Engineer Scotty left a long and thoughtful comment on my recent post about vehicles for roads, streets and country roads. The whole thing is worth reading, especially for short-term recommendations, but I want to focus on this part:
But we still have need for intra-city mobility of the sort that cannot be accommodated with streets alone. Even if SOVs were banned, trucks and busses still need some way to get around to serve medium-distant trips. Cities need arterials of some sort to function.
I think that when he wrote that, Scotty hadn't yet read my subsequent post where I argue that trucks and buses will always kill and intimidate, and the only way to transport large loads and large groups of people without disrupting the street is by train.
The problem with urban "arterials" of any kind is that they don't work the way everybody seems to think they do. As I argued back in 2008, the nucleus of a city is a transportation pivot of some kind: a crossroads, an intermodal transfer point like a port or a railroad station, the entrance to a pass or a valley, or even just a rest stop.
It is the very fact of having to slow down, stop or rest that makes a city what it is. If the travelers and drivers don't stop, they won't spend money, and there is no reason for businesses to set up there. If there are no businesses, there are no jobs and thus no reason for anyone else to live there.
Bypasses kill cities in two ways. They can simply remove any reason to slow down or stop at all, so that every customer just goes flying by, and all the commerce goes to another city. Or they can move the crossroads away from the old downtown to a new place on the outskirts, which becomes the new downtown (if you can call it that).
Yesterday, the Urbanophile tweeted links to a Detroit Free Press article about the vast open spaces in that city, and a Chicagoist post about similar developments on a smaller scale in Chicago's South Side. My reply was simple: both places have been bypassed. They're no longer at any kind of pivot point on the way to anywhere. They no longer have access to anything useful. So why would anyone want to live or make anything there?
Of course, Scotty isn't talking about a bypass, but some kind of moderate arterial where through traffic is prioritized over local side traffic. This is the same problem, just to a lesser degree. The easier you make it for traffic to pass through the city - the more you facilitate mobility - the less commerce you have. In cities, mobility is the enemy of commerce.
I'm simplifying, you understand. A city can't be a complete bottleneck to the point where people make their own bypasses, for example by going through other cities. The people who sell things to travelers, well they have to get those things somehow. Sometimes you need to go outside easy walking distance to another neighborhood, and you don't want that to be too difficult.
That kind of wider access can be provided by transit. The ideal urban transit is the subway, because it's mostly invisible, but you can get a lot with elevateds and "green tracks" trolleys. Bicycles, of course, are a great way of providing medium-range mobility.
In the past the push for bypasses came from two directions: the shippers and travelers who didn't want to be slowed down, and the neighbors who didn't want traffic. But the reason the neighbors didn't want traffic was because the traffic was dangerous, noisy and polluting. Cities like Detroit and neighborhoods like Bronzeville will never come back until through traffic can cross them at a safe, human scale.
The key is to balance mobility and access with commerce, comfort and safety. Speed can be mitigated by the use of rails instead of tires, and lightweight vehicles like bicycles and pushcarts. Separate faster rail traffic from slower foot traffic if you must, but if you make your city too easy to go through or around, then you risk making it irrelevant.