2013.09.22
Travel Behavior and Network Model Courses@The University of Tokyo

Reassessing accident risk indicators:
A risk-benefit perspective
The University of Tokyo
Makoto Chikaraishi

Contents
1. Reassessing accident risk indicators– Conceptual discussions– Empirical analysis

2. Needs for further activity-based research– Evaluative aspects2

Conventional risk analysis
Fact-Value separation(e.g., US National Council, 1983; European Commission, 2003)

Step1: Risk assessment

Step2: Risk management

Capturing phenomenal factby quantifying risk

Making value judgmenton the phenomenal fact

In case of accident risk analysis…

In case of accident risk analysis…

Measuring accident risks

Policy discussions

3Perspective-dependency
How to quantify the risk depends on our perspective/
value judgment on the risk
Examples:• Can accident risk be regarded as a voluntary risk?
• Vehicle accident risk can be discussed separately from othertravel modes’ accident risk?
• What is the benefit from travel (what is the fundamental
reason for accepting accident risk)?

4

Conventional risk analysis
Fact-Value separation(e.g., US National Council, 1983; European Commission, 2003)

Step1: Risk assessment

Step2: Risk management

Capturing phenomenal factby quantifying risk

Making value judgment
on phenomenal fact

In case of accident risk analysis…

In case of accident risk analysis…

Measuring accident risks

Policy discussions

Reflect a certain perspective/value judgment(not value-free risk quantification)

5

Risk-benefit perspective
In general, we accept a certain level of accident risk toobtain certain benefits– Demand side• Road users’ perspective/value judgments– Supply side• Governments’ perspective/value judgments

6

Research Objectives
1. What kinds of value judgments we have made?
– Reviewing historical shift in governments’ perspective on
accident risk and the benefit from travel– Clarifying (hypothetical) current government perspective

2. How have we reflected the judgments inquantitative accident risk analysis?
– Reassessing the conventional accident risk indicator andproposing an alternative indicator– Conducting empirical analysis in the US context
7

Historical shift in governments’perspectives on traffic safety
Since 1990s
Since 1960sSince 1950s
Car companies’
responsibilities

Statement on the
responsibilities ofvehicle manufacturesand safety
Drivers’organizations
responsibilities Ex) Vehicle Safety
Drivers wereStandards
considered to becompletelyresponsible

Early 20th century

Multiplestakeholders’
responsibilities
Statement on the all
stakeholders related
to traffic safetyincludingnational/local
governments
Ex) Traffic Safety
Policies Basic Act

Changes inconcept“It can never be
ethically acceptedthat people are
killed or seriously
injured by roadtravel”
Ex) Vision Zero8Historical shift in governments’perspectives on traffic safety• Vision Zero– Emphasizing the importance of forward-looking
responsibility (Fahlquist, 2006)• Backward-looking responsibility– Who should take a responsibility on the accident alreadyhappened» Responsibility ascriptions to be fair/morally justified• Forward-looking responsibility– Who can actually control the accident risks» Responsibility ascriptions to practically solve problems
Ex) Improving public transit service, bike lane, etc.
9Value judgments on benefits• The implicit (practical) value judgment– benefit of travel is higher than accident costs• Travel– Sphere of citizenship as the right to participate insocial, economic, political, and cultural activities
that are essential to living (Vasconcellos, 1997)• Accessibility (not mobility)• The value of activities
10

The (hypothetical) current
governments’ perspective
In public policy discussions on traffic safety, the“forward-looking responsibility” approach should
be focused on to reduce the number of accidents
in a proactive way, while safety measures thatreduce the level of accessibility should not beconsidered. Providing access to desired
destinations is the primary goal of transportationsystems―and because of this―accident risk canbe justified.

11

Definition of accident risk
ACAR E
AR:
AC:
E:

Conventional risk indicator

Number of fatalities
Total vehicle distance travelledInternational Transport Forum (2012)
Papadimitriou et al. (2013)

Accident Risk
Total number of accidents
Total exposure, or total number of trials

12The state-of-the-artof accident risk indicator
Accident Risk =Number of fatalities
Total vehicle distance travelled

Theoretical basis: (value-free) objective fact
– Papadimitriou et al. (2013)“The number of trials, which is defined as the number of times road
users are exposed to possible accidents, should be the best
theoretical measure of risk exposure.”“The most appropriate measures of exposure are vehicle- andperson-kilometres of travel, because they are closer to thetheoretical concept of exposure and can be available, in theory, to asatisfactory level of detail.”13

Problems of conventional indicator
AShoppingHome

Conventional indicator:
A is safer than BWorkB

Population = 1,000,000
Av. car travel distance = 20 [miles]
Av. number of trips = 3# of accidents = 100

CHome

Shopping

HomeShoppingWork

Population = 1,000,000
Av. car travel distance = 5 [miles]
Av. number of trips = 3# of accidents = 50

