visibility safety · 2019. 12. 10. · reflective sign sheeting and all-weather pavement markings,...
TRANSCRIPT
Science of Retroreflectivity
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3M Signing Systems
Visibility & SafetyLife of the Roadfor the
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3M, the innovation leader in traffic safety for more than 65 years, now combines comprehensive services with high-performance materials to provide safety and visibility for the life of the road. From installation, maintenance, asset management and flexible funding programs to state-of-the-art reflective sign sheeting and all-weather pavement markings, the 3M difference is clearly visible.
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Luminance
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Coefficient of Retroreflection: RA Luminance / Illuminance cd / m2 / lux
Intensity of a Light Source = Candela cd
Illuminance: Light falling on a unit area lux
Luminance: measured brightness of the reflected light from a surface
Intensity / unit area cd / m2
Units of Measurement
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How the Technology Works
Glass Bead Technology
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How the Technology Works
Microprism Technology
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Cone of Returned LightRetroreflector
Reflected Light in Divergence Cone
Light Source Direction
Most retroreflectors also have diffuse reflection. Diffuse reflected light is simply “wasted”, as it does not help the nighttime driver. Higher efficiency sheetings divert more of this otherwise wasted light toward the driver.
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Pattern Of Retroreflected Light Returned To Driver from Left Headlight
Cone of Returned Light
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Observation Angle
Observation angle (α)
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Specification Observation Angles Applied to an Overhead Sign Approach
750ft(230m)
350ft(107m)
175 ft (53m)
Dover
0.50o
0.20o
1.0o 2.25o
0.50o 1.0o Dover
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Entrance Angle
Entrance Angle (β)
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As a Vehicle Approaches a Sign Along a Curved Roadway
• Sign offset
• Distance of vehicle to sign
• Travel lane of vehicle
• Curvature of road
• Twist of sign on post
Entrance Angle Varies…
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Multi-lane Traffic
Entrance Angle increases as vehicle approaches sign
Curved Roads
EXIT
Critical Signs at Intersections
12°26°
420
140
450
Entrance Angle Scenarios
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15%
22% 100%
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Sign Surveys Medium sized city, 147,000 population
Sign Position Number Percent Right Side 17,124 61.5 Straight Ahead 338 1.2 Overhead 13 0.0005 Median 1,198 4.3 Left Side 9,162 32.9 27,835
Over 37% of signs in disadvantaged locations
U.S. Data on Sign Position
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The First Retroreflective Device
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• Fluorescent Technology 1990’s
• Diamond Grade™ VIP Sheeting 1990’s
• Diamond Grade™ LDP Sheeting 1980’s
• High Intensity Grade Sheeting 1970’s
• Engineer Grade Sheeting 1940’s
• Exposed Lens Sheeting 1930’s
Traffic Signing Safety Solutions
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Total Light Return Efficiency
14%
Tot
al R
etro
refle
ctiv
e Ef
ficie
ncy
All 30% to smaller observation angles
30%
Tot
al R
etro
refle
ctiv
e Ef
ficie
ncy
15% to larger observation angles
Cone of Retroreflection
Cone of Retroreflection
Minimal retroreflection to larger observation angles
All truncated cube corner sheetings have nearly 30% retroreflective efficiency.
Only 15% light remains for smaller observation angles
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Total Light Return Efficiency
60%
Tot
al R
etro
refle
ctiv
e Ef
ficie
ncy
30% to larger observation angles
30% to smaller observation angles
Cone of Retroreflection
Increasing the overall total retroreflective efficiency ensures serving the greatest segment of nighttime driver population without the need for sign lighting.
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Because what you see during the day ...
Is not always what you get at night !
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▪ Aging Population ▪ Growing Truck Fleet ▪ VOA Headlamps ▪ Environmental
Impact ▪ Rural Roadways ▪ Specifications
Signing Considerations & Trends
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ASTM Specification▪ Type I = Engineer Grade - Enclosed Lens ▪ Type II = Super Engineer Gr. - Enclosed Lens ▪ Type III = High Intensity - Encapsulated Lens ▪ Type IV = High Intensity Prismatic ▪ Type V = Delineators - Metallized Prismatic ▪ Type VI = CWZ Roll-Ups - Prismatic ▪ Type VII = Prismatic ▪ Type VIII = Prismatic ▪ Type IX = Prismatic ▪ Type X = Prismatic ▪ Type XI = Diamond Grade.
