By Dom Nozzi
Mobility is about moving people and goods from place-to-place.
Accessibility is something that is easily approached, entered, obtainable, or attained.
Todd Litman (http://www.vtpi.org/tdm/tdm84.htm):
Accessibility Versus Mobility
Cities and other major Activity Centers tend to have a relatively poor vehicle mobility (due to congestion), but are economically successful due to excellent accessibility (activities are clustered together and there are many travel options). This indicates that in the game of economic competitiveness, accessibility trumps mobility.
This suggests that Traffic Congestion itself is not necessarily a major constraint on economic activity provided that land use patterns minimize the amount of driving needed to reach common activities and destinations, and that travelers have good Transport Options to choose from. Roadway level of service or average per-mile vehicle operating costs are less important indicators of transport system performance than average per-capita commute travel time and total per-capita transportation expenditures (Measuring Transport). Smart Growth strategies that result in more accessible land use may be the best way to improve transport and increase economic productivity, because they reduce the average distance between destinations and therefore total travel costs, while a congestion reduction strategy may provide little or no economic benefit overall if it stimulates sprawl which reduces overall accessibility in a community.
Transportation should be evaluated based on Accessibility, rather than treating mobility as an end in itself.
The following is from http://www.m-bike.org/blog/2009/06/09/accessibility-vs-mobility/
Traffic engineer Ian Lockwood...highlighted a key concept for bicycle, pedestrian, and transit advocates. Current U.S. traffic engineering culture pursues greater mobility, i.e. how fast someone can get between places. That’s often why they are stuck thinking primarily about cars, wider roads, higher speeds, and interstate expressways. Lockwood says we should … focus on accessibility instead. In doing so, we’d try to rein in sprawl…and improve transportation options.
Perhaps given our automotive heritage, Detroit seems particularly focused on mobility. A recent Brookings Institute report found Metro Detroit led the nation in job sprawl. Seventy-seven percent of our jobs are more than 10 miles from the city center.
The fundamental purpose of a transportation system is to provide people with comfortable access to what we want, in a short amount of time. Whether or not we use motorized vehicles to accomplish this access, is a secondary concern. In fact, energy independence would be best served if we used motorized vehicles as little as possible. Measuring convenient human access in terms of the speed of motorized light duty vehicles, is indirect and counterproductive. This is because transportation planners often minimize transportation choices in order to reduce delays per VMT — for example by shortening signal light durations, removing pedestrian crosswalks, and minimizing bike lanes. While increasing vehicular mobility, these measures reduce the safety and convenience enjoyed by pedestrians and cyclists.
…imagine an urban street grid completely dominated by cyclists and pedestrians, with many nearby destinations, and a few vehicles wending their way through the crowd at 3 mph. Such an area would generate a poor “mobility” score, even though it serves many people with quick, convenient errands, while consuming minimal energy.
…If the human goals for a transportation system also include human health and safety, community-building, minimal environmental impact, cost-effectiveness and delight, these goals can all be included in appropriate metrics. Transportation systems could easily be evaluated and designed in terms of all of these measures of human quality of life — for a tiny percentage of the cost of most vehicle-based “transportation improvement” projects. Since people pay for these systems, why settle for anything less?
If the fundamental goal of a transportation system is happy vehicles, then conventional vehicle-based metrics for mobility and congestion are appropriate. However, if the fundamental goal is happy people, then we need innovative performance metrics.