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FHWA Home / Safety / Pedestrian & Bicycle / Guide for Scalable Risk Assessment Methods for Pedestrians and Bicyclists

Guide for Scalable Risk Assessment Methods for Pedestrians and Bicyclists

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STEP 4. SELECT EXPOSURE MEASURE

Step 4 in the scalable risk assessment process is to select a specific exposure measure to be used in the calculation of risk values. There are several different categories of exposure measures that attempt to quantify the level of contact that pedestrians and bicyclists have with potentially harmful safety outcomes. The five categories of exposure measures included in the guide are briefly defined below.

The selection of an exposure measure will depend upon several criteria, such as the use of the risk values (Step 1), the geographic scale (Step 2), and other criteria. This chapter of the guide introduces the various categories of exposure measures and provides guidance on selecting the most appropriate exposure measure given criteria (such as geographic scale and analytic method). Table 11 contains a selection matrix to help analysts choose an exposure measure best suited for their analysis. Note that each exposure measure will be for a defined time period that matches other variables in the risk definition (such as crashes or other risk indicators). Table 12 provides guidance on the strengths and limitations of each category of exposure measure.

Table 11. Selection Matrix for Exposure Measures
Category of Exposure Measure Typical measures Typical scale Typical data sources
Point Segment Network Region
Distance Traveled Miles of travel to a small extent to a great extent to a great extent to a great extent
  • Site counts or demand estimation models, multiplied by segment length
  • Sometimes travel surveys
Miles crossed per entering vehicle to a moderate extent      
Time Traveled Hours of travel to a small extent to a small extent to a great extent to a great extent
  • Travel surveys
  • Sometimes site counts combined with crossing time or average travel speed data.
Product of crossing time and vehicle volume to a small extent to a small extent    
Volume/
Count
Volume/count to a great extent to a great extent    
  • Site counts
  • Demand estimation models
Product of pedestrian /bicyclist volumes and motor vehicle volumes to a moderate extent to a moderate extent    
Trips Made Number of trips     to a great extent to a great extent
  • Travel surveys
Population Number of people that walk or cycle on regular basis     to a great extent to a great extent
  • U.S. Census data products
Percent of the population that walk or cycle on regular basis     to a great extent to a great extent

Legend: open circle (black circle with white center) = to a small extent; half circle (black circle with left half solid black) = to a moderate extent; closed circle (black circle with black center) = to a great extent.
Note: Each exposure measure will be for a defined time period that matches the risk definition.

Source: Partially adapted from Greene-Roesel et al., Estimating Pedestrian Accident Exposure: Protocol Report, March 2007.

Table 12. Strengths and Limitations for Exposure Measure Categories
Category Strengths Limitations
Distance Traveled
  • Most commonly used measure for motor vehicle exposure.
  • Can be used at facility-specific and areawide geographic scales.
  • Calculation typically based on simple data inputs (counts and segment lengths).
  • Better represents quantity of travel than population or volume/count-based measures.
  • Not the best measure for comparing risk between different modes, due to shorter distances travelled for pedestrians and bicyclists.
Time Traveled
  • Better measure for comparing risk among different travel modes, as it accounts for different prevailing travel speeds (and therefore travel times) for each mode.
  • Can be used at facility-specific and areawide geographic scales.
  • Better represents quantity of travel than population or volume/count-based measures.
  • The number of walking and bicycling trips are often underreported in travel surveys.
  • The amount of time traveled for walking or bicycling can be overestimated in surveys, especially if based on self-reporting or recall.
Volume/
Count
  • Simple exposure measures that are based entirely on site counts.
  • No assumptions are made about distance or time traveled.
  • Does not account for the distance or time traveled by pedestrians or bicyclists, which is an important weighting mechanism in exposure measures.
  • Less meaningful when aggregated to areawide geographic scales.
Trips Made
  • Trip reporting is common in travel surveys and also emerging GPS-based smartphone applications.
  • Provides meaningful exposure measures for areawide geographic scales.
  • Trip-based measures can be subdivided (i.e., by trip purpose) for more detailed exploratory analysis.
  • Walking and bicycling trips are often underreported in travel surveys.
  • A large number of survey respondents are needed to adequately represent the full population.
  • Trip-based measures are not meaningful for facility-specific geographic scales.
Population
  • Typically easy and low-cost to obtain; available for most areawide geographic scales.
  • Travel surveys can be used to subdivide the total population to the portion that walk or cycle on a regular basis.
  • Does not account for the number of trips or intensity (i.e., distance or time traveled).
  • Simple population-based measures may not even account for the portion of the population that walks or cycles (e.g., available census data are limited to commute trips made by the working population).
  • Does not capture visitor or tourism trips/travel.

Source: Partially adapted from Greene-Roesel et al., Estimating Pedestrian Accident Exposure: Protocol Report, March 2007.

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Page last modified on September 19, 2018
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