Work
Population = 1,000,000
Av. car travel distance = 1.5 [miles]
Av. number of trips = 3# of accidents = 25

Conventional indicator:
B is safer than C
14

Problems of conventional indicator• Delimitation issue– Conventional: Focus only on car travel
 focus on whole transport systems• Exposure metric– Conventional: Vehicle kilometer (mobility-based) Need to focus on the benefit from travel(accessibility-based), e.g., number of trips

An alternative accident risk indicator:
Accident Risk =Number of fatalities
Number of trips made by all modes15

An alternative risk indicator
AShopping

HomeWork
Population = 1,000,000
Av. car travel distance = 20 [miles]
Av. number of trips = 3# of accidents = 100

CHomeShopping

Alternative indicator:
B is safer than A
Conventional indicator:
A is safer than B
B
HomeShoppingWork

Population = 1,000,000
Av. car travel distance = 5 [miles]
Av. number of trips = 3# of accidents = 50WorkPopulation = 1,000,000
Av. car travel distance = 1.5 [miles]
Av. number of trips = 3# of accidents = 25

Conventional indicator:
B is safer than C
Alternative indicator:
C is safer than B

16

Empirical Data
• Accident data (2010)– Fatality Analysis Reporting System (FARS)• Number of fatalities• Exposure data (2009)– National Household Travel Survey (NHTS)• Travel distance• Number of trips

17

Risk indicators to be compared• Conventional accident risk indicator
Accident Risk =Number of fatalities
Vehicle travel time

• Alternative accident risk indicator
Accident Risk =Number of fatalities

Number of trips made by all modes

18Accidents by population density
Motor vehicle occupants' fatalities
Non-motor vehicle occupants' fatalities
Share of non-motor vehicle occupants' fatalities

250%
40%

1.5

30%
120%

10000-

4000-9999

2000-3999

0%1000-1999

0500-99910%

100-499

0.5

Share of non-vehicle
occupants’ fatalities (%)

60%

-99

The number of fatalities per
10 thousand population per year

2.5

Population density (population per sq miles)19

100%
90%
80%
70%
60%
50%
40%
30%
20%
10%0%

4
3.5
32.5

# of non-vehicle trips (per capita per day)# of vehicle trips (per capita per day)
Modal share of car

21.5
1
0.5

Population density (population per sq miles)

10000-

4000-9999

2000-3999

1000-1999

500-999

100-499

0-99

Average number of trips per day

4.5

Modal share of car (%)

Exposures (the number of trips)

20

100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%

108
64

Average travel distance per trip (all travel modes)
Average travel distance per trip (car only)
Share of car travel distance per day

2Population density (population per sq miles)

10000-

4000-9999

2000-3999

1000-1999

500-999

100-499

0-99

Average travel distance per trip (mile)

12

Share of car travel distance per day

Exposures (travel distance)

21

Comparison of risk indicators# of fatalities per 100 mil vehicle miles

1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0

Alternative indicator# of fatalities per 10 mil trips (by all modes)

Conventional indicator
1.8
1.6
1.4
1.2
10.8
0.6
0.4
0.20Rankings of accident risks
National averageAge 65 or overConventionalrisk indicator

Alternativerisk indicator

Conventionalrisk indicator

Alternativerisk indicatorLess than 100

7747100-499

665

6500-999

35

141000-1999

24352000-3999

13234000-9999

426210000 or over

51

7

1

1: lowest risk ---------------7: highest risk

23

Summary• Reassessing accident risk indicator– A conventional indicator may not be really consistent withthe current governments’ perspective– We may have to focus on (1) the safety of whole transportsystems, and (2) the benefit from travel in thequantification of accident risks• There would be no value-free risk indicator Making clear “perspective” we are taking is one of
the crucial aspects in accident risk analysis• Limitation of empirical analysis: benefit calculation24

Needs for further activity-based research

25

Risk-benefit perspective• Difficulties in the calculation of benefits– Although the number of trips may be one of the proxy
indicators of benefits, further improvement is certainly
needed• Activity-based approach– “choices to travel are conceptualized as being dependent
on the value of participating in the activities that travel
makes possible” (Garling, Axhausen & Brydsten, 1993) The benefits from travel may not be able to be
calculated from travel behavior per se26

Activity-based approach• Limitations– Most studies have focused on behavioral descriptions (fact)
rather than evaluative aspects (value), while the central
foundation of activity-based approach is in the value ofparticipating in activities• Possible research directions– Valuing activity participation as a productive outcome• The myth of travel time saving (Metz, 2008)27

Possible research directions• Valuing productive time spent– Utilizing utility-based time use model• The productivity of time (Becker, 1965)– Study on the value of unpaid work
• Replacement cost method (if we ask professional service…)• Opportunity cost method (if we work…)– Questionnaire based methods– etc.

• Demand side perspective– Quality of life

• Supply side perspective– ?????

28

Appraisal requirements and capabilities
under each perspective (Jones, 2009)29