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Reflectivity Specification
Current Common ASTM Spec Angles
Angles Describing a more Comprehensive Set of Roadway Situations
0.2 0.5 1.0 1.5
-4
+30
+45
Minimum RA
Ent
ranc
e A
ngle
sObservation Angles
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Total Sign Visibility
▪ ATA support for a 1.0 observation angle ▪ Truck traffic fatalities represent a significant percent of
total fatalities across the country (15%) ▪ Portable retroreflectometers are commercially
available for measuring 0.50 and 1.00 observation angles at -40 entrance angle.
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Camera 21 inches above
Headlight
Camera 50 inches above
Headlight
Larger Observation Angle for Trucks
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Type I Eng Gr
Type III HI
Type VIII Prism
Type IX VIP
Type IX VIP
Type IX VIP
Viewed at .2 and -4
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Type I Eng Gr
Type III HI
Type VIII Prism
Type IX VIP
Type IX VIP
Type IX VIP
Viewed at .5 and -4
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Type I Eng Gr
Type III HI
Type VIII Prism
Type IX VIP
Type IX VIP
Type IX VIP
Viewed at 1.0 and -4
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75% of roads in the U.S. are two-lane highways
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Most Fatalities Occur on Two-Lane Roads
Number of Lanes
Num
ber o
f Fat
aliti
es, 1
999
Source: The Federal Highway Program and Highway Safety: An Economic Analysis, William R. Buechner, Ph.D., American Road & Transportation Builders Association, March 31, 1998.
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Two Out of Three Deaths Occur in Rural Areas(National Safety Council)
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Driver CuesDaytime
Many cues available Driver task relatively easy
Nighttime Few cues remain Task more difficult
U.S. Nighttime Accident Rate is 4 Times the Daytime Rate
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Fluorescent Yellow Signing Benefits▪ Optimal daytime visibility, superior to standard yellow colors,
through the use of durable fluorescent colorants: Improves Safety and positive driver behavior.
▪ Optimal nighttime sign visibility through DG3 retroreflective optics providing superior luminance for all vehicle types without external lighting.
▪ The result: improved 24-hour visibility of all yellow warning signs - especially critical warning signs that demand early detection and recognition.
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▪ The visual properties of combined performance signs (fluorescent + retroreflective) are strong enough to have significant influence on driver attention and behavior ▪ Fluorescent retroreflective signs provide drivers with more time to:
▪ see ▪ decide ▪ act safely
▪Warning signs using fluorescent yellow should have a positive impact on safety
Key Study Findings
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Where are we today?
§ Local travel/rural roads
§ Round-the-clock construction projects
§ Disparity in vehicle size
§ Changing headlight design and performance
§ Aging driver population
§ Increasing Commercial Vehicle and Freight Transportation, SUVs and LTVs – ever increasing need for higher observation angle retroreflectivity
Trends Relevant to Signing Systems
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The Design of DG3
▪ Full Cube Construction versus Truncated Cube Construction (HIP) ▪ Where to put the recovered light… ▪ Let research define DG3 performance ▪ Design the luminance response to meet the needs of drivers
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The Supply / Demand Continuum…
What do drivers really need from signs?
Can passive, retroreflective signs deliver?
Meeting Driver’s Needs
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Visibility of traffic signs…
▪ Drivers see signs in terms of Luminance ▪ Luminance is the physical measurement of brightness. … understanding Luminance
Meeting Driver’s Needs
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Luminance: Physics: the luminous intensity of a surface in a given
direction per unit of projected area; standard unit of measure for luminance is candela per square meter (cd/m2).
Plain English: the physical measure of brightness
Meeting Driver’s Needs
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Minimum vs. Optimum ▪ What is the minimum brightness needed in a traffic sign? ▪ What is the optimum brightness needed in a traffic sign? ▪ What determines minimum, what determines optimum?
