Traffic, Mobility and Accessibility
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Victoria Transport Policy Institute
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Updated
22 July 2008
This chapter describes different ways to measure transportation system performance, discusses their different perspectives and assumptions and how they affect planning decisions, and identifies best practices for more objective and comprehensive analysis.
Transportation Performance Indicators
Perspective: Traffic, Mobility and Accessibility
Trade-offs Between Different Types of Accessibility
Planning and Investment Decisions
Road Safety Analysis (Safety Evaluation)
Unit Costs of Incremental Peak-Period Vehicle Trips
(Transportation Costs)
Florida Multi-Modal Quality-Of-Service Standards (FDOT,
2002)
San Francisco Multi-Modal Level-Of-Service (Hiatt, 2006)
The Human Capacity Manual
(www.walksf.org/essays/pedCountEssay.html)
References And Resources For More Information
Management experts often say that, “you can’t manage what you can’t measure.” What is measured, how it is measured, and how data are presented often affect how problems are defined and solutions selected. A particular solution may appear best when measured one way, but undesirable when measured another way.
For example, a baseball player’s performance can be evaluated based on batting averages, base hits, runs batted in, and ratio of wins to losses, plus various defense statistics that depend on the player’s position. Performance statistics can be calculated per at-bat, per inning, per game, per season, or for a career. A player can be considered outstanding according to one set of statistics but inferior according to another.
Similarly, people’s economic status can be evaluated based on hourly, monthly or annual wages; income per worker, per adult or per household; gross, net or disposable income; wealth (income plus assets minus debts); or purchasing power (which takes into account relative wages and living costs). Independent contractors are paid high hourly rates, but often only work part-time and must cover their own overhead expenses, so their net income is lower then colleagues with a steady salary. A multi-worker family will be considered poorer if income is measured per capita or per worker than per household. Residents in some communities have high wages, but their purchasing power is low due to relatively high living costs. Some households that own expensive homes and vehicles have negative wealth because their debts outweigh their assets. A person can be considered rich by one set of statistics, but poor by another.
These are just two examples of how different measurement methods and units can affect how things are evaluated. Often, no single unit conveys all the information needed for evaluation. Different measurement units represent different perspectives and highlight different features. Decision-makers may need to consider a variety of different statistics. A coach needs to consider several different statistics when evaluating how a particular player fits into a team. Policy makers need to consider various measures of income and wealth to determine which households should be considered economically disadvantaged. It is important that people using such information understand the different perspectives and assumption implicit in the units they use.
As Ken Alder explains, “Measures are more than a creation of society, they create society,” and fundamentally affect the relationships between people. Scientists and planners have long realized that how objects and activities are measured can affect how we think about and solve certain problems. This is an important factor in transportation planning. When transportation system quality is measured in one way, conditions may seem unsatisfactory, justifying certain improvements, but when the same conditions are measured in another way, the same improvements may appear unjustified and harmful.
This chapter discusses different methods used to measure transportation, the different perspectives they represent, how the selection of one or another method can affect transportation and land use planning decisions, and best practices for evaluating transportation activities and systems.
Performance indicators are practical ways to measure progress toward objectives (Evaluation). Per capita travel statistics, traffic counts, Level-of-Service (LOS) ratings, cost per mile, and customer satisfaction survey results are examples of performance indicators used for transport planning.
In most cases, no single indicator is adequate, so a set of indicators that reflect various objectives and perspectives are used. Which indicators are selected and how they are weighted and presented implicitly defines the value placed on different objectives. For example, if a community values nonmotorized transport it is important to include suitable indicators of pedestrian and cycling conditions in transportation planning.
