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FHWA Home / Safety / Pedestrian & Bicycle / Synthesis of Methods for Estimating Pedestrian and Bicyclist Exposure to Risk at Areawide Levels and on Specific Transportation Facilities

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Executive Summary

Background

The Federal Highway Administration (FHWA) and other federal agencies routinely work with state and local agencies to improve the safety and connectivity of bicycling and walking networks. Part of this effort has been to promote a data-driven approach to identifying and mitigating safety problems.

Pedestrian and bicyclist exposure to risk is often mentioned as a missing piece of the puzzle. The lack of readily-available pedestrian and bicyclist exposure data often make it difficult to accurately identify and then prioritize high-crash (or high-risk) locations or interpret year-to-year trends in citywide, state, or national crash statistics. Even when pedestrian or bicyclist exposure data are used, inconsistency can be present in the formulation and calculation of exposure measures between regions. Exposure has been defined based on direct counts, population, hours of travel, miles of travel, and others. Having diverse measures and definitions make direct comparisons difficult, if not impossible.

Pedestrian and bicyclist exposure is also a critical element in better understanding pedestrian and bicyclist crash causation. For example, exposure is a key variable in developing pedestrian and bicyclist safety performance functions, which are used to identify those factors that best predict when and where crashes are likely to occur. If exposure is not included, then the effects of other factors may be biased or overstated due to the omission of a key variable (exposure).

Overall Project and Task 3 Objectives

In May 2016, the FHWA’s Office of Safety initiated this project to develop a standardized approach to estimate pedestrian and bicyclist exposure to risk in the form of a Scalable Risk Assessment Methodology (ScRAM). This approach will make it easier to assess pedestrian and bicyclist exposure to risk and inform project priorities and funding decisions, and will include several methods according to the scale of the needed exposure estimate.  The project objectives are three-fold:

  1. Synthesize and document existing technical resources.
  2. Develop scalable risk assessment methods for practitioners to use to calculate pedestrian and bicyclist exposure to risk that can be applied to a facility, corridor, network/system, or regional level.
  3. Widely promote the use of these risk assessment methods through outreach, training, and technical assistance.

This report documents the findings of Task 3, which sought to review and synthesize the variety of methods used to estimate and evaluate exposure in pedestrian and bicyclist safety analyses (i.e., first objective in bullet list above). Later tasks in this project will develop and promote the risk and exposure assessment methods (i.e., second and third objectives in bullet list above).

Definitions and Concepts

Chapter 2 summarizes basic definitions and concepts for risk and exposure, and then discusses these terms in the context of pedestrian and bicyclist safety analysis. It is important to define these terms and related concepts in the early stages, such that subsequent development work in this project has a clear and unambiguous foundation.

Definitions of Risk and Exposure in Transportation Safety

In the literature, most authors agree on the theoretical definition of risk as a measure of the probability of a crash to occur given exposure to potential crash events. The relationship between risk and exposure is implied in some definitions, but more explicit in others; exposure is a normalization factor (i.e., denominator) to equalize for differences in the quantity of potential crash events in different road environments. There is also general agreement in the literature on this relationship between risk and exposure:

Formula. Risk is defined as the number of expected or measured crashes, by kind and severity, divided by exposure.

Most theoretical definitions of exposure in the literature are similar, in that exposure is a measure of the number of potential opportunities for a crash to occur. However, there is wide divergence on operational definitions of exposure, and an even wider range of exposure measures being used in practice. For example, exposure measures in the literature have been quantified in terms of:

Several key factors may explain this wide divergence in exposure measures used:

Definitions of Risk and Exposure in Public Health

Much similarity exists between how risk is conceptualized and estimated between the fields of public health and transportation safety. There are many examples in the literature of epidemiological studies that use similar methods for estimating time at-risk or some other metric needed to compute risk or rates. Differences between the two fields exist for terminology such as relative risk, which needs to be considered when interpreting and comparing findings across the two fields.

Importance of Geographic Scale in Exposure Analysis

Many articles in the literature emphasize the importance of geographic scale in estimating exposure. Different data sources and methods are available or feasible at different geographic scales, and this is likely to explain some of the wide divergence in the exposure measures used in pedestrian and bicyclist safety analyses. In this report, exposure scale is defined as the geographic level for which an exposure measure is desired. The literature review indicated that most exposure analyses could be grouped into one of these four scales (see Figures ES-1 and ES-2):

  1. Regional (like city, county, metropolitan statistical area, or state).
  2. Network for various area definitions (like traffic analysis zone, Census tract, or Census block group).
  3. Road segment (typically between major intersections or nodes).
  4. Point or street crossing (intersection or mid-block).
Figure ES-1. This figure illustrates two areawide scales—Regional and Network—that were found to be common in pedestrian and bicyclist safety literature. The Regional scale is the most spatially aggregate scale, and includes geographic definitions like city, county, metropolitan statistical area, state, and others. The Network scale is more granular than Regional scale, and includes US Census geographies (such as Census blocks, block groups, or tracts) and traffic analysis zones.

