Transcript PowerPoint Presentation - NEVADA SPEED MANAGEMENT …

H
NEVADA SPEED
MANAGEMENT WORKSHOP
Reno, NV
April 4, 1996
Davey L. Warren
Office of Safety and Traffic Operations R&D
Federal Highway Administration
Realistic Speed Limits
Why & How
Speed limits and the law
 Speed limits, speed and accident risk
 Speed limits and driver acceptance
 Design speed vs posted speed
 Methods for setting speed limits
 Recommended approach
 Implementation & federal assistance

SPEED
LIMIT
85
PERCENTILE
Speeding Problem
Rational for government regulation of speed
 Speed a factor in 10% of accidents & 30% of
fatal crashes
 Severity increases with speed
 Speed limits widely disobeyed
 Conflict between access, safety, and mobility
 What is a safe speed & who decides?

Why Do People Speed
Speed set too low
 Road design conducive to high speeds
 Perceived risk of accident or citation from
speeding is low (everyone is doing it)
 Fast is consistent with social values
 General conflict between mobility & safety

Basic Tenets of Law
Laws protect the public by regulating
unreasonable or unsafe actions
 Actions of a reasonable person should be
legal
 Most people drive in a safe and reasonable
manner
 Law cannot be effectively enforced without
the public consent & voluntary compliance

Speed Limit Laws
BASIC SPEED RULE -Driver must travel at a speed
that is reasonable and proper for conditions and not
endanger others or crash.
SPEED ZONING -Gives highway agency or localities
authority to post limits higher or lower based on an
engineering study, and to set them to indicate the maximum
reasonable and safe speed.
Speed Laws
PRIMA FACIE LIMIT
 Exceeding limit is
evidence of traveling at
unsafe speed
 Recognizes no one speed
is safe for all conditions
 Post for prevailing
conditions
 Check against traps
ABSOLUTE LIMIT
 Illegal to exceed
 Reasonable and safe
speed not relevant
 Easier to enforce
 Not fair unless speed
limit set for optimal
conditions
Methods Used To Set Speed Limits

Arbitrary
....lack funds & personnel

Political Pressure
....based on complaints and reactions to accidents

Traffic engineering
....based on traffic and road characteristics...but
still somewhat subjective and arbitrarily applied
Current Practice
85th percentile speed
Roadside development
Accident experience
Adjacent limits
10 mph pace
Roadway geometrics
Average test run speed
Pedestrian activity
SPEED
LIMIT
85
PERCENTILE
States
100
85
79
71
67
67
40
40
Locals
86
77
81
45
34
57
50
50
Requirements of Speed Limits
Be fair
 Be related to risk
 Be credible and
accepted by drivers
 Enforce control over
unreasonable behavior

SPEED
LIMIT
85
PERCENTILE
Speed and Accident Risk
12

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Safest near mean
speed of traffic
Roads with highest
speeds are safest
Limit should be set at
upper end of low risk
band
Relative Risk
ACCIDENTS
10
Freeway
Main Rural
Two Lane
8
OVERTAKINGS
6
4
2
0
-20 -15 -10 -5
0
5
10 15 20
Deviation from mean speed, mi/h
Accident Risk in Built-up Areas
14
12
10
8
6
4
2
0
5
15
50
85
Percentile Speed
95
100
Speed of traffic best indicator of
reasonable and safe speed
8
Relative Risk
SPEED
LIMIT
85
PERCENTILE
6
ACCIDENT
INVOLVEMENT
4
2
OVERTAKINGS
0
-20
-15 -10 -5
0
5
10 15
Deviation from mean speed, mph
20
Driver Acceptance



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%Compliance
30% nationwide
40
Range from 3-100%
35
Worse with low limit
30
Better in west/midwest 25
20
Better w/prima facie
15
10
Only 1 speed zone in
5
10 better than 50
0
percent compliance
25 30 35 40 45 50
Posted Speed
Group Mean %
Most Limits Set Way Below Prevailing Speed
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
Prevailing speeds 8-12
mph above limit
Trucks 3 mph slower
Speeds vary 4-8 mph
over a 24-hr day
No difference between
weekday & weekend
60
Operating Speed
85 %tile
Average
50
40
30
20
10
0
25 30 35 40 45 50
Speed Limit, mi/h
Factors Influencing Speeds