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TDM Performance Indicators Below are common performance indicators used to Evaluate TDM programs. These indicators can be defined for a particular time (such as peak-hour) and geographic location (such as a particular destination, district or region). · Awareness – the portion of potential users who are aware of a program or service. · Response – the number of people who respond to an outreach effort, such as asking for information on alternative travel modes in response to a promotion campaign. · Participation – the number of people who use a service or alternative mode. · Utilization – the number or portion of trips that use a travel service or alternative mode. · Mode split – the portion of travelers who use each transportation mode. · Mode shift – the number or portion of automobile trips shifted to other modes. · Average Vehicle Occupancy (AVO): Number of people traveling in private vehicles divided by the number of private vehicle trips. This excludes transit vehicle users and walkers. · Average Vehicle Ridership (AVR): All person trips divided by the number of private vehicle trips. This includes transit vehicle users and walkers. · Vehicle Trips or Peak Period Vehicle Trips: The total number of private vehicles arriving at a destination (often called “trip generation” by engineers). ·
Vehicle
Trip Reduction – the number or percentage of automobiles removed from
traffic. · Vehicle Miles of Travel (VTM) Reduced – the number of trips reduced times average trip length. · Energy and emission reductions – these are calculated by multiplying VMT reductions times average vehicle energy consumption and emission rates. · Cost Per Unit of Reduction – these measures of cost-effectiveness are calculated by dividing program costs by a unit of change. For example, the cost effectiveness of various TDM programs could be compared based on cents per trip reduced, or ton of air pollution emission reductions. However, cost-effectiveness analysis that only considers direct impacts and a single objective may overlook additional costs and benefits to participants and society. For example, two TDM programs may have the same direct costs per unit of emission reduction, but differ significantly in terms of consumer costs, consumer travel options, traffic congestion, parking costs, crash risk and land use impacts. Evaluation
studies can compare performance indicator values before-and-after, over time
(for example, over months or years), with-and-without (for example, comparing
performance indicators at a worksite or area that has a TDM program with
otherwise comparable sites that do not have such programs, or with regional
averages). A variety of methods can be used to collect the data needed for
performance evaluation, including general travel Statistics,
participant Surveys, parking lot counts, traffic
counts, and focus groups. Before-and-after and with-and-with comparisons
require the collection of good baseline data or the use of readily-available
statistics. |
Different performance indicators reflect different perspectives about the nature of transport. Three perspectives, called traffic, mobility and accessibility, are described and compared below in terms of how they view users, modes, land use, transport problems and solution, and how they are measured.
Traffic refers to vehicle movement. This perspective assumes that “travel” means vehicle travel and “trip” means vehicle-trip. It assumes that increased vehicle mileage and speed benefits society.
From this perspective, transportation users are primarily motorists (including drivers, passengers and businesses that rely on commercial deliveries). Non-motorists are considered a relatively small and unimportant minority, defined as members of households that do not own an automobile.
This perspective focuses on automobile travel. It places little value on transit and cycling, since they represent a small portion of vehicle-mileage. It considers walking primarily as a way for motorist to access parking facilities or as a form of recreation, and so devotes little transportation funds to nonmotorized facilities.
This perspective evaluates land use primarily in terms of proximity to highways and parking supply. The best location for a public facility is along a major arterial or freeway intersection, in an area with abundant parking supply. Downtown locations are undesirable due to excessive roadway congestion and parking costs.
This perspective defines transportation problems in terms of costs, barriers and risks to motorists. It favors solutions that increase road and parking capacity, roadway traffic speeds, vehicle ownership, and the affordability of driving. From this perspective, the best way to benefit non-drivers is to help them become motorists, by making automobile and taxi travel convenient and inexpensive.
Vehicle traffic is relatively easy to measure. Most jurisdictions have data on motor vehicle registrations, drivers licenses, and vehicle mileage. Performance indicators include traffic volumes, average traffic speeds, roadway Level of Service (LOS), congestion delay, parking supply, vehicle costs and crash rates.
Table 1 Roadway
Levels of Service (TRB 1994)
|
LOS |
Maximum
Flow Cars/lane/hour |
Average
Speed Km/hour |
Maximum
Density Cars/land/km |
|
A |
720 |
96.5 |
7.5 |
|
B |
1,200 |
96.5 |
12.4 |
|
C |
1,650 |
94.9 |
17.4 |
|
D |
1,940 |
91.7 |
21.1 |
|
E |
2,200 |
88.5 |
24.9 |
This table shows Level of Serve (LOS) values used to
evaluate traffic conditions.
Mobility refers to the movement of people or goods. It assumes that “travel” means person- or ton-miles, “trip” means person- or freight-vehicle trip. It assumes that any increase in travel mileage or speed benefits society.
From this perspective, transport users are mainly motorists, since most person- and ton-miles are by motor vehicle, but recognizes that some people rely on non-automobile modes, and some areas have large numbers of transit, rideshare and cycling trips. It recognizes that a significant portion of people use non-automobile modes at least occasionally.
This perspective considers motor vehicles most important, but also values transit and ridesharing on congested corridors, and recognizes that walking and cycling may be important in areas such as college towns and resort communities. It supports an integrated view of the transportation system, with attention to connections between different modes. For example, it recognizes that most transit trips involve at least one walking link, and so walking and transit are complementary travel modes. It justifies devoting a modest portion of transport funding to transit, HOV and cycling.
