Transportation Theory

As a prelude to some further examination of the transportation issues in Northwest Houston, I wanted to take a few minutes to distill some transportation theory. The theory I’m going to talk about today is my own expansion of the well-established idea of a transportation hierarchy. I came up with this as part of my graduate research in Real Estate Development at Texas A&M, and as far as I know this is an original idea, so please let me know if you’ve read something extremely similar someplace else, or if you want to use this idea elsewhere.

The inspiration for my theory came from my observation of two well-established transportation ideas:

The first is the suburban transportation hierarchy, which consists of local, collector, and arterial streets. As you move up the scale from local to arterial, the streets get bigger and faster. The idea is that the local streets provide the greatest access, the collectors balance access and speed, and arterials provide less access but high speed.

The second is the Ferrovia Statale, the Italian train system. The FS consists of three main classes of trains. Local trains make every stop along a medium-short route, Intercity trains stop only at medium/large cities along a longer route, and Eurostar trains stop only at major cities and cover very long routes. The local trains are the slowest, the IC trains are in the middle, and the Eurostar is pretty fast. Also, the locals are very cheap, IC’s mid price, and Eurostars are a little pricey.

This pattern is pretty common, actually, and I’m sure there are other good examples. The reason for this is that there are three fundamental tiers of transportation, each with a different function, and each with a different need. The three tiers work something like this:

Tier One

Tier one travel is long distance travel from one economic area to another. This is not just a long trip, it specifically is a long trip from primary market to another. Therefore, this is not a trip from the Woodlands to Galveston, both of which are submarkets of Houston. The nearest Tier 1 destinations from Houston would be College Station, Beaumont, and Victoria, then the larger cities beyond (Austin, San Antonio etc.). For passenger travel, T1 trips are almost always related to special business or recreation trips, such as visiting family in another city or attending a major sporting event. Most freight is Tier 1, moving from a manufacturing center to various other markets where it is to be sold. A T1 trip is likely to involve at least one overnight stay, and is also likely to involve above average daily spending.

The critical indicator for a Tier One trip is SPEED. Above all else, people who are making a Tier 1 trip want to get there as fast as possible.

Tier Two

Tier two travel is medium distance travel within a single economic area. This can be a long trip, such as a trip from Jersey Village to Clear Lake. This includes the majority of business travel, especially daily commutes and business related deliveries. This does not include day to day errands and basic shopping (like grocery shopping), but it can include major shopping trips, such as a trip to Ikea, or an Apple Store, both of which draw shoppers from an entire region. These trips are likely to be regular events, but scheduled more flexibly than a T1 trip.

The critical indicator for a Tier Two trip is RELIABILITY. Above all else, people who are making a Tier 2 trip want to know in advance exactly how long a trip will take.

Tier Three

Tier Three trips are short trips within a certain travel-time radius of the home. These tend to be frequent, short, daily trips for things that people are less willing to ‘go out of their way’ for. Great examples are trips to the grocery store, or trips to restaurants, parks etc. These are the most frequent and most numerous of all trips. These trips are usually not made in a hurry, though they might be. These also include most social trips, people’s friend networks are usually contained within a certain travel tolerance. These are the most pervasive kind of trip: even little kids make tier three trips when they walk down the street to see a friend.

The critical indicator for a Tier Three trip is CONVENIENCE. Above all else, people making a Tier 3 trip want the trip to be easy.

Hybrid Trips

It is possible for a single trip from the home to include hybrid elements. For instance, a person who is heading out from Houston to College Station to go see a football game might stop at the Fairfield Outlet Mall on the way. The key distinction is that the Tier 1 element (traveling to see a football game) is rigidly scheduled (the game happens when it happens), and it involves going from one economic area to another. The Tier 2 trip is included only because it is on the way.

Higher tiers tend to dominate lower tiers in people’s decision making. In the example above, a person would be very unlikely to take a major detour from their Tier 1 trip to accomodate a Tier 2 need because their travel motivation is speed. Likewise, when commuting to and from work, people might take a detour that they know will take ten minutes, but they will be much less likely to take a detour if it involves driving in an unfamiliar area (risk of getting lost) or in a highly congested area (risk of getting stuck in traffic) because their primary motivation is reliability. People want to get to work (or home from work) at a certain time.

