, Sapporo does better than all North American cities for sustainability, save, maybe, New York. What gives? Well, here is an image from Google Maps of Sapporo, with different road categories in different colors:
Sapporo’s urban area
The A pin is the downtown area. The highways are in orange, the yellow roads are prefectural (I think) roads, but they’re not anything we’d call highways here. They are almost indistinguishable from regular streets, with max speeds of 50, even 40, km/h (30 and 25 mph) and regular intersections rather than interchanges. This is an example of one of them:
One of the roads marked in yellow in the previous image
The only real highway goes around the urban core, making it nearly useless for commuters. Worse, it’s tolled, HEAVILY tolled. About 25-30 cents per kilometer driven (40-50 cents per mile). So the remoteness of the highway and the tolls make it much, much less disruptive than our own highways in North America.

The point is that Sapporo’s streets have incredibly high capacity, but that doesn’t seem to induce demand too much, because speed, not capacity, is what really induces demand. Since Sapporo’s streets are low-speed due to low speed limits and frequent intersections, they don’t make walking, biking or transit uncompetitive. The only significantly faster road is a heavily-tolled remote highway made for long-distance trips only. The result is pretty good in keeping car mode share down by keeping other modes of transport viable.

Coming back to Laval…

OK, so let’s back up to Laval for a while. Let’s zoom in on the area in question:
Proposed new street connection in red
Now, the point is, there are already lots of high-speed stroads and highways in Laval. The damage is done. The streets are often poorly connected and uses are kept separate, but even there, they’ve started building higher density residential areas, especially since the subway was opened (the orange line on the image). The proposed street would connect two existing streets to make one unbroken street at reasonable speeds (50 km/h). So it’s not an high-speed road, the danger of induced demand is not really there, especially that there are plenty of high-speed roads around, so, a bit too late for that.
The main effect I see is that it would end a detour of 1,2 km by the south (0,75 mile). This has little impact for car drivers, but for pedestrians, cyclists and transit riders, this is a significant deal. This detour to go to the schools to the left, or the shopping center, costs them 15 minutes (for pedestrians), 5 minutes (for cyclists) and 3-5 minutes (for buses). The transit operator will now be able to run buses along the entire street. So it’s a massive deal for active transport, much bigger than for cars. It’s a good step towards finishing a street grid rather than relying on canalizing all the traffic on a few stroads and highways, and a very good idea, especially if they build a parallel bike lane while they’re at it.


So induced demand, it’s real, but it’s about speed, not capacity per se. It appears to be about capacity only because high-speed roads attract a lot of users, more than they can support, the result is congestion which effectively reduces their speed. So we shouldn’t worry about highly porous street grids, even if they have extremely high capacities, as long as the streets themselves are calmed and slow.

Acerca de salvolomas

Asociación vecinal, cuyo objeto es preservar la colonia habitacional unifamiliar, sus calles arboladas con aceras caminables, con trafico calmado, seguras para bici, parques, areas verdes, centros de barrio de uso mixto accesibles a pie y oficinas solo en áreas designadas.
Esta entrada fue publicada en Automóvil, Calles, Trafico, Transporte y etiquetada , , . Guarda el enlace permanente.


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