Highway-oriented development: know thine enemy

People in urbanist circles talk a lot about TOD, Transit-Oriented Development. I've personally described one clear example of it when talking about streetcar suburbs. But I think it's important to look at other forms of development too, especially the current type of development which might be called car-oriented development. It's very important because:

1. It's important to understand how we've built cities if we want to reform them into something better, to see how they can be gradually migrated to a better form of development
2. Car-oriented development has been amazingly effective at spreading itself, trying to understand how it propagates and sets the stage for its success can be useful to conceive of how TOD can hopefully replace it
3. To understand its pattern of replication so as to know when we are doing it, so we can avoid that trap

You'll note that in the title I spoke of "Highway-Oriented Development" rather than simply of "Car-Oriented Development". I'll come to this later.

Basics of car-oriented development

• Residential areas

Despite the association of sprawl with low-density suburbs full of single-family detached houses with quarter-acre lots, that is not an essential condition of car-oriented development.

These two images are from Swindon, UK - 85% car mode share

Mississauga, Toronto's car-dependent edge city

What COD requires is to have plenty of residential parking everywhere so each working adult can find a parking spot for his car. That means in general having at least two parking spots per house, with on-street parking to deal with the overspill. This requirement does cap density, but it still allows for some. For instance, a 6-meter wide (20-foot) townhouse on a lot 30-meter deep (100-foot) could have a driveway that could have 4 cars parked on it, given a large enough setback. Such a house could get a density of around 40 units per hectare. 40 units per hectare with single-family houses can yield a population density of around 9 000 to 10 000 people per square kilometer (22 000 to 25 000 per square mile). Even single-family detached houses can achieve decent density, the Japanese achieve 30 to 40 units per hectare with SFD housing, mainly by having shallow lots (around 15- or 20-meter deep lots, 50- or 70-foot).

To scale, 9 houses in Japan (3 deep plus a street, 3 across) vs 3 houses in Vancouver, Canada

With multi-family housing, density can be even higher. This often looks like towers in parking lots, but density can even reach 100 units per hectare (40 units per acre), or even more if you use storied parking, either in aboveground structures or underground parking lots.

Of course, if you want to avoid having cars everywhere, making areas ugly as sin, and don't want to use expensive underground garages, the density limit of parking requirements is even tighter.

Likewise, in consideration of the residents, there needs to be ways to force through traffic off of residential streets, often resulting in cul-de-sac and the like.

• Retail and job centers

Too often, people tend to look only at residential areas. There is almost a taboo around commercial ventures, as if the home was sacred but commerce was profane. Anyway, here again, the COD requires parking spots, plenty of parking spots. Buildings can be high, but the higher they are, the bigger the parking lot around them. The form these take then is highly hostile to non-drivers in most cases: buildings are spread apart, there is little density and parking lots are between the door of the building and the street to have car drivers walk as little a distance as possible to it and to clear up the street for signs to advertise to passing car drivers what business is on the side of the street.

The location of these centers is also very important, and tends to depend on three important factors:

1. The catchment area
2. The local traffic
3. The capacity of the road network

The catchment area is simply the number of residents who inhabit close enough to the store or office or factory to consider patronizing the place or working there, forming a pool of potential customers or employees. This can be approximated by asking the question: "how many people live within a XX-minute drive from here?" This time varies depending on what store we're talking about, people may tolerate a 15-minute drive to go to a major mall and a 30-minute drive to go to work, because they expect a lot of benefits from those trips, but they will have less tolerance if they're just looking a buy a pint of milk from a corner store. Businesses will thus favor areas that are accessible to a greater number of people within as short a time as possible.

The local traffic is sought especially by small stores, because it opens the door to a lot of "pass-by" customers, people who may not make a trip just to go there but who will stop and shop there if it's on their way. Donut/coffee shops thrive on pass-by customers for example.

The road capacity of course matters because if it's insufficient to deal with the expected traffic, then some customers will turn back, or some employees will actively seek other jobs.

All of these factors combine to explain why stores locate on arterials, these areas tend to be accessible to the largest amount of people, to already have plenty of traffic and relatively high capacity.