From this perspective, convenient highway access and parking is most important, but transit and HOV access are also desirable in areas where density and demographics concentrate enough riders. The best location for public facilities has a combination of convenient roadway access, adequate parking, transit service, and cycling routes.
A mobility perspective defines transportation problems in terms of constraints on physical movement, and so favors solutions that increase motor vehicle system capacity and speed, including road and parking facility improvements, transit and ridesharing improvements, high-speed train, aviation and intermodal connections. It gives little consideration to walking and cycling except where they provide access to motorized modes, since they represent a small portion of person-miles. From this perspective, the best way to benefit non-drivers is to improve motorized transport, including automobile, transit and taxi modes, with more modest consideration of walking and cycling.
Mobility is measured in person-miles, ton-miles, and travel speeds. Mobility is sometimes measured door-to-door, taking into account each link of a trip, including walking to a parking lot or transit stop. Current travel data tends to underrepresent non-motorized travel, short trips, travel by children and lower-income people, and recreational travel, but newer travel surveys can help overcome these constraints (Stopher and Greaves, 2007). In recent years improved techniques have been developed to evaluate Transportation Diversity, Transit and Nonmotorized travel. Transportation engineers now have standardized methods for calculating pedestrian, cycling and transit Level of Service, just as they do for automobile traffic (IHT, 2000; FDOT, 2002; Mitchell and Milam, 2006), although these are not yet widely used.
Accessibility (or just access) refers to the ability to reach desired goods, services, activities and destinations (collectively called opportunities). Access is the ultimate goal of most transportation, except a small portion of travel in which movement is an end in itself (jogging, horseback riding, pleasure drives), with no destination. This perspective assumes that improved access benefits society, and mobility is one way to achieve this goal. This perspective considers vehicle traffic a subset of mobility, and mobility a subset of accessibility.
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Other Meanings of “Access” The
word “access” can have other specific meanings in transportation planning. In
pedestrian planning it refers to Accessible Design or Universal
Design, which refers to facilities designed to accommodate people with
special needs, including those with disabilities. For example, a pathway
designed to accommodate people in wheelchairs may be called “accessible.” In
roadway engineering “access” refers to connections to adjacent properties,
such as driveways and private roads. A “limited access” highway has minimal
connections to adjacent properties, while a local road provides direct
access. Access Management refers to programs to limit
the number of driveways and intersections on highways to improve traffic flow
and safety. |
From this perspective, transportation users consist of any person or businesses that wants to reach a good, service, activity or destination. It recognizes that most people use a variety of access options.
This perspective considers all access options as being potentially important, including travel options such as Transit, Ridesharing and Nonmotorized Modes; mobility substitutes such as Telework and Delivery Services; and strategies to increase land use Accessibility such as Smart Growth and Location Efficient Development. It supports an integrated view of transportation and land use systems, with attention to connections among modes and between transport and land use patterns. It values modes according to their ability to meet users’ needs, and does not necessarily favor longer trips or faster modes if shorter trips and slower modes provide adequate access. It considers Walkability to be a particularly important mode, because walking provides Basic Access, including connections between modes and to destinations. It supports the broadest use of transport funding, including mobility management and land use management strategies if they increase accessibility.
From this perspective, land use is as important as mobility in the quality of transportation, and different land use patterns favor different types of accessibility. The distribution of destinations, land use mix, network connectivity and walking conditions all affect transportation system performance. The best location for public facilities has a combination of convenient proximity, roadway access, transit service and walkability.
Accessibility-based planning expands the range of transport problems and potential solutions that can be considered. From this perspective, transport problems include any cost, barrier or risk that prevents people from reaching desired opportunities. Solutions can include traffic improvements, mobility improvements, mobility substitutes and more accessible land use.
Accessibility is evaluated based on the time, money, discomfort and risk (the generalized cost) required to reach opportunities. Individuals often think of it in terms of convenience, that is, the ease with which they can reach what they want. Accessibility is relatively difficult to measure because it is affected by a variety of transportation, economic and geographic factors. For example, access to employment is affected by an individual’s physical and economic abilities, the quality and cost of travel options that reach worksites, the feasibility of telework (which may allow employment for a firm that is physically difficult to reach), and the geographic location of suitable jobs. Activity-based travel Models and integrated transportation/land use models using detailed travel survey data are most suitable for quantifying accessibility. Although access is a well-recognized concept in the disciplines of geography and urban economics, it is a new concept for many transportation practitioners. In recent years transportation professionals have started exploring the implications of basing transport planning on access rather than traffic or mobility (BTS, 2001). Improved techniques are being developed to better evaluate Transportation Diversity, Transit and Nonmotorized travel, as well as Land Use Factors that affect transport. The Accessibility chapter describes how to calculate an Accessibility Index.
