Curve
ATSSA 2012 Conference
Introduction to High Friction Surfaces
Provide Skid Resistant Pavement Surfaces 23 United State Code 402 Highway Safety Program Standard 12 Every State shall have a program to improve highway safety such that "there are standards for pavement design and construction with specific provisions for high skid resistance qualities. Each State shall have a "program for resurfacing or other surface treatment with emphasis on correction of locations or sections of streets and highways with low skid resistance and high or potentially high accident rates susceptible to reduction by providing improved surfaces.“
Provide Skid Resistant Pavement Surfaces 23 Code of Federal Regulations 626.3
"Pavement shall be designed to accommodate current and predicted traffic needs in a safe, durable, and cost effective manner." To provide a safe pavement, wet pavement friction should be provided.
Provide Skid Resistant Pavement Surfaces
• • •
Curves may justify a higher threshold value for a frictionrelated parameter. Higher speed facilities may justify a higher threshold value for a friction-related parameter. An increased probability of wet-weather conditions would justify a higher level of texture or higher threshold value for a friction-related parameter.
Crashes at Horizontal Curves
Pavement Friction Management
T 5040.38
“Research
June 17, 2010
has shown that curves and
intersections tend to lose friction at a faster rate than other roadway locations and thus justify a higher friction demand.”
Provide Skid Resistant Pavement Surfaces
Crash Types addressed by improving pavement friction: • wet weather • other skidding (e.g. too fast for conditions) • curves
Provide Skid Resistant Pavement Surfaces
Crash Types addressed by improving pavement friction: • wet weather • other skidding (e.g. too fast for conditions) • curves
http://safety.fhwa.dot.gov/speedmgt/ref_mats/fhwasa1/112
Proportion of Fatal Crashes Occurring on Wet Pavement
Source FHWA-SA-11-21
So……. How is that working!
Horizontal Curves and Safety 6.7 7
Average crash rates for horizontal curves is about 3 times that of tangent segments
Crashes/km
6 5 4 3
2.21
2 1 0
Tangent Segments
Curves and Transitions
Source: Glennon, et al, 1985 study for FHWA
2008 National Fatal Crashes 34,017 Fatal Crashes 17,818 Roadway Departures Roadway Departure Crash - A non-intersection crash in which a vehicle crosses an edge line, a centerline, or otherwise leaves the traveled way. 1-12
2008 Horizontal Curve Crashes
28%
Straight
72%
Curve
Alignment 2008 Fatal Crashes
Unknown
Curve Straight
Urban
Rural
0
2000
4000
6000
8000 10000 12000 14000
2008 Rural Fatal Curve Crashes
FHWA Roadway Departure Strategic Plan Roadway Departure Fatalities by Most Harmful Event (FARS 2007-2009) 2,210
Overturn Opposite Direction
4,156
Trees, Shrubs
2,702 18,530
2,861
Signs, utility poles, traffic signals
3,694
Other Fixed Object Barriers
11,452 14,374
Embankments, Ditches, Boulder, Snowbank Other
Roadway Departure Fatalities Most Harmful Event (FARS 2007-2009) Nearly ¾ of Roadway Departure Fatalities are from 3 crash types.
Overturn Crashes
31%
Source: FARS
Opposite Direction Crashes
24%
NCHRP Report 500, Volume 6 Reported that only 4.2% of head-ons involved a passing vehicle.
Source: FARS
Tree Crashes 80%
% Fatal Crashes 68% 64%
60% 51%
19%
49%
48%
40% 31%
20%
0%
Source: FARS
Where Can High Friction Surfaces Benefit Safety? 1. Horizontal curves 2. Approach to intersections When the pavement has: a) low friction b) marginal friction effected by weather c) Friction values not compatible with approach speeds and geometrics
What is a High Friction Surfaces?
