Visibility Safety · 2019. 12. 10. · reflective sign sheeting and all-weather pavement markings, ... Type V = Delineators - Metallized Prismatic ... optics providing superior luminance

Science of Retroreflectivity

© 3M 2008. All Rights Reserved.

3M Signing Systems

Visibility & SafetyLife of the Roadfor the

3M Signing Systems

2© 3M 2008. All Rights Reserved.

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


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


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


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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


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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?


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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.


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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.


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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:


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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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0

10

20

30

40

50

60

70

80


Lu

min

ance

[cd

/m2 ]

White DG3 [cd/m2]


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0

10

20

30

40

50

60

70

80


Lu

min

an

ce

[c

d/m

2 ]

White DG3 [cd/m2]


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:


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The trouble with vehicle lighting –

Diminishing light for traffic signs…


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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


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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


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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…


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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


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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?


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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


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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


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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?


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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

Documents

Visibility Safety · 2019. 12. 10. · reflective sign sheeting and all-weather pavement markings, ... Type V = Delineators - Metallized Prismatic ... optics providing superior luminance