Meeting Driver’s Needs
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Driver Needs Depend on Age
▪ Age creates a different driver
▪ Visual deterioration is a natural aging process
▪ Older drivers require more reaction time
▪ Serving older drivers promotes having the safest driving environment possible
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Older Drivers Need: • More light to see and read a sign • Light need doubles every 13 years. • More reaction time
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USA Today October 5th, 2000But the study suggests that older drivers have trouble navigating the roads, which were not designed with them in mind. For many aging motorists, signs are hard to read, lanes are too narrow and left-hand turns are very difficult. Intersections are a particular problem. According to the study... Half of all fatal accidents involving older drivers occur at intersections, compared with about 25% for other drivers.
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The Needs of The Older Driver
60 Year-Old40 Year-Old
20 Year-Old
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FHWA Research - Minimum Luminance Levels: ▪ The minimum luminance* to read a sign at 40ft/inch
legibility index is 2.4 cd/m2. ▪ This applies to dark rural conditions with static viewing ▪ Below this level, it is physically not possible to read the sign no
matter how much time is given to read.
Human Factors Research - Optimal Luminance Levels: ▪ Optimal luminance levels range from 24 cd/m2 to 343 cd/
m2 in human factors literature. ▪ This is the level where reading is effortless and accurate
*Carlson, P. J., Hawkins, G. H. “Updated Minimum Retroreflectivity Levels for Traffic Signs”, FHWA-RD-03-081, USDOT Federal Highway Administration, June 2003.
Meeting Driver’s Needs
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▪ Researchers add the following*: ▪ “The FHWA minimum performance levels can be considered the
absolute minimum, as they do not address all of these critical issues [headlamps emitting little up-light for signing, heavy vehicles, and older drivers], especially in combination.
▪ If, in consideration of combined factors such as glare, complex backgrounds, and heavy vehicles,…, then the desired luminance levels could increase dramatically from the absolute minimum of about 2.4 cd/m2 to a range between 30 and 80 cd/m2 for candidate desired levels.”
* Carlson, P. J. “A Proposal for Performance-Based Sign Sheeting Criteria”, 17th TRB Visibility Symposium, Washington, D.C., 2005.
Meeting Driver’s Needs
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The IES Lighting Handbook▪ In Section 22: Roadway Lighting – under “Illuminated
Roadway Signs,” the handbook: ▪ Breaks down ambient lighting levels into three categories:
• Low: Rural area, no street lighting • Medium: Areas with small commercial development, some lighting
at intersections • High: Many street lights, plus lit advertising signs
▪ Recommended levels for externally lighted roadway signs: • Low: 22 – 44 cd/m2
• Medium: 44 – 89 cd/m2 • High: 89 – 178 cd/m2
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Understanding a luminance curve:
Meeting Driver’s Needs
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Luminance for a Right Shoulder Sign, SUV/Van, 50 Twist, UMTRI US 25 Percentile Headlamp
0
10
20
30
40
50
60
70
80
01002003004005006007008009001000Distance to Sign [ft]
Lu
min
ance
[cd
/m2 ]
White DG3 [cd/m2]
Anatomy of the Luminance Curve
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Luminance for a Right Shoulder Sign, SUV/Van, 50 Twist, UMTRI US 25 Percentile Headlamp
0
10
20
30
40
50
60
70
80
01002003004005006007008009001000Distance to Sign [ft]
Lu
min
ance
[cd
/m2 ]
White DG3 [cd/m2]
Anatomy of the Luminance Curve
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Luminance for a Right Shoulder Sign, SUV/Van, 50 Twist, UMTRI US 25 Percentile Headlamp
0
10
20
30
40
50
60
70
80
01002003004005006007008009001000Distance to Sign [ft]
Lu
min
an
ce
[c
d/m
2 ]
White DG3 [cd/m2]
Anatomy of the Luminance Curve
0.2 0.5 1.0 1.5
-4 ?+30 ?+45
Minimum RA
Ent
ranc
e A
ngle
s
Observation Angles
6.82° 640’Left Headlamp
1080’
5.84°
Right Headlamp
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Research and traffic control device performance…
Eight important reports:
Meeting Driver’s Needs
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The trouble with vehicle lighting –
Diminishing light for traffic signs…
Meeting Driver’s Needs
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First signs of trouble… “Characteristics and Needs for Overhead Guide Sign Illumination
from Vehicle Headlamps” FHWA-RD-98-135, Russell, et al, Kansas State University
#1
Meeting Driver’s Needs
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FHWA-RD-98-135, Russell, et al, Kansas State UniversityPage 88, Conclusions: …50% of the 1500 vehicles tested did not provide
sufficient illumination to meet a minimum luminance of 3.2 cd/m2 from overhead guide signs made of high intensity sheeting.
