Travel time reliability is the measure of a transportation mode’s ability to move people between two places in a predictable span of time. Everyday factors affecting the reliability of transport systems (highways, street networks, rapid transit, bike paths, etc.) include weather, operator error, technological system error, and neglect.
The social, environmental, and economic implications of travel time reliability are immense (and beyond the scope of this post). For an individual citizen, the availability of reliable transportation is a prerequisite for freedom itself – freedom of movement.
Philadelphia’s KYW Newsradio 1060 (AM) traffic report – “Traffic and Transit on the Two’s” – provides regular data points to support my assertion that mass-transit is substantially more reliable compared to highways. Nearly every report details a litany of jams on the region’s major highways; and nearly every report concludes with: “mass-transit, no reported delays.” (It is important to note that SEPTA’s rail system is both massive and ancient.)
The comparison is well illustrated on the Ben Franklin Bridge, which carries a heavy rail commuter line, a bicycle/pedestrian walkway, and seven vehicular lanes (I-676). 100,000 vehicles travel the bridge’s seven lanes on a typical day; and the malfunction of a single machine can impact the lives of thousands of people.
While enjoying an Easter bike ride over the bridge to visit my girlfriend’s family in Camden, I witnessed the perfect anecdote (pictured above). The motor vehicle of some unfortunate soul had caught fire in the westbound lanes, and three seemingly endless lines of cars became stuck for hours.
Any number of other events could have led to the same result: a drunk driver, someone texting, a spent catalytic converter or fuel pump, someone running out of gas, etc. Any of these relatively probable occurrences could impact the lives of tens of thousands of people.
It takes only one human error or mechanical breakdown – out of 100,000 vehicles every day – to shut down the system for hours. And unlike a gridded street network, alternate routes are literally few and far between.
Contrast this to the PATCO High-Speed Line, the mass transit subway traveling over the same bridge. No doubt – a single malfunction or operator error could inconvenience thousands of people, but the chances for this to occur are minute in comparison to I-676.
First of all, a relatively small number of people “operate” the PATCO system. These people are trained professionals working under strict rules and regulations. They are screened for drug use, and prohibited from using their cell phones.
Furthermore, the mechanical and electrical systems employed in railroading, which provide for a significant degree of automation, have been perfected over 150 years and continue to improve. (“Cars that drive themselves” are mere experiments.)
Of course transit systems experience delays on the regular. But these occurrences are infrequent when set against the massive amount of time (and money) people waste inching along “expressways.”
Chain Link Metaphor
A useful metaphor is to think of highways as relying on a chain composed of a large number of weak links, and rapid transit systems relying on a smaller number of strong links.
Highways have a great many chances to fail, and each chance is relatively probable. Rapid transit systems have much fewer chances to fail, and each chance is relatively improbable. I suspect a thorough quantitative comparison would uphold the metaphor.
The Reliability of Non-Motorized Transportation
The metaphor breaks down when applied to non-motorized transport, which occupies a reliability category of its own. Because we lack comprehensive biking infrastructure in the United States (such as the Dutch enjoy), the point is limited to sidewalks, where present.
If one chooses to arrange their life such that everyday trips can be accomplished on foot, they will enjoy 100% travel time reliability.
As observers from Jane Jacobs to Jeff Speck have pointed out, people might not choose to walk unless the trip is safe, useful, comfortable, and interesting. But these qualities in a neighborhood require minimal public investment, and usually arise from the mere existence of healthy communities.
Alas, if walking is the most reliable form of transportation with respect to travel time, and if it is followed closely by biking, with transit a distant third, and highways dead last, then, we conclude that travel time reliability correlates directly with energy efficiency and cost effectiveness, and correlates inversely with environmental impact and required public investment.