• The road network

COD depends on a road network that can support a lot of traffic. For instance, a grid of wide arterials with 2 or 3 lanes per direction, if not more. This can allow plenty of people in cars on the road at the same time. Residential streets can be narrow if you want, as traffic is channelized on arterials, as long as residential streets connect to arterials, it is fine.

Street-based car-oriented development

First, what does a car-oriented city based on regular streets look like?

Cities following a street-based COD actually develop not that much unlike traditional cities, in that they would tend to be relatively round, with a strong center. Why? Because if all streets go at roughly the same speed, say around 30 km/h on average (including stops), then growth will tend to happen in a concentric fashion to reduce the travel time to all other areas of the city, just as they would in traditional cities where most people walked. The main difference is one of scale.

Likewise, the center of a city is by definition the most accessible part of it. It is the part of the city that is closest to every other part of the city on average. Also, supposing an uniformly distributed population, the center also has the highest road capacity of all areas as exactly one quarter of the population is likely to access it by the north, by the east, by the west and by the south:

The center doesn't just reduce the maximum distance, but also distributes people coming into it to 4 directions equally, in this case, if every street in has a capacity of 2 000 vehicles per hour and there are 6 000 vehicles per hour coming into the downtown area, then each street is at 75% of capacity

Meanwhile, a retail and jobs center that would not be at the center would face a much higher traffic coming from one direction than from the other directions, risking congestion and becoming undesirable to go to for people who are stuck on that particular way in.

An off-center location results in a lot more people coming in from one or two directions, in this case, using the data from the previous image, the west access is probably at 150% of capacity, the north access is at 100% of capacity while the east and south access are at 25% capacity, so most people coming to the center would come through congested streets

Major stores would likely be located in the center, with some proximity stores on arterials all over the city.

As a city grows, it may come to limits on growth, the arterials leading to the center may become congested, the city may grow beyond what would be considered acceptable travel distance (in minutes) from the center, which can be dealt in a variety of fashion: creating new centers on the periphery, with new residential developments to follow, creating a polycentric city, investing more in transit to increase road capacity for peak hours, widening streets, etc...

A street-based car-oriented city is not so bad. Sure, residential and commercial forms give priority to cars rather than to humans, but since there is no high-speed road and speed is largely the same on all arterials, there is a focus on reducing distance and on being more compact rather than spread out. Transit can also be effective with bus lines on arterials following the same routes as cars. There is no street off-limits to pedestrians and cyclists, though crossing some streets may be long and unpleasant, it remains a possibility.

Highway-oriented development 

Highway have a big impact on traffic and development patterns. Since they are much faster than regular streets, people often have an advantage to take detours by the highway instead of traveling slower on city streets. The result of highways is thus a concentration of traffic on the highway, drawing them away from arterials. Highway interchanges also become the most accessible places with the biggest catchment areas of the entire region:

10-minute driving distance between an interchange versus an arterial far from highways

Consequences on development are dramatic:

• Opening the way for more sprawl

One of the big effects of highways is to open up new lands for development. People, even those who want to be far from activity centers, want to be within reasonable distance (in minutes) from the rest of the region they settle in. If roads are limited to average speeds of 25-30 km/h (15-20 mph), then that limits how far cities can sprawl, some kind of effective informal urban growth boundary. By adding high-speed links with two or three times the speed, this opens up the amount of land that can be effectively developed without being too far from the rest of the city. This makes land cheap, and what happens when something is cheap? People feel free to use it a lot, in this case with large lots and big parking lots.

• Draining traffic off of existing commercial arterials

High-speed roads, especially limited access roadways, will likely drain traffic by offering much faster travel. To some, it may superficially seem like a good thing, it is not. As I said earlier, stores want local traffic to have "pass-by" customers, if much of traffic is drawn away from the arterials, this will take away much customers from small stores that may then close, leaving arterials built into residential areas without much stores. Even if arterials can be seen from highways, highway traffic is fast and cannot easily stop.