(Before the use of VOA headlamps on vehicles)
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Making matters worse…
▪ 1997 NHTSA allowed “visually/optically aimable” (VOA) headlamps. ▪ Outer lens labeled VOL or VOR ▪ Sharp “cut-off” on the left or the right ▪ No special instruments required for aiming ▪ Beam pattern resembles European headlamps
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More trouble…
“A FIRST LOOK AT VISUALLY AIMABLE AND HARMONIZED LOW-BEAM HEADLAMPS” UMTRI 2000-1, Sivak, et al, University of Michigan Transportation Research Institute
#2
Meeting Driver’s Needs
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UMTRI 2000-1Page 11, table 5: Comparisons of the VOL lamps with conventional U.S., European, and
Japanese lamps… ▪ Headlamp illumination on a retroreflective sign; right shoulder at
150m (~500ft): ▪ Reduced by 53%
▪ Headlight illumination on a retroreflective sign; center overhead at 150m (~500ft): ▪ Reduced by 28%
▪ Headlight illumination on a retroreflective sign; left shoulder at 150m (~500ft): ▪ Reduced by 42%
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UMTRI 2006-31A more recent research report on headlamp illumination on traffic
signs:
“Recent Changes in Headlamp Illumination Directed Toward Traffic Signs”, UMTRI 2006-31
▪ The results imply that with current HID low beams, “sign retroreflective efficiency would need to be increased to maintain the effectiveness that a given sign had a decade ago with tungsten-halogen low beams. This applies to overhead signs …, and to right and left shoulder-mounted signs ….”
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99 Mustang at 60 feet - New Style Headlights
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92 Taurus at 60 feet - Old Style Headlights
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Type IXVIP
Type IIIHI
Type IEng Gr
Current US Headlights VOA Headlights
Type IXVIP
Type IIIHI
Type IEng Gr
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Understanding when signs must perform…
Meeting Driver’s Needs
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“Line of Sight Distances to Signs” Hummer et al; TRB-05-1473 , North Carolina State University.
Are signs obstructed in the real world?
#3
Meeting Driver’s Needs
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When are signs available …
▪ 79% of all signs viewed were obstructed at some point during the approach
▪ 97% of urban signs were obstructed ▪ Real-world sight distances are shorter than theoretical
sight distances ▪ Signs in urban areas are most affected by obstruction
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Where do drivers actually read traffic signs?
Meeting Driver’s Needs
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“Driver Eye Fixation and Reading Patterns while Using Highway Signs under Dynamic Nighttime Driving Conditions: Effects of Age, Sign Luminance and Environmental Demand”
Schieber, Frank; Heimstra Human Factors Lab – University of South Dakota, TRB 2004-001951
#4
Meeting Driver’s Needs
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Schieber’s Methodology
▪ Live field tests ▪ Both rural and urban sites ▪ State-of-art eye scanning technology used ▪ Young and old (65 years and older) studied ▪ Eye fixation time/distance recorded
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Where do drivers read signs?
▪ Drivers begin to read signs close to the theoretical legibility threshold [40 ft/in of letter height (young), 33 ft/in (older)]
▪ Drivers fixate on a sign for ~3 seconds to gather information from the sign
▪ Older subjects fixated closer to the signs
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“Traffic Sign Luminance Requirements of Nighttime Drivers for Symbolic Signs” Schnell et al, Operator Performance Lab - University of Iowa, TRB 2004
How bright should a traffic sign be? #5
Meeting Driver’s Needs
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How bright should a traffic sign be?
▪ Optimum luminance should be ~80 cd/m2.
▪ Confident legibility requires being ~25% closer to the sign than the theoretical legibility threshold.
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Summary of Human Factors Research
▪ Vehicle headlamps are putting much less light on traffic signs than ever before
▪ Most traffic signs are obstructed on approach
▪ Drivers read signs closer than the threshold legibility distance of 40 foot/inch.