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
Number of access
points along road
Commercial
development
Severe curvature
Level of accidents
Adverse weather
Minimum Design Speed
All geometric elements must meet or exceed
 Only when constrained by terrain or
development is minimum used
 Maximum safe speed for worst driver under
bad road conditions
 Reaction time of drunk drivers, stopping
with bald tires on slippery road, average
comfort level of blind folded passengers in
1930 vintage car

Raising limit has little effect on
prevailing speed of traffic
After Speed, mi/h
75
65
55
+5 mi/h
45
+10 mi/h
35
+15 mi/h
25
25
35 45 55 65
Before Speed, mi/h
75
Lowering limit doesn’t slow traffic
65
Speed After, mi/h
60
55
50
45
40
- 5 mi/h
35
-10 mi/h
30
-15 mi/h
25
25
35
45
55
Speed Before, mi/h
65
Little or no change in behavior
5
Change in 85th Percentile Speed, mi/h
4
Maximum
Increase
3
2
Average
Change
1
0
-1
-2
Maximum
Decrease
-3
-4
-5
-15 & -20
-10
-5
+5
Change in Posted Speed Limit, mi/h
+10 & +15
Compliance before and after change
Percentage of Driver Compliance
100
93
Before
90
After
82
80
68
70
66
60
55
50
40
30
36
39
26
25
20
15
10
0
-15 & -20
-10
-5
+5
+10 & +15
Change in Posted Speed Limit, mi/h
Accidents Before & After
50
Upper Estimate
40
Percent Change in Crashes
30
+27.5
20
10
Best Estimate
+6.9
+5.3
0
-10
-20
-30
-10.4
Lower Estimate
-11.3
-25.2
-40
-50
Lowering Speed Limit
Raising Speed Limit
What We’ve Learned

Most speed limits set unreasonably low
Make technical violators out of large percent of motorists
driving at safe speed

Current speed limits do not reflect accident risk
Inconsistent with traffic law system
Misallocates enforcement resources

Most motorists drive at a speed road and traffic
permits regardless of posted speed
Don’t automatically drive 5 mph over limit

Speed limits have lost their informational value
Engineering of credible limits which are appropriate and
enforceable the key to managing risk
Recommended Procedure

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24hr free flow speed
Round up
1/2 mile interval
500ft from jct. & curves
Dry roads, typical traffic
No other adjustments
SPEED
LIMIT
85
PERCENTILE
Design Speed
vs
Maximum Curve Speed
Design
Speed
Min. Tolerable Min.
E
Lateral Gs Radius
Impending Impending
Skid (wet) Skid (dry)
Rollover
Car
40 mi/h
0.06
0.15
63.3 mi/h
87.5 mi/h
98.1 mi/h
70 mi/h
0.06
0.10
2,083’ 120.7 mi/h
177.0 mi/h
198.4 mi/h
509’
Car: f(dry) = 0.65
Source: FHWA-RD-89-226
Special Zones

School zone • Base on average speeds when children present
• Restrict to 15 -30 minutes periods when kids present

Work zones
• Base on average speed during good conditions
• Use warning sign for hidden hazards & post advisory
speed at average speed of traffic when hazard exits
• Vary limit based on speeds when work in progress
Residential Areas

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Post at 85% rounded
to nearest
If lower limit desired,
must change the road
environment
Road humps, traffic
circles, etc.
Confine engineering
measures to small area
General Approach
Engineer rational speed limits
 Educate and inform public

Hazards of traveling too fast or too slow
Limits engineered to be safe & reasonable

Strict enforcement and adjudication
Eliminate tolerances & inform judiciary

Evaluate
Effects on drivers, safety, police, courts
SPEED
LIMIT
85
PERCENTILE
Key Benefits

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Respect for law and enforcement
Effective allocation of resources
Deterrent to excessive speed by clearly separating
occasional violator from majority
Reduce conflicts caused by large variations in
travel speeds
Improved road safety and better use of roads
SPEED
LIMIT
85
PERCENTILE
Federal Fund$ Available
Engineering
Education
Enforcement
Evaluation
Next Step-Application to practice
Mileage of roads to be rezoned by class
 Frequency and duration of speed studies
 Public information and outreach plans
 Enforcement and judicial cooperation
 Evaluation
 Estimated total cost and cost share

Where to get more information
Greg Novak
Federal Highway Administration
705 N. Plaza Street, Suite 220
Carson City, NV 89701
( 775.687.1204
fax 775.687.3803
Email: [email protected]
SPEED
LIMIT
85
PERCENTILE