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VIEW POINT Transportation accessibility


The metrics: reloaded T


Performance-based management is paramount, says Mike McGurrin as he delves into the cloudy waters of transportation metrics


he Moving Ahead for Progress in the 21st Century Act (MAP-21) became law on 6 July 2012. MAP-21 codifies the increasing emphasis on performance-


based transportation management that has been building for many years. Performance-based management can be a powerful tool for guiding effective investments, but only if the metrics used are well-aligned with the desired outcomes. Paraphrasing management consultant Peter Drucker, the proper metrics will ensure that transportation agencies are doing the right thing, not just doing the thing right. What is the goal (desired outcome) of transportation?


Are we using metrics that fully capture that goal? Most transportation is what economists refer to as a “derived demand.” Transportation is just a means to an end. We rarely transport people or goods just to move them, but rather to provide services, obtain work, or achieve some other primary purpose. Improved access can be provided by reducing congestion,


but it can also be provided through changes in land use, by increasing connectivity in the transportation system, or by providing substitutes for transportation, through the move- ment of information. Therefore metrics that focus only on congestion and its associated costs only address a subset of the problem and of potential solutions. Mobility metrics, such as Annual Hours of Delay (AHD) used in programs such as the US Congestion Management and Air Quality (CMAQ) program, defined as travel time above a congestion threshold (defined by State DOTs and MPOs) in units of vehicle hours of delay are incomplete. Such measures focus only on delay above a travel time based on free-flow or other set value. As long as traffic moves freely, it doesn’t matter, according to these measures, how long the total trip time is, or, in fact, whether or not the trip is even necessary. Metrics such as delay, the travel time index, or buffer times all share this shortcoming (not to mention that they are focused on a single mode of travel). These metrics are well-understood and relatively easy to measure, but focusing exclusively on them can lead to misdirected and inefficient investments. Because transportation is a derived demand, the more


appropriate goal is to minimize the total time and cost asso- ciated with travel, not just those associated with congestion. There is a concept accompanied by objective, quantitative metrics that focus on this goal. This is the concept of trans- portation accessibility, also called access to destinations.


38 thinkinghighways.com Vol 8 No 3 North America


DEFINING ACCESSIBILITY Accessibility is defined as the ability to reach desired goods, services, activities, and destinations. Accessibility can be improved by reducing congestion, but it can also be improved by adding additional links in transportation networks (eg, new highway bridges or increased service on metro lines), by locating services closer to households (through land use changes) or by substituting the movement of information for the movement of people and goods (eg, telework). Acces- sibility is a comprehensive outcome-focused measure that captures the effects from any of these changes. Academics have developed numerous mathematical for-


mulas for calculating accessibility metrics. Among the easiest to understand are cumulative opportunity metrics. Cumula- tive opportunity metrics simply count the number of desti- nations of interest within a set travel time or distance. For example a cumulative opportunity model for jobs in a neigh- borhood using a 30 minute travel time threshold, simply counts the number of jobs that can be reached within a 30 minute commute from that neighborhood. The accessibility


3D heat map shows access to jobs in Washington, DC


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