Figure ES-1. Areawide Scales (Regional and Network) in Pedestrian and Bicyclist Safety Literature

Figure ES-2. This figure illustrates two facility-specific scales—Segment and Point—that were found to be common in pedestrian and bicyclist safety literature. Segments are typically homogenous links between street intersections, whereas points are typically intersection or driveway crossings.

Figure ES-2. Facility-Specific Scales (Segment and Point) in Pedestrian and Bicyclist Safety Literature

Future methodological development in this project could benefit from the use of clear, unambiguous terms for various scales. For example, the Highway Capacity Manual is widely used for street and highway analysis and provides clearly-defined terms for various roadway system elements, such as points, segments, facilities, corridors, areas, and systems.  Similarly, the United States Census Bureau has defined several different area geographies, including Census tracts, block groups, and blocks. Also, some metropolitan planning organizations define traffic analysis zones for use in their travel demand models. Traffic analysis zones are typically composed of multiple Census blocks, but sometimes deviate from Census geography units to accommodate local conditions.

When and Where Does Exposure Occur for Pedestrians and Bicyclists?

Most theoretical definitions of pedestrian and bicyclist exposure include references to contact with harmful vehicular traffic or opportunities for a pedestrian or bicyclist crash. For pedestrians, this occurs most explicitly during a street crossing. But several authors in the literature have posed a series of questions about when and where pedestrians or bicyclists can be considered exposed. For example, are pedestrians exposed while walking along a sidewalk that is separated from motor vehicle traffic? Are bicyclists exposed when they travel in a bike lane immediately adjacent to a motor vehicle travel lane, but then not exposed if they are riding in a separated bikeway?

In most cases, the feasibility and practicality of data collection has been used to operationalize the theoretical definition of exposure. Data cannot be collected on all pedestrian, bicyclist, and motor vehicle movements at all locations at all times. Therefore, most operational definitions of exposure have been based on pedestrian and bicyclist activity data that are already available (e.g., from travel surveys) or can be feasibly measured or estimated (e.g., from direct counts or models).

Exposure Analysis at Areawide Levels

Chapter 3 summarizes many examples of exposure analyses that were conducted at areawide levels. In this report, areawide is a generic term that includes all geographic scales that are not facility-specific. The term areawide in this chapter includes geographic scales (some not explicitly defined) such as networks, systems, regional, city, state, etc.

The areawide exposure analyses in the literature were most often performed to quantify big picture trends in pedestrian and bicyclist safety. For example (see Chapter 3 for more details):

Data Sources and Methods

Most of the exposure analyses at areawide levels have used travel survey data from one or more of these sources:

Most methodologies for areawide exposure analyses fall into the category of sketch planning, which is defined as methods to estimate existing or future demand that are simpler alternatives to developing complex travel demand models.  Often, sketch planning methods are implemented in spreadsheets or geographic information systems using existing travel survey and other data. In some cases, the results from multiple surveys are combined to provide a more complete picture of pedestrian and bicyclist activity. In a few analyses, ACS data (which includes only primary journey-to-work trips) was combined with NHTS data (which includes all trips) to provide an accounting of all pedestrian and bicyclist trips. Also, locally collected survey data (from either regional household travel surveys or other localized data collection) has been used to supplement the federally collected ACS and NHTS data. For example, Chapter 3 summarizes an analysis for the Nonmotorized Transportation Pilot Program that uses spreadsheet-based calculations to combine ACS, NHTS, and locally collected count data to derive areawide exposure estimates. Chapter 3 contains several other similar examples whereby data from one or more travel surveys were used to develop areawide exposure estimates.

Exposure Measures

The units used in areawide exposure measures varied widely. Since the primary data source for areawide exposure analyses were travel surveys, and most travel surveys gather data on specific trips, many analyses used the number of pedestrian and/or bicyclist trips. However, some analyses focused on only journey-to-work trips (directly from ACS data), whereas other analyses included total trips (from NHTS data or combining ACS and NHTS data). In other analyses, the pedestrian and bicyclist trips were converted to pedestrian and bicyclist miles of travel using estimated trip length data. A limited number of analyses estimated pedestrian and bicyclist hours of travel using the number of trips and estimated trip times.

The geographic scale of available travel surveys is an obvious limiting factor in the scale and choice of exposure measures. The most common types of travel survey data do not have facility-specific trip information (although emerging crowdsource methods may address this in five to ten years), so exposure measures are limited to the areawide geography defined in the travel survey data that are being used. Travel surveys also do not include any data about nonmotorized traffic interactions with motorized vehicles, such that certain exposure measures (such as the product of pedestrian or bicyclist volume and motorized vehicle volume, (P or B) × V) cannot easily be estimated. These limitations notwithstanding, travel survey data can be a useful input to areawide exposure analyses when big picture exposure trends are desired for more aggregate geography and time intervals.

