Tuesday, May 21, 2019

Spatial Interactions Models, Travel Time, and Kids

In spatial interaction theory, there is a thing called the gravity model: That the interaction between two objects is proportional to the square of the distance between them. The longer the distance, the less the interaction. And it's pretty accurate, in terms of travel time: the frequency of a trip is inversely proportional to the travel time required. But when each trip is so costly (in time), you consequently make fewer of them.

Kids make every trip take longer (even if it's only getting them buckled in). So by adding a kid to your life, you've (effectively) moved further away from everything. Hmm...So it's not by accident that larger families move further out: Space is cheaper, and trip-times are already long. Adding 5 minutes to a 45 minute trip is nothing. Contrast: If I need to run to to the grocery store, it's a three minute drive. And a choice of four full-service supermarkets.

For my wife, grocery shopping is practically part of her commute: she just overshoots the stop nearest our home on TRAX by two stops, shops and returns.  My family in NYC likewise shops daily. I think...this is the cause of the misapprehension of suburbanites. The idea of shopping daily appalls, because if you live in suburbia, grocery shopping is a major undertaking, which takes a lot of time. Doing it daily seems appalling. But for my brother in Brooklyn, the bodega across the street is closer to him than most people's bedrooms are to their garages.

I admit to being uncertain of the differences in difficulty/time consumption in buckling a kid into a car-seat vs. a stroller. I assume them to be roughly equal, with stroller packing/unpacking similar to opening/closing doors with sufficient care as to not slam little fingers in them.

But this does sort of explain the emergence of massive strollers in NYC: It's not just a stroller: It's also a grocery cart--a place to hang bags and stash parcels so that the mother doesn't have to carry them.  

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