Surrogate measures in traffic safety analysis · Validity of Surrogate Safety Measures: ......
Transcript of Surrogate measures in traffic safety analysis · Validity of Surrogate Safety Measures: ......
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Surrogate measures in traffic
safety analysisALIAKSEI LAURESHYN
Technology & Society, LTH
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What is ”surrogate”?
”surrogate” = not accidents
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What is wrong with ”accidents” then?
• Few and random
• Under-reporting
• Scarce/biased information
• Process?
• Retro-active approach
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What can we learn from ”not-accidents”?
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“Pyramid” concept
Hyden, 1987
Undisturbed passages
Slight injury
Severe injury
Fatal
Accidents
Serious conflicts
Slight conflicts
Potential conflicts
Damage only
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Concept of ”severity”
• Nearness-to-collision?
• Consequences IF a collision?
• Both? How to “weigh” together?
From Vision Zero perspective:
• Nearness-to-severe injury
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How to measure severity?
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• TTCmin
• TA – Time-to Accident
• TET, TIT
• …
Time-to-Collision (TTC)
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• PET
• TAdv – predicted PET
• Time Gap
• Headway
• …
Post-Encroachment Time (PET)
1
2
1
2
t 1 t 2
PET = t 2 - t 1
conflict zone
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Deceleration
• Deceleration-to-Safety (DST)
• Max deceleration
• Proportion of stopping distance
• Jerk
• …
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Delta-V
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Other
• Subjective scores
• Combined indices
• Traffic conflict techniques
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Conflict techniques (TCT)
• Swedish – Hydén (1987)
• Dutch (DOCTOR) – Kraay et al. (1986)
• US – Parker & Zegeer (1989)
• British – Baguley (1984)
• Canadian – Sayed & Zein (1999)
• Finnish – Kulmala (1984)
• French – Muhlrad & Dupre (1984)
• Belgian - Mortelmans et al. (1986)
• German – Erke (1984)
• Czech – Kocárková (2012)
• Austrian - Risser & Schutzenhofer (1984)
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Swedish TCT
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Validity & reliability
• Reliability – can we measure accurately enough?
• Validity – do we measure the desired property (safety)?
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Relation between conflicts & accidents
Traffic
dynamics
Undisturbed
ConflictEvasive
actionCollision
Traffic
continuation
successful
not successful
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Relation between conflicts & accidents
Traffic
dynamics
Undisturbed
ConflictEvasive
actionCollision
Traffic
continuation
successful
not successful
Traffic
dynamics
Undisturbed
Critical
event
Evasive
actionCollision
Traffic
continuation
Conflict
Normal
trafficsuccessful
not successful
not successful
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Exposure, conflicts, accident
Amundsen & Hydén, 1977
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Exposure, conflicts, accident
Amundsen & Hydén, 1977
Statistical “trials”
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Conflict
Probability pProbability (1-p)
AccidentAvoided
accident
Observed in
conflict studies
But why should it have any
explanatory power?
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Conflict
Probability pProbability (1-p)
AccidentAvoided
accident
Definition should
start here
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Conflict
Probability pProbability (1-p)
Severe injury/
injury accident
Avoided
injury
In a view of Vision Zero…
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Relation between conflicts & accidents
𝜆 = 𝑐 ∙ 𝜋
λ – expected number of accidents;
c – conflicts;
π – accident-to-conflict ratio.
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Extreme value theory
TTC = 0
PET = 0Observations
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Causal model
Initial conditions
[U]
Outcomes
[Y]
Evasive actions
[X]
v1 v4v3v2
0
0,1
0,2
0,3
0,4
0,5
0,6
4 4,5 5 5,5 6 6,5 7 7,5 8 8,5 9
Pro
bab
ilit
y
Deceleration, m/s2
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Validity
Validity of Surrogate Safety Measures:
• Absolute product validity
• Relative product validity
• Process validity
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Validation
Product validity:
• Observed conflicts vs. reported accidents - linear regression
• Observed conflicts vs. SPF - linear regression
• Ratio observed conflicts/expected accidents – variance
• SPF based on conflicts as input
• Extreme value theory methods
• Before-after studies based on conflicts vs. accident history
• Accidents vs. conflicts as functions of traffic flow
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Current status
0
5
10
15
20
25
30
35
Nu
mb
er
of
pu
blic
ati
on
s
Publication year
Included
Not located
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Analysis tools
• Human-based
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Analysis tools
• Human-based
• Semi-automated / watchdog
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Analysis tools
• Human-based
• Semi-automated / watchdog
• Fully automated
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Looking forward ...
• Good evidence for the sound approach (from the past)
• Rapidly developing technologies for data collection
• New traffic conditions -> new validations
• New tools -> new validations
• VRU to be better included
• Many accidents outside intersections -> single accidents
• Triangulation of methods
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I am still not persuaded
this is unsafe.
Let’s wait for some
more planes to crash…
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What was not mentioned…
• Micro-simulations of conflicts
• Naturalistic studies
• Behavioural observations