FN ≥ 60 Aggregate with resistance to polishing
Friction and Crash Risk Relationship
Source NCHRP 108
Pavement Friction Verses Tire Slip
Source NCHRP 108
Conceptual Relationship Between Friction Demand, Speed and Friction Availability
Source NCHRP 108
Skid related crashes are determined by many factors: •Road Geometry •Driver Actions •Weather Conditions •Traffic Characteristics •Vehicle Speeds
Source NCHRP 108
Basis for AASHTO Curve Design Model Is Driver Comfort Although the curve design policy stems from the laws of mechanics, the values used in design depend on practical limits and factors determined empirically over the range of variables involved.
AASHTO Horizontal Curve Design Model
e+f = V2/15 R e = superelevation f = side friction factor V = design speed (mph) R = radius of curve (ft)
Improving Friction to Keep Vehicles on the Roadway NCHRP 500 Volume 7
Improving Friction to Keep Vehicles on the Roadway AASHTO Design assumes vehicles: • do not exceed the design speed • traverse the curve following a constant radius.
Likelihood of skidding increases when these assumptions are violated. Several studies have shown that under real world conditions both of these assumption are violated
AASHTO Design Model Side Friction Factor - ‘f’
Superelevation – “e” “e” design values established by policy, with maximum values (.06, .08, .10, .12) set for consideration of icy conditions
Curve Tracking • Actual Vehicle Path Does Not Follow a Perfect Circle • Drivers ‘Overshoot’ Curve (track path is sharper than radius) • Path Is Spiral • Path Overshoot Behavior Is Independent of Speed
Driver tracks a ‘critical radius’ sharper than that of the curve just past the PC
Speeds on Curves Driver Behavior Vs. Model Assumptions Curves driven faster than Policy assumption
Curves driven slower than Policy assumption
When crash types were examined for these drivers excessively speeding, researchers found that speeding was the leading cause of single-driver right or left roadside departure with traction loss and the third leading cause of head-on crashes.
…primarily on curves, at night, on local or collector roadways, and during clear weather.
Truck Operations on Curves • Trucks with high centers of gravity may overturn before losing control due to skidding • Skidding trucks may lead to overturn • Trucks on downgrade curves generate greater lateral friction demand (Superelevation Is Not As Effective) • Margin of safety for ‘f’ is lower for trucks • Friction demand varies per tire Reference NCHRP 505
What about Friction Demand?
When the: • superelevation, • radius • approach speed is known
Solve for friction demand Fs = V2 - e 15R
Assessing Curve Severity and Design Consistency Using Energy- and Friction-Based Measures Michael P. Pratt and James A. Bonneson
Transportation Research Record 2075
June 26, 2008 Oldham Era Rain Increases Danger on KY. 22 Crews work to clear the roadway after a head-on collision during rush hour Friday on KY- 22 at Bridge Hill. The crash left four people injured. Another crash took place Sunday afternoon.
Bridge Hill - Oldham County, KY 22
• signing work was done 4 to 5 years prior to the HFS • rumble strips were installed 3 to 4 years prior.
Bridge Hill - Oldham County, KY 22
Here is some information about the location we discussed. Scott Pedigo, P.E., PTOE Central Office Traffic Operations Oldham County, KY 22: 8/1/07 thru 8/1/09: 59 total crashes (1 Fatal and 23 Injury crashes),
Bridge Hill- Oldham County, KY 22
Bridge Hill - Oldham County, KY 22
Bridge Hill - Oldham County, KY 22
Here is some information about the location we discussed where KYTC installed High Friction Surface and received great results. Scott Pedigo, P.E., PTOE Central Office Traffic Operations Oldham County, KY 22, 8/1/07 thru 8/1/09: 59 total crashes (1 Fatal and 23 Injury crashes), 2 crashes since
Total Project Cost = $66,500
Horizontal Curve HFS Installation
Penn DOT High Friction Surface Installation Northampton County SR 611 South of Easton Contact: Stephen Pohowsky Safety Program Specialist – District 5
Mixed 2 part epoxy resin is hand spread with a serrated squeegee.
