by David Levinson from Transportist.org
This text was copied from the source above mentioned. All Rights Reserved
Cities are organised so that many people reach one another in a short amount of time. Residents reach other people, places, goods, and services on foot, or by bike, bus, train, ferry, or car. People don’t need planes or very fast trains to travel between places within a city, even though planes and very fast trains are faster than walking, bikes, buses, trains, ferries and cars. Cities optimise what people can reach in a given amount of time, in the face of modest speeds. We see this when we compare the average speed of travel inside Sydney — about 30 km/h by car after considering traffic signals and congestion — versus the 100 km/h that they can travel on rural highways. Rational people pay dearly to live in Sydney or Melbourne compared to rural Australia. This is not a criticism of Sydney or Melbourne. Despite their extreme congestion the access these and other great cities provide, and the value of that access, is what makes cities great.
Accessibility measures how many potential destinations (jobs, workers, stores, doctors, etc.) someone can reach from a particular point in a given travel time (say 30 minutes) by a particular mode at a certain time of day. The cumulative opportunities measure of accessibility is like the meter or kilogram in the metric system, it means the same thing regardless of where you are. We can talk to a politician and show her how many jobs can be reached from a location in 30-minutes by transit at 8:00 a.m., and we can compare that number with any other point, where the accessibility may be higher or lower. We can compare Los Angeles and San Francisco, or compare Los Angeles in 2019 with Los Angeles in 1973. In Australia, the ’30-minute city’ has been adopted by the Greater Sydney Commission, the planning agency for the Sydney region, as a centrepiece of its 40-year plan. The aim is that residents of Sydney can reach one of three important regional centres in less than a half-hour by walking, biking, or public transport. Doing so requires the thoughtful application of knowledge at hand, using modes of transport technology that have been around for more than a century. This includes wise choices about big investments in subways or elevated highways, and intelligently making small decisions about streets, intersections, and transit stops.
Not everyone works, or needs to get to or from work in a half hour. Different places have proposed 5-, 10-, 15-, and 20-minute neighbourhoods as well. For instance, the ‘pint-of-milk test’ (in New Zealand, the beverage in question isn’t milk) asks whether you can purchase a pint of milk within a 10-minute walk of your home; and a modified version of that test asks if can you do it at a place that doesn’t also sell petrol. The related concept of a 20-minute neighbourhood is about ‘living locally’ by giving residents the opportunity to access all the services they need with a 20-minute round-trip walk, cycle, or public transport trip. While the 30-minute city tends to focus on work and includes travel by motorised vehicles, these other tests ask about life’s other activities and emphasise non-motorised travel. If you can walk to a pint of milk or the local hotel or pub within 10 minutes, and get to your major services within 30 minutes, you are doing better than the average 62-minute trip now experienced by public transport-riding Sydneysiders.
One way to examine accessibility is to measure how much additional accessibility some project or new service will provide. Another way is to examine how much accessibility is lost because an improvement has not been made.Prospect Theory teaches that we feel losses more significantly than gains. So, for example, if destinations reachable in 30-minutes is considered to be 100 percent of accessibility, if you lose 10 minutes of time out of that 30 minutes (because of delay or circuitous routes arising from poor system design), you lose more than half of your accessibility. That loss is so large because accessibility increases as a non-linear function of time. The area of the accessibility ring from 20 to 30 minutes (blue in Figure 1) is much larger than from 0 to 10 minutes (green), or 10 to 20 minutes (red). If we lose five minutes, we lose 30 percent of our accessibility, as shown in Figure 2. Every second counts. Even if a policy or design sacrifices only 30 seconds, this extra delay costs people not only their travel time, but a sacrifices opportunities they could have reached within that travel time.