• Attracting commercial businesses and employment centers to highway exits

This is a particularly bad outcome. As I said, once you put in highways, the places with the biggest catchment areas become highway exits and interchanges, and highways draw in traffic off of arterials. Highways also have high road capacity per lane and many authorities will widen them significantly to avoid congestion. Big catchment area, plenty of traffic getting on or off the highway at a few exits, high road capacity... highway exits are prime locations for businesses, especially big box stores who need large catchment areas to have the customer pool needed to sustain themselves.

The result is that you will see malls and big box stores springing up at highway exits while arterial commercial streets will weaken. As much as we can find strip malls ugly, they are still a lesser evil compared to big box stores located at highway exits.

What is the big problem with highway locations? Well, first of all, they tend to be located far from population centers, at the periphery of urban areas, not at the center. Second, the highway itself creates a barrier imposing a huge detour to get to the area for those on the wrong side of the highway, including for pedestrians walking along extremely wide arterials with plenty of fast traffic. Third, unlike the earlier arterials, it is hard to serve an highway with transit. On the arterials, buses could stop frequently, and even with parking lots, people would still be in face of the businesses, but buses can't stop on highways in general, even if they did, they would leave riders in an eminently unpleasant location, far from everything.

So not only are these commercial areas distant from population centers, there are barriers imposing detours, walking or biking there is uncomfortable if not outright dangerous and transit service will usually be terrible. The only ones advantaged by locations at highway exits are car drivers.

Here is an example of highway-located malls in Atlanta to illustrate my point:

Note how commercial areas (that can be easily identified by the prevalence of gray color due to the parking lot pavement and the roof of big box stores and malls) congregate directly at the highway exits, with residential areas (identified by the green with small gray line patterns) further out. In the second case, the mall is even enclosed by two highways running parallel to each other for a while, forming a significant barrier to direct trips.

Now, let's see examples of malls in street-based car-oriented developments. Once again, I turn to Japan, because that's where this kind of development is most seen, even European cities use relatively higher speed arterials (though nowhere near the highways of North America).

An AEON mall in Obihiro, put inside a residential area, do not mistake it for a huge parking lot, it's just the parking on the roof that gives that impression

You can see the AEON sign in the background, with a 8-story residential building in the foreground

Another AEON mall marked by the red "F", in Asahikawa

What it looks like from the street

These two examples of malls in car-oriented Japanese cities. We may not like how they look like, but they are built inside residential areas and can be walked or biked to, unlike the Atlanta malls. The price of land compelled the owners to build parking on higher stories to be able to fit their stores in smaller lots. I'm not saying this is perfect, not at all, but they are way less bad than highway-oriented developments.

Conclusion: how to avoid HOD and what we can learn from it

Highway-oriented development is not just a form of development that favors cars, it is one that essentially punishes any other mode of travel. A street-based car-oriented development at least allows for other modes of travel to exist and even the possibility of conversion if need be. HOD has no redeeming features whatsoever for sustainability and multi-modality. To avoid it, there are two main solutions:

1. Do not build any highways inside urbanized areas, or even high-speed arterials (what Charles Marohn at Strongtowns calls Stroads). Build highways so that they go around cities and do not penetrate them, with very few exits very spread apart, this will reduce the catchment area of highways and avoid them being used for local trips. Local car traffic should be dealt with by a tight grid of lower-speed arterials with frequent intersections (high capacity, low speed, low cost).
2. Toll highways high amounts, 10-20 cents per km (15-30 cents per mile). This can be justified by the high cost of building and maintaining them. Tolling highways will significantly reduce demand for it, and commercial developers will want to avoid locations that require people take highways to access stores and job centers, because the tolls will be a constant irritant to potential customers.

As to what we can learn from HOD... there is a strong analogy to make between highways and rapid transit. Both offer much higher speeds and higher capacity than usual modes of transport, both have limited access (RT has great stop spacing, highways have few exits and entries) and both tend to draw users away from other axes of transport (RT eats local transit ridership, highways drain traffic from local arterials). So if you have a form of rapid transit like subways, you should try to replicate a bit the form of development seen at highway exits: large stores and malls at stations for maximum catchment area and pass-by potential customers. With the difference that residential areas must be much closer for RT than for highways.

As bad as it is for sustainability and as wasteful as it is, HOD is incredibly successful at imposing itself, so taking inspiration from it for TOD makes all the sense in the world.