Context-Sensitive Streets

Continuing in our discussion of some of the basic ideas behind good urbanism, today we’re going to look at Context-Sensitive Streets (or CSS). This idea came about after many years of study on the neighborhood impacts of very wide suburban thoroughfares, and the design requirements that mandated them. The definitive source of information on Context-Sensitive Street design is the design manual produced by the Institute for Transportation Engineers, which is available for download here (warning: large PDF). For advanced information and examples, I encourage you to check out the manual.

The Conventional Street Hierarchy

The first thing to understand is the Conventional Street Hierarchy, which consists of Arterial, Collector, and Local streets. Arterials are the largest, collectors are in between, and locals are the smallest. In a typical city, arterial streets are either four or six lanes wide with a median and turning lanes. Collectors are typically 2-4 lanes with a turn lane, and locals are two wide lanes with room for parking on both sides.

In the image below I’ve shown a randomly selected portion of Northwest Houston, and highlighted the ‘arterials’ in red, and collectors in orange. What you can see is the typical suburban pattern, a very small number of roads serve as the only real connection points to the outside world. These roads handle tremendous volumes of traffic because of the wide areas that they serve, which have no alternative.

The Justification

These places were designed this way on purpose, and they are deliberately a bad network. Why? Because people don’t want excessive traffic flying down their streets. Why is this a problem? People have a long way to go, and they’ll drive as fast as ‘feels safe’. So with a wide, wide road and nothing placed near the street (so that it can be widened further in the future), people feel safe going 45-55 miles an hour in many neighborhoods. Fatalities from auto-pedestrian accidents increase exponentially as you pass 10 miles an hour, and when you get past 25 there’s little chance the pedestrian will survive. Why do we want neighborhood streets to be built for cars to thrive instead of for people to thrive?

Consider the two pictures below:

A completely car-oriented street in Houston

West Alamaba Street (from Google street view)

The first picture is an unfortunate example of a totally car-oriented residential street in suburban Houston. It’s one of the worst I’ve seen, but the street section is pretty typical. The second picture is West Alabama street near the University of Saint Thomas. The pavement is almost exactly the same, but the context is completely different. People drive faster on the residential street than they do on West Alabama (even on off-peak times with zero congestion), mainly because the context surrounding West Alabama subconsciously communicates ’slow down’ to a driver. The stripe in the middle helps as well.

But this is a good example for two reasons. First, why should two streets have identical pavement when they serve totally different functions? They shouldn’t. Second, streets and their context are inseperable, the context impacts the way people use the street, and the street influences what people do outside the right-of-way.

Historic Context

Looking back at the history of street design, for all of America’s early history streets were simply unpaved open spaces between properties. The streets were mainly full of people walking, but also used by horses, carriages, and later bicycles and cars. Only in the busiest urban areas were streets usually paved. Even when streetcars came on the scene, most of the street outside of the train tracks was still shared between pedestrians, horses, bicycles and cars, with pedestrians accounting for the vast majority of traffic. People parked in the street if they had a car or a carriage, but these were few.

In the historic context, a fifty foot wide space between properties made sense. Less developed areas had less traffic and buildings farther back from the street, more urban areas tended to build right up to the street. But when your traffic is mainly people on foot, noise and safety are not as big of a problem. The problem is that as cars took over, the way the 50 foot space was used no longer fit the neighborhoods. By dedicating the vast majority of this space to paved main lanes for cars, a ‘raceway’ is created.

Since that time there have been many design band-aids used to try and fix the problem. Sidewalks are now required, which improves pedestrian safety. Certainly sidewalks are a critical part of a modern road, since cars and pedestrians cannot share the same space unless the cars are going very slowly. But the main solution was to cut off all the streets, therefore limiting the bulk of all traffic to a very small number of through streets, thus ‘freeing’ the neighborhoods of unwanted cars.

Quit treating the symptoms, treat the disease!

The biggest problem facing urban America is badly behaving automobiles. Now, plenty of people have called for the elimination of cars entirely. We’re not even close to that technologically or culturally, so there’s really no point debating the merits of cars. What we need to do is build a street network that accomodates cars in a way that is compatible with the needs of pedestrians, cyclists, and transit, and which is appropriate to the development around it.

Context-Senstive Streets are the solution. These treat the disease (dangerous driving and auto-only system), which eliminates the main problems with through streets (the danger and noise of high-speed traffic), which means we can open up the network to create a large number of alternate routes. This dramatically improves circulation, lowers congestion, and makes alternative transportation more viable.

An illustration from the ITE Manual showing the difference between suburban design and traditional urban design.

What do Context-Sensitive Streets look like?

Well, it depends on the context! A great place to turn for examples is the Institute for Transportation Engineers. They’ve produced a report that is really excellent, and sumarizes CSS better than I could. You can read the whole report here.

The important thing is, CSS is specifically designed to tie into the surrounding context. The ITE used the best research available on urban patterns, the Transect, and tied their street design model into it. So when you’re in a T-3 area your street might look like this…

And when you’re in a T-5 area your street might look like this…

In CSS, the travel lanes and sidewalks are designed as one integrated street. This is a tremendous benefit to adjacent property owners, because they can count on a street that is safe, multi-modal, and consistent throughout its length.

CSS in Houston

Today’s post is meant only as an introduction to the concept of Context-Senstive Streets. In the future we’ll look at more detailed applications of this idea in the city, but for now let me leave you with three important thoughts:

First, Houston’s planning commission is working on Urban Corridor Planning right now, and one of the major components are some CSS inspired street designs. They’ve come up with some good ideas, and if you’re not already, it’s worth staying informed on that ongoing effort.

Second, these ideas are currently seen as relevant only in the area immediately surrounding light-rail stations. That’s an important step in the right direction, but we really need to apply the concept of Context-Sensitive design to the entire city. Developers equipped with a versatile, quality design manual like the one produced by the ITE can and will build better neighborhoods, and we need to get Houston’s outdated street design requirements out of their way.

Finally, techniques like CSS allow the city to make smarter investments. Smaller residential streets cost less to build and maintain, in addition to their safety and quality of life benefits. Great streets also attract investment from the private sector and create lasting value for the city. This is a win-win, and the City needs to step up and be a leader on this issue.

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I hope you enjoyed this introduction to Context-Sensitive Streets! Let me know what you thought by leaving a comment!