Skid Testing & Crash Results
Kentucky Compared to Other States Most “Deadly”
Source: Readers Digest, Based on Fatalities Per 100 Million Vehicle Miles Traveled April 2010;
Least “Deadly”
HFS Installation in yellow
Bellevue Washington
Provide Skid Resistant Pavement Surfaces
Installation of a Microtexture surface on a horizontal curve Prior to the installation, this location had 45 accidents.
Installation by hand
Source NCHRP 108
Hamilton County OH
Hamilton County OH
Page 101
Page 107
Introduction to High Friction Surfaces
Provide Skid Resistant Pavement Surfaces 23 United State Code 402 Highway Safety Program Standard 12 Every State shall have a program to improve highway safety such that "there are standards for pavement design and construction with specific provisions for high skid resistance qualities. Each State shall have a "program for resurfacing or other surface treatment with emphasis on correction of locations or sections of streets and highways with low skid resistance and high or potentially high accident rates susceptible to reduction by providing improved surfaces.“
Provide Skid Resistant Pavement Surfaces 23 Code of Federal Regulations 626.3
"Pavement shall be designed to accommodate current and predicted traffic needs in a safe, durable, and cost effective manner." To provide a safe pavement, wet pavement friction should be provided.
Provide Skid Resistant Pavement Surfaces
• • •
Curves may justify a higher threshold value for a frictionrelated parameter. Higher speed facilities may justify a higher threshold value for a friction-related parameter. An increased probability of wet-weather conditions would justify a higher level of texture or higher threshold value for a friction-related parameter.
Crashes at Horizontal Curves
Pavement Friction Management
T 5040.38
“Research
June 17, 2010
has shown that curves and
intersections tend to lose friction at a faster rate than other roadway locations and thus justify a higher friction demand.”
Provide Skid Resistant Pavement Surfaces
Crash Types addressed by improving pavement friction: • wet weather • other skidding (e.g. too fast for conditions) • curves
Provide Skid Resistant Pavement Surfaces
Crash Types addressed by improving pavement friction: • wet weather • other skidding (e.g. too fast for conditions) • curves
http://safety.fhwa.dot.gov/speedmgt/ref_mats/fhwasa1/112
Proportion of Fatal Crashes Occurring on Wet Pavement
Source FHWA-SA-11-21
So……. How is that working!
Horizontal Curves and Safety 6.7 7
Average crash rates for horizontal curves is about 3 times that of tangent segments
Crashes/km
6 5 4 3
2.21
2 1 0
Tangent Segments
Curves and Transitions
Source: Glennon, et al, 1985 study for FHWA
2008 National Fatal Crashes 34,017 Fatal Crashes 17,818 Roadway Departures Roadway Departure Crash - A non-intersection crash in which a vehicle crosses an edge line, a centerline, or otherwise leaves the traveled way. 1-12
2008 Horizontal Curve Crashes
28%
Straight
72%
Curve
Alignment 2008 Fatal Crashes
Unknown
Curve Straight
Urban
Rural
0
2000
4000
6000
8000 10000 12000 14000
2008 Rural Fatal Curve Crashes
FHWA Roadway Departure Strategic Plan Roadway Departure Fatalities by Most Harmful Event (FARS 2007-2009) 2,210
Overturn Opposite Direction
4,156
Trees, Shrubs
2,702 18,530
2,861
Signs, utility poles, traffic signals
3,694
Other Fixed Object Barriers
11,452 14,374
Embankments, Ditches, Boulder, Snowbank Other
Roadway Departure Fatalities Most Harmful Event (FARS 2007-2009) Nearly ¾ of Roadway Departure Fatalities are from 3 crash types.
Overturn Crashes
31%
Source: FARS
Opposite Direction Crashes
24%
NCHRP Report 500, Volume 6 Reported that only 4.2% of head-ons involved a passing vehicle.
