Memo: Sustainable Transportation 7

Part of a series of “reading memos” that offer a brief summary of interesting academic content along with my personal reflections. This one covers Chapters 9, 11, and 13 (cars and transportation demand management) from Jeffrey Tumlin’s Sustainable Transportation Planning.


People have strong emotional attachment to their cars, equating freedom of mobility with personal liberty. Yet indulging in cars kills 40,000 Americans per year and has a myriad of externalized costs (e.g., congestion, air pollution, noise, waste, etc.). Principles for designing for cars including considering the local context (i.e., rural-urban transects), remembering that speed (over 18mph) kills, not trying to build more travel lanes to get rid of congestion, and making cities delightful for pedestrians first.

The many costs of driving

Existing design manuals for roads typically feature guidance for design vehicles, design speed, lane width, number of lanes, intersection design, corner sight distance, corner radii, right-turn lanes, curb extensions, access management (driveway design), grade separations, emergency service vehicles, and traffic calming. Traditionally, the Four-Step Travel Demand Model has helped forecast future travel demand between travel analysis zones (TAZ) but activity-based models (looking at individual and household travel choices) may better address traveler behavior.

Example of a curb extension with “pockets” for parking

Freeways have many disadvantages, but their negative impacts can be mitigated by running them below grade, capping them with a park, wrapping them with a wall or active uses (shops), and/or configuring their off-ramps for a safe transition to the city grid (one-way couplets to disperse traffic, gateway treatments, multiway boulevards, and pedestrian-scale designs).


Traditional carsharing features cars leased or owned by a carsharing organization (CSO), spread across a geographic area, reservable on the Internet or by phone, which must be returned to their original location. Peer-to-peer (P2P) carsharing allows car owners to share their private vehicles for revenue. Carsharing members drive ~44% less than if they owned their own cars and each shared car removes between 9-20 private vehicles from the road.

Carsharing is most successful where there is sufficient density (10+ units per acre), mixed uses, high off-street parking rates, and at least 40% of residents taking trips to work in nonauto modes. Municipalities can support carsharing by reserving parking for shared cars, defining/permitting carsharing by right in zoning ordinances, encouraging/requiring programs to be integrated into new developments (including with public access), and converting municipal fleets to shared cars. Communities can also support jump-start programs by guaranteeing revenues in return for placement of shared cars, or leverage money from federal or developer sources.

Transportation Demand Management

Traffic congestion is a concern because it takes up valuable time, increases local pollution, reduces capacity of the overall network, and limits future economic expansion. It’s not really possible to “solve” congestion (as it is a result of economic growth), but cities can strategically concentrate it in limited areas (e.g., freeway ramps), help people avoid trips altogether by focusing on accessibility (proximity of goods and services) over mobility, and influence people to choose more efficient travel modes (considering their most important deciding factors being time, quality, and cost). Transportation Demand Management (TDM) refers to strategies that encourage shifts of auto trips to alternative modes or out of peak periods.

The most effective TDM strategies are those that are mandatory for all developments

Broadly successful TDM programs include offering employers deeply discounted transit passes in exchange for universal enrollment of all employees, requiring employers to offer an equal transportation fringe benefit/subsidy for employees who park as well as nondriving employees, revising development standards to require provision of bicycle storage/showers/lockers, supporting carsharing programs, and opening a transportation resource center (storefront). Direct congestion/cordon pricing (i.e., a toll ring) around a city’s downtown is likely to be the most effective strategy, but its controversial nature suggests that congestion-based (dynamic) parking pricing would be a more politically acceptable alternative.


I completely agree that governments subsidize driving far too much, but it seems that attempts at incremental taxation (to address the negative externalities) are few and far between. For example, the federal gas tax in the United States has not been raised in almost 30 years, and suggestions of a VMT-based tax are met with outrage from all political sides. The continued subsidization of electric automobiles in the future (versus other modes of transport) also portends many more generations of auto-centric road infrastructure.

The lesson that removing freeways does not require major shifts to transit (and that spillover traffic can be absorbed) was surprising and somewhat counter-intuitive. The natural assumption is that the loss of so much road capacity would lead to traffic chaos without already established transportation alternatives, but it seems that congestion has the ability to settle itself into a new equilibrium without too much additional effort.

When it comes to carsharing, I was formerly an avid user both at home (in an apartment complex with limited parking) and when traveling (opting to use my membership sporadically instead of taking a traditional car rental). However, I noticed that the condition of the shared vehicles deteriorated over time (poor maintenance and limited cleaning) while costs kept increasing. Whenever I rented one, I also felt a lot of pressure to return the shared vehicle to its original location at a given time, which always made the experience stressful. The advent of one-way carsharing (where vehicles don’t need to be returned to their original location) with companies such as car2go was more attractive, but the business model seems unstable as several competitors have since left the market in North America.

While I’m familiar with the concept of induced demand (leading to perpetual traffic congestion no matter how many lanes are added to roadways), I didn’t realize it applied with the addition of any type of transportation infrastructure – including new transit lines and bikeways. It was a bit counterintuitive to think that a new rail line, for example, would not meaningfully relieve traffic congestion on roadways because any freed capacity would subsequently be consumed again. This is quite disappointing as I do often see transit infrastructure projects selling this false idea of relieving congestion, but at least their other touted benefits (more opportunity for economic activity and development) are still valid.

The political fights over congestion pricing in the United States are quite perplexing given the proven (exceptional) efficacy of this TDM strategy all over the world. One would think all of the potential trade-offs have been considered and negative impacts mitigated at some point already. However, given the tendency of policy makers in the United States to ignore what happens abroad (or espouse the belief of American exceptionalism), I imagine the implementation of congestion pricing here is still quite a long way off.

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