Transcript Introduction - Web Publishing

Statistics of Speed Limits
Aren Cambre
EMIS 7370
Fall 2003
Agenda
• Fundamental Concepts
• Actual Practice
• Speed and fatalities?
• That IIHS study
Fundamental Concept #1
• Speed variance increases crashes
Solomon (1964 & Cirillo 1968)
West and Dunn (1971)
Other Studies
• Fildes, Rumbold, and Leening (1991)
– Above mean speed = higher crash risk
– Below mean = insignificant
• Harkey, Robertson, and Davis (1990)
– Found same U-shaped speed/crash
relationship as Solomon and Cirillo
Fundamental Concept #2
• Crash severity increases exponentially
with speed.
Crash Severity Graph
• Note how fatality
probability declines
over time
Fundamental Concept #3
• Speed limits have little effect on actual
speeds.
• Urban areas: no effect.
• Rural areas: modest effect.
• Most comprehensive studies of 1995
repeal of federal speed limit controls:
no significant speed changes
Contemporary Example
• Environmental Speed Limits in 9county DFW area:
– All 70 and 65 MPH limits reduced by 5 MPH
– 1 year later
• Speed checks show average 1.6 MPH speed
drop
• Much of drop attributable to suburban
development
Fundamental Concept #4
• Traffic enforcement has an effect on
speeds and safety.
However!
• Effect limited to vicinity of enforcement
activity.
• Halo effect.
• Laws must appear reasonable to
everyday drivers.
Setting Speed Limits
• Speed limit should only criminalize
speeds associated with excessive
crash risk.
Important Observations
• Excessive crash risk
in top 5%-10% of
drivers
• Speeds are normally
distributed
• 85th percentile
speed = top speed
of “flow of traffic”
Texas speed zoning procedure
Measure passenger vehicle speeds
Determine 85th percentile speed
Lower speed by up
to 7 MPH
Unusual
crash or
roadway
problem?
Set speed limit
Lower speed by up
to 10 MPH
TxDOT intends for this path to be
taken the vast majority of the time,
but that’s not what actually
happens.
State Law
• Speed limits capped 70 MPH in most
cases, 75 MPH in particular desolate
counties.
• All speed limits prima facie.
Analysis of Dallas-area
statistics
• Speed limits on highways outside I635/I-20 loop are 30th percentile
If properly set…
Roadway
Current Limit
Correct Limit
I-35E between
60 MPH
70 MPH
I-635 west of I-35E
60 MPH
75 MPH
I-45 @ Ennis
65 MPH
80 MPH (?)
downtown and 635
US-75 in Collin County 65 MPH
75 MPH
Centennial
Blvd. in Richardson
45 MPH
40 MPH
Excessive Speed Related
Fatalities?
• Common sense says that speed causes
most fatalities.
• Is it true?
Excessive Speed Related
Fatalities?
• In Dallas County, 88 fatalities in first 4
months of 2003.
• 42 (47%) were “speed-related.”
Dallas Speed-Related Fatalities
Speed-related accident factors
12%
17%
DUI
Speed alone
ROW violation
61%
10%
Motorcycle
Dallas Speed-Related Fatalities
• Speed was not primary factor in 95% of
Dallas-area highway deaths!
• NHSTA definition of speed-related
fatality:
– Driving too fast for conditions
– Exceeding posted limit
• Under-posted limits = more “speeders”
= more speed-related crashes!
The recent big news
My Own Analysis
2.5
2
1.5
Stay at 65
Change to 70
Change to 75
1
0.5
0
1994
1995
1996
1997
1998
1999
2000
2001
2002
Summary
• Statistics is highly relevant to speed
limits.
• The statistics are surprising.