Marchetti's Constant

Reading about Marchetti's constant today, which suggests that all humans strongly prefer to spend about an hour a day in travel. (Half hour in, half hour out). This supposedly holds across all cultures, in all contexts.

Walking about 5 km/hr, and returning back to the cave far the night, gives a territory radius of about 2.5 km and an area of about 20 km^2.

20 km^2 is about 5000 acres. Conveniently, Salt Lake City's blocks are about 10 acres, so I have a ready conversion metric, so that means 500 blocks, which is a gird 22 blocks on a side. #

This range supposedly applies to villages with agricultural fields and the area contained within the outer walls of ancient cities. This implies that it applies to modern cities as well, or rather the walkable portions of them. Which implies a size limit for Transit Oriented Developments as well. Given a half hour time budget for commuting, of which at least some is taken up by the transit trip itself (say half). This suggests that a 10 minute walk*** to a transit station may actually be more appropriate. Indeed, Calthorpe's original TOD concept* had a 10 minute walk to a heavy rail, from a max distance of about 2000'.

This suggests that the 'Density Gradiant' near transit stations should be an extremely steep one, with elevator apartments (3+ stories) adjacent, and single family homes a half mile away. Because people beyond that half mile aren't likely to walk to the station*.

TOD (Transit Oriented Development) is frequently done badly. Largely because we lack these metrics. If TOD is going to work, the majority of residential units have to be next to the station. Not behind the parking lot, not beyond a belt of adjacent commercial development. Right next to it. Let the Park&Riders walk through that access path to the station. Direct them down a single avenue, running the gauntlet of convenience retail every day. Done right, ground-floor retail could actually be made to work, and in  way that inconveniences the minimum number of apartment dwellers.

In a way, NAM** does us a bad turn. The idea was for transit oriented metropolis, with independent cities clustered around transit stations. Each with their own 'neighborhood center' core. of retail and office development. Nothing like that has ever been built. A 'Transit Metropolis', where transit is embedded in the urban fabric, would look very different.

Recall that most 'New Urbanism'**** is greenfield development, not infill. Even if we fix the street connectivity and raise the density, any given metro area has less than 50 transit stations. 50 possible TODs, each with about 288 acres within 2000' of the station. Some tiny fraction of the total urban area.

*Certainly, some will, but they will be the fast walkers: Young, able, athletic.  Biking to station probably occurs from outside the half mile circle. Presuming a bike to travel 3 times as fast as a walker, say the 'Bike Zone' extends out to 1.5 miles.
**The Next American Metropolis
***People coming from longer distances walk faster. I stroll at about 3.0 miles per hour, and race-walk at about 4.0 miles per hour.
**** AKA 'New Suburbanism'

#How big can a CBD (Central Business District) get before it is 'too big'? It can't scale in size to population, certainly.

The paper is happily available online here