Table 2 summarizes differences between these three ways to measure transportation.
Table 2 Comparing Transportation Measurements
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|
Traffic |
Mobility |
Access |
|
Definition of Transportation |
Vehicle travel. |
Person and goods movement. |
Ability to obtain goods, services and activities. |
|
Unit of measure |
Vehicle-miles and vehicle-trips |
Person-miles, person-trips and ton-miles. |
Trips. |
|
Modes considered |
Automobile and truck. |
Automobile, truck and public transit. |
All modes, including mobility substitutes such as telecommuting. |
|
Common performance indicators |
Vehicle traffic volumes and speeds, roadway Level of Service, costs per vehicle-mile, parking convenience. |
Person-trip volumes and speeds, road and transit Level of Service, cost per person-trip, travel convenience. |
Multi-modal Level of Service, land use accessibility, generalized cost to reach activities. |
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Assumptions concerning what benefits consumers. |
Maximum vehicle mileage and speed, convenient parking, low vehicle costs. |
Maximum personal travel and goods movement. |
Maximum transport options, convenience, land use accessibility, cost efficiency. |
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Consideration of land use. |
Favors low-density, urban fringe development patterns. |
Favors some land use clustering, to accommodate transit. |
Favors land use clustering, mix and connectivity. |
|
Favored transportation improvement strategies |
Increased road and parking capacity, speed and safety. |
Increased transport system capacity, speeds and safety. |
Various strategies to increase transport and land use system capacity, efficiency and safety. |
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Implications for TDM |
Considers vehicle travel reductions undesirable, except where congestion is extreme. |
Supports TDM strategies that improve personal and freight mobility. |
Supports TDM whenever it is cost effective. |
This table compares the three major approaches to measuring transportation.
For example, from a traffic perspective, the best location for a public school (or other major public facility) is adjacent to a major roadway at the urban fringe where land is available for abundant parking, and most school transportation resources will be devoted to accommodating the needs of parents who chauffeur their children to school. This assumes that most staff and students will arrive by private automobile. From a mobility perspective, the best location is on a major urban street with adequate parking, frequent public transit service, and perhaps a bike lane, and school transportation resources can be devoted to accommodating both private automobile trips and school bus services. This assumes that most staff and students will arrive by automobile, but some will bicycle or use transit. From an accessibility perspective, the best location for a school may be within a residential neighborhood, even if driving is inconvenient there, because most students and some staff will walk or bicycle, and school transportation resources can be devoted to School Trip Management.
Different transport modes play different roles in providing mobility and accessibility. For example, nonmotorized modes serve shorter-distance trips and motorized modes serve longer-distance mobility. Some modes are more suitable for people with physical disabilities or low incomes. Some modes are particularly important for certain industries.
Standard transport statistics indicate that motor vehicles are by far the most important form of transport, implying that other modes do little to provide accessibility. Travel surveys indicate that in most North American communities more than 90% of households own an automobile, and that more than 90% of trips are made by automobile, while only about 5% of trips are made by nonmotorized modes and less than 2% are made by transit. This suggests that the only way to significantly improve transport is to improve automobile travel, and that 90% or more of transport funding should be devoted to automobile-oriented improvements.
But the high priority given automobiles and the low priority given other modes is partly an artifact of how data are collected and presented. Most travel surveys only count the primary mode used between relatively large Transportation Analysis Zones (TAZs), and some only count peak-period travel or commute trips. Most travel surveys are biased in ways that undercount shorter trips and travel by lower-income people (Stopher and Greaves, 2007). As a result, they undercount shorter trips (those occurring within a TAZ), nonmotorized links of motorized trips, off-peak trips, non-work trips, travel by children, and recreational travel. Although only about 5% of trips are made exclusively by nonmotorized modes, about four times as many involve at least some walking or cycling on public right-of-way. For example, most surveys would not count a walk from a parking space to a destination, or a walk from work to a nearby diner for lunch. If a traveler cycles 10 minutes to a bus stop, rides a bus for five minutes, and takes another 5-minute walk to their destination, this bike-transit-walk trip is usually coded simply as a transit trip, even though the nonmotorized links take more time than the motorized link.
Although most households own an automobile, many members of automobile-owning households cannot drive or must share a vehicles with other drivers. Motorists often use alternative modes when their automobile is unavailable due to a mechanical failure or other problems, and an increasi