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Summary of Human Factors Research Obs angle 500’ = 0.700
400’ = 0.900
300’ = 1.200 200’ = 1.750
500’ = 0.350 400’= 0.450 300’ = 0.600 200’ = 0.900
500’ = 0.300 400’ = 0.400 300’ = 0.500 200’ = 0.750
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Observation Angles when Reading Signs
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What happens when optimum performance sign sheeting is used?
Meeting Driver’s Needs
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Optimum Results“Reducing Crashes at Controlled Rural Intersections” Harder, et al,
University of Minnesota for MnDOT
“Reducing Crashes at Rural Thru-STOP Controlled Intersections” Howard Preston and Richard Storm, CH2M Hill
#6
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Reducing Crashes…Quotes from appendix…
▪ “At intersections with crashes, the use of more and larger STOP signs appears to reduce the number of Ran the STOP crashes.”
▪ “The use of brighter retroreflective sheeting material appears to reduce the frequency of both total crashes and right angle crashes. The highest usage of Diamond Grade sheeting was at intersections with no crashes and the lowest usage was at intersections with multiple Ran the STOP crashes.”
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Reducing Crashes…
▪ “STOP AHEAD signs were in place at all but one intersection. At intersections with crashes, it appears that the use of larger, brighter advance warning signs reduces the frequency of Ran Thru the STOP crashes.”
▪ “Increasing the conspicuity of traffic control devices by using bigger, brighter or additional signs and markings appears to lower the frequency of Ran the STOP crashes.”
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Reducing Crashes
“The Safety Effects of Traffic Sign Upgrades” Ripley, D. A.; H.R. Green and Associates, Presented at 2004 ITE
Annual Meeting
#7
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Ripley – Crash Reduction
▪ Sioux City, IA, reduced crashes by ~30% in the 3 years after upgrades
▪ Insurance Council of British Columbia (ICBC) concluded a Benefit-Cost Ratio for sign upgrades at greater than 10:1
▪ Mendocino County (California) showed a 46% crash reduction over a 6-year period
▪ Putnam County (New York) showed a 25% crash reduction in the three years after the upgrades
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TABLE IV-7 Highway Safety Improvements With The Highest
Benefit-Cost Ratios 1974 - 1995Rank Improvement Desc. Benefit/Cost Ratio
1 Illumination 22.8 2 Upgrade Median Barrier 22.6 3 Traffic Signs 22.4 4 Relocated/Breakaway Utility Poles 17.7 5 Remove Obstacles 10.7 6 New Traffic Signals 8.5 7 Impact Attenuators 8.0 8 New Median Barrier 7.6 9 Upgrade Guardrail 7.5 10 Upgrade Traffic Signals 7.4 11 Upgrade Bridge Rail 6.9 12 Improve Sight Distance 6.1 13 Median for Traffic Separation 6.1 14 Groove Pavement for Skid 5.8 15 Improve Minor Structure 5.3 16 Turning Lanes and Channelization 4.5 17 New RR Crossing Gates 3.4 18 New RR Crossing Flashing Lights 3.1 19 Pavement Markings and Delineation 3.1 20 New RR Crossing Lights and Gates 2.9
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What are we to conclude…
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Logical Conclusion
“Progress toward performance-based retroreflectivity criteria for traffic signs”
Carlson, Paul; Texas Transportation Institute, 2005 TRB Visibility Symposium
#8
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Conclusion
▪ 0.20 observation angle numbers are overstated in importance as it relates to how, when, and where signs are used by the drivers
▪ In urban settings 0.50 to 1.50 observation angles are critical to motorists needs (sign retroreflectivity is often measured at 0.20)
▪ In urban areas most signs do not provide the needed luminance levels at the distances where they are viewed and the driver processes information
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Conclusion
▪ Signs should perform to address the needs of the 85th percentile driver, aged 65 or older
▪ Proposed FHWA retroreflective minimums are a good start toward assuring signs are adequate, but fall well below the level that human factors research has determined is needed
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Solution:
Since there is less light than ever available to illuminate signs…
Use a reflective sheeting that has the best efficiency in returning what little light comes from the headlights back to the motorist.
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The answer is3M Diamond GradeTM DG3
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Working Together to Enhance Safety