Urban layout has vast consequences for mobility. Everything from density, street layout, street design, the size of the urban area, building setbacks, parking requirements, to transit systems effects how we get around. To complicate things, all these factors not only effect mobility, but they effect the mobility caused by the other factors. To complicate yet further, all these factors have additional consideration on top of mobility. This makes considering a large transit investment something which absolutely must be done in the context of all the other factors.

 

The way I define the utility of mobility is by origin destination pairs for a given cost, be that cost time or money. One of the biggest goals of urban design, transit, cars, etc, is to increase mobility, i.e. increase the origin destination pairs over cost.

 

One of the simpler factors in mobility is simply the size of the urban area. All other urban design factors held still, a bigger urban area can increase mobility by offering additional origin destination pairs at the fringes. It can also reduce mobility by causing congestion, increasing the cost of going between the pairs in the smaller area. The larger size can also mean that transport infrastructure necessary to handle the increased congestion has to be larger, resulting in larger cost. A larger urban area can also have adverse effects on air quality and access to nature. This is important because investing in transport infrastructure which enables larger urban areas may or may not increase mobility, but will likely have other bad impacts.

 

Another of the more straightforward factors is density. Increase in density increases pair density. This can also create congestion which dilutes the mobility increase. Infrastructure to handle the increased traffic is often more expensive due to increased cost of acquiring land or tunneling to not disrupt current uses. Increased density tends to have relatively little impact on nature, but can worse air quality just like larger urban area.

 

Street layout  can be thought of by the extremes, a grid or medium size streets vs cul de sacs off of curvy arterials. A grid provides a direct relatively short linear path to a large amount of origin destination pairs. Cul de sacs mean many pairings which are geographically close are far in distance by the road. Grids evenly distribute traffic across the road system. Cul de sacs funnel the traffic onto a few major roads, heavy with the traffic for everyone must pass the same road. The pluses of the grid is it increases mobility, especially for pedestrians. The paths are shorter and the quiet streets are easy to pick for walking. For cars the increase in mobility from a grid is less clear. Cars also need to travel longer distances, but they may be able to travel at faster speeds if they are on a road designed for primarily for speed. Cars are slowed by every intersection, and cul de sacs have less signaled intersections, but few curvy arterials tend to make for huge intersections with many complicated light cycles which can take much longer all at once. The primary reason to design urban areas as cul de sacs is to make it so little traffic passes by the majority of buildings. If we work with the assumptions that all residents have cars and that the city isn’t big enough for that to make serious congestion, than cul de sacs can be used with little impact on mobility. But if residents may have otherwise not had a car, or the city grows to the size where congestion from everyone driving causes major congestion, then cul de sacs come at the a high time and fiscal cost to the residents.

 

Another important aspect of mobility is infrastructure designed for increasing mobility. Counter intuitively, it can often decrease mobility. A large road or a passenger rail line can significantly increase mobility in the direction of the infrastructure, but mobility crossing the infrastructure can be greatly inhibited. On the extreme, freeways can block crossing for miles. For someone on foot, that effectively halves the destinations reachable from places right against the freeway. Car infrastructure can even only have minimal impact on increasing car mobility. Because the marginal monetary cost to a driver of using a particular road is none, if there is significant demand for a road, congestion increases till the time cost mitigates demand. An increase in road capacity can create only a small decrease in congestion if the travel demand is flexible. Whenever considering investing in infrastructure, we must not only compare the increase in mobility with the fiscal cost, but also with the loss of mobility caused by implementation.

 

Mass transit can also improve mobility. For trips were car is not an option, transit is often faster than walking. In congested cities, if the transit has its own right of way (ROW), such as subway tunnel or bus lanes, mass transit can be faster than driving. High speed rail improve intercity mobility by being faster than driving. Sometimes transit increases mobility for only a small number of people. If the demand for busses isn’t high enough for the buss to come frequently, the wait to get on the buss can reduce utility till only people with no other options will take it. Sometimes, a small increase in demand can cause a large increase in frequency because the time cost of using the bus goes down. Mass transit can improve mobility, but only if there is enough demand and proper infrastructure is in place for the public transit to be time competitive with other options.
Things start getting really complicated when considering how all these factors effect each other. How will allowing an increase in density effect mobility. Maybe it will decrease the mobility by increasing congestion. But maybe it will increase mobility by increasing demand enough to justify more effective public transit. But what if the streets aren’t in a grid, because they were made when density was lower, the density may not allow for the increase in transit because transit requires walking and non-grid layouts reduce walking mobility. Etc, etc. No part of the system can be considered in isolation. This quick list of factors is merely a starting point for thought, investigation and considerations on how we can get to as many places as possible.