Personal Preferences

Every individual has a travel tolerance, or an amount of time that they are willing to spend in transit. This tends to be highly mode-dependent. For instance, I personally enjoy a ten minute walk. Many people do not. I do not enjoy an hour long drive, but some people do. Some examples:

• When I lived in Europe and took the train from Castiglion Fiorentino to Florence, the trip took a little over an hour. That trip never bothered me, because I would read or draw during the trip and it went by very quickly.
• Living in Houston I drive to College Station sometimes, which also takes a little over an hour. I find that trip to be extremely boring, and I really don’t enjoy it. Some people like this, though.
• Also, I don’t really enjoy flights lasting more than about 3 hours. I find sitting still for that long gets pretty uncomfortable. However, there are plenty of people who fly longer than 3 hours all the time and it doesn’t bother them.
• When it comes to commuting, I find that 20-30 minutes is about as much as I can handle. That’s pretty common, though there are many people who are willing to commute up to an hour each way. There are not many people who will commute farther than that.

My personal travel tolerance is therefore highest by train or by foot, then by plane, then by auto as a passenger, and lastly by auto as a driver. Like I said, I get bored really easily while driving, but that’s just me.

Travel tolerance applies to all facets of of life.

Social networks a a good example of how this plays out. In a city the size of Houston it’s easy to have friends who live 30 or 40 minutes away from you. You’ll quickly notice, however, that some of these people don’t mind coming to your place to hang out, and others will protest that it’s too far. Most people find their best friends will live within a mutual travel tolerance, ie close enough that neither person minds going to the other’s neighborhood to socialize. Weaker friendships, when someone lives outside your travel tolerance but within theirs (or vice versa), and when people live outside of mutual travel tolerance, close friendships are uncommon.

Of course, long-term, long-distance friendships do exist, and frequent socializing isn’t the only indicator of a healthy friendship. Also, technology has begun to expand people’s radius of friendships by creating artificial proximity. But the basic principle remains, people tend to be friends with people who live within each others travel tolerance.

Why this matters

The reason this is important is that is provides a vital lens to examine our transportation policy and design. When we’re building a road, a sidewalk, or a light-rail line, it’s important for us to decide what transportation tier we are attempting to serve, and build accordingly. There are two important principles that come out of this theory:

1. Because each tier of transportation has a different critical factor, conflict between tiers sharing the same space is common.

The daily commute is the easiest example to consider. Every morning there are millions of people driving around Houston, and they’re doing three very different things. Some are traveling through Houston on their way to another place, a Tier One trip. Many people are traveling to and from work, a Tier Two trip. Most people are running errands, just going about their daily life, a Tier Three trip. The problem is, in places where the street network is not highly connective, all of these people end up competing for the same road and freeway space, and their competing interests cause behavior that can make the congestion worse.

For instance, when the freeway is highly congested many Tier 2 commuters will stick to the frontage road, just because it’s less stressful. This conflicts with Tier 3 users running errands at shops along the frontage road. Also, Tier 3 users who are familiar with the local traffic pattern might choose to use the freeway just to skip one traffic light, which adds to the congestion on the on/off ramps.

2. All three tiers must be present in a well connected network for any transportation system to function well.

The simplest reason why the automobile is the dominant form of transportation in this country is that it is one of the only travel modes than can operate effectively across all three transportation tiers. The problem with cars is that they take up a lot of space and function best at low densities. When the car was invented, America was a principally agrarian nation with a widely-distributed, low-density population. We fell in love with the car in that era. The problem is, the space requirements of cars (parking and freeway lanes) are increasingly costly to provide as cities get larger and larger. Our nation is now predominantly urban, and the pattern of urbanization shows no sign of reversing. As cities grow larger, population density naturally increases, and an auto-dependent transportation system is less and less effective.

The problem is, people have a hard time coming up with alternatives. When we think about building a new transportation system, we have to remember that there must be compatible networks across all three tiers if a system is going to function well.

All right, that’s enough for one post. I’ll continue with some other theoretical frameworks in the days ahead, and look at how these can be applied to regional mobility in Houston. Stay tuned, and please share your thoughts by leaving a comment!