Exposure Analysis on Specific Transportation Facilities

Chapter 4 summarizes many examples of exposure analyses that were conducted on specific transportation facilities. In some cases, exposure estimates are calculated for specific facilities, but also aggregated to various areawide geographies.

The facility-specific exposure analyses were most often used to identify high-priority locations for pedestrian and bicyclist safety improvements and were typically conducted for an entire city. In some cases, the facility-specific information was also aggregated to provide overall trends for certain road types or for subareas within a city.

Data Sources and Methods

Most of the facility-specific exposure analyses used pedestrian and bicyclist count data from one or both of these sources:

For direct measurement of pedestrian and bicyclist counts, much progress has been made in the past ten years. Several companies now offer automated count equipment that helps to make pedestrian and bicyclist counting more efficient and cost-effective. The National Bicycle and Pedestrian Documentation Project provided early guidance and helped to promote count data collection. Since then, FHWA has included a chapter specifically devoted to nonmotorized traffic monitoring in their 2013 edition of the Traffic Monitoring Guide. In 2014, NCHRP Report 797: Guidebook on Pedestrian and Bicycle Volume Data Collection was published and is a comprehensive resource on pedestrian and bicycle count data collection. This direct measurement approach (calculating exposure from systematic traffic monitoring programs) is the current state-of-the-practice for motor vehicle exposure estimation.

For estimating pedestrian and bicyclist counts, direct demand models have been the most widely used models for facility-specific exposure estimation thus far, and typically use regression analysis to relate directly measured counts to other measured attributes of the adjacent environment (e.g., land use and form, street type, etc.). Assuming that these measured attributes are available citywide, the regression model allows one to extend the sample of facility-specific counts to all facilities citywide. Chapter 4 provides details on other types of modeling approaches that have been used on a limited basis or could be used for facility-specific exposure estimation. These approaches include regional travel demand models, geographic information system (GIS)-based models, trip generation and flow models, network analysis models, discrete choice models, and simulation-based traffic models.

Exposure Measures

Similar to the areawide exposure analyses, the units used in facility-specific exposure measures varied widely. Since the primary data source was pedestrian and bicyclist count data (rather than surveyed trip data in areawide exposure analyses), the units of exposure typically were a volume count for specified time period or a distance traveled (calculated by multiplying a count by a street crossing width or road segment length). In a few cases where the exposure values were very high, the exposure was given in units of 1 million or 10 million (e.g., 1 million pedestrian miles traveled, 10 million pedestrian crossings).

Unlike areawide exposure analyses, several of the facility-specific exposure analyses did account for the interaction of nonmotorized and motorized traffic in their exposure measure. Several analyses computed the product of pedestrian or bicyclist traffic and motorized traffic ((P or B) × V) at intersections or other street crossings. A few other analyses used exposure measures such as pedestrian crossings (or pedestrian miles) per entering motorized vehicle. Thus, having more granular exposure data on specific facilities does provide a better opportunity to quantify the level of interaction between pedestrian or bicyclist traffic and motorized vehicle traffic.

Risk Factors Other than Exposure

Chapter 5 summarizes many risk factors (other than exposure) for pedestrian and bicyclist safety. Findings from most of these studies indicate association, not causation, between potential risk factors and crash outcomes. The risk factors were categorized into two basic groups:

  1. Disaggregate risk factors are facility-specific (e.g., facility quality or condition) or individual-specific attributes (e.g., age, socioeconomic, behavioral).
  2. Aggregate risk factors are associated with areawide geography (e.g., land use, urban form).

For disaggregate risk factors, many studies have shown that poor facility conditions (e.g., no or inadequately designed pedestrian and bicyclist infrastructure) are a significant risk factor, as well as facilities where high-speed, high-volume motorized traffic routinely interacts with pedestrian and bicyclist traffic. Significant risk factors for individuals are age (i.e., children and seniors), intoxication, nighttime visibility, and distracted behavior.

For aggregate risk factors, many studies have documented the effects of land use (in particular, population density) on pedestrian and bicyclist safety. Several studies have also found that neighborhoods with lower-income and minority communities have a higher risk for pedestrian and bicyclist crashes.

Conclusions

The project team reviewed and synthesized over 280 research and technical documents (see Bibliography) on pedestrian and bicyclist risk and exposure and developed these key conclusions:

Next Steps

Based on the findings and conclusions in this Task 3 report, the TTI-led project team will develop a conceptual framework and design for risk exposure estimation at several different geographic scales (Task 4.A. of this project). The conceptual framework will be based on best practices as identified in this report, as well as other practices and processes that may be in development (such as those from NCHRP 17-73, Systemic Pedestrian Safety Analyses). The first draft of the conceptual framework will be available for review in May 2017.

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Page last modified on March 31, 2017
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