Source: FARS
Tree Crashes 80%
% Fatal Crashes 68% 64%
60% 51%
19%
49%
48%
40% 31%
20%
0%
Source: FARS
Where Can High Friction Surfaces Benefit Safety? 1. Horizontal curves 2. Approach to intersections When the pavement has: a) low friction b) marginal friction effected by weather c) Friction values not compatible with approach speeds and geometrics
What is a High Friction Surfaces?
FN ≥ 60 Aggregate with resistance to polishing
Friction and Crash Risk Relationship
Source NCHRP 108
Pavement Friction Verses Tire Slip
Source NCHRP 108
Conceptual Relationship Between Friction Demand, Speed and Friction Availability
Source NCHRP 108
Skid related crashes are determined by many factors: •Road Geometry •Driver Actions •Weather Conditions •Traffic Characteristics •Vehicle Speeds
Source NCHRP 108
Basis for AASHTO Curve Design Model Is Driver Comfort Although the curve design policy stems from the laws of mechanics, the values used in design depend on practical limits and factors determined empirically over the range of variables involved.
AASHTO Horizontal Curve Design Model
e+f = V2/15 R e = superelevation f = side friction factor V = design speed (mph) R = radius of curve (ft)
Improving Friction to Keep Vehicles on the Roadway NCHRP 500 Volume 7
Improving Friction to Keep Vehicles on the Roadway AASHTO Design assumes vehicles: • do not exceed the design speed • traverse the curve following a constant radius.
Likelihood of skidding increases when these assumptions are violated. Several studies have shown that under real world conditions both of these assumption are violated
AASHTO Design Model Side Friction Factor - ‘f’
Superelevation – “e” “e” design values established by policy, with maximum values (.06, .08, .10, .12) set for consideration of icy conditions
Curve Tracking • Actual Vehicle Path Does Not Follow a Perfect Circle • Drivers ‘Overshoot’ Curve (track path is sharper than radius) • Path Is Spiral • Path Overshoot Behavior Is Independent of Speed
Driver tracks a ‘critical radius’ sharper than that of the curve just past the PC
Speeds on Curves Driver Behavior Vs. Model Assumptions Curves driven faster than Policy assumption
Curves driven slower than Policy assumption
When crash types were examined for these drivers excessively speeding, researchers found that speeding was the leading cause of single-driver right or left roadside departure with traction loss and the third leading cause of head-on crashes.
…primarily on curves, at night, on local or collector roadways, and during clear weather.
Truck Operations on Curves • Trucks with high centers of gravity may overturn before losing control due to skidding • Skidding trucks may lead to overturn • Trucks on downgrade curves generate greater lateral friction demand (Superelevation Is Not As Effective) • Margin of safety for ‘f’ is lower for trucks • Friction demand varies per tire Reference NCHRP 505
What about Friction Demand?
When the: • superelevation, • radius • approach speed is known
Solve for friction demand Fs = V2 - e 15R
Assessing Curve Severity and Design Consistency Using Energy- and Friction-Based Measures Michael P. Pratt and James A. Bonneson
Transportation Research Record 2075
June 26, 2008 Oldham Era Rain Increases Danger on KY. 22 Crews work to clear the roadway after a head-on collision during rush hour Friday on KY- 22 at Bridge Hill. The crash left four people injured. Another crash took place Sunday afternoon.
Bridge Hill - Oldham County, KY 22
• signing work was done 4 to 5 years prior to the HFS • rumble strips were installed 3 to 4 years prior.
Bridge Hill - Oldham County, KY 22
Here is some information about the location we discussed. Scott Pedigo, P.E., PTOE Central Office Traffic Operations Oldham County, KY 22: 8/1/07 thru 8/1/09: 59 total crashes (1 Fatal and 23 Injury crashes),
Bridge Hill- Oldham County, KY 22
Bridge Hill - Oldham County, KY 22
Bridge Hill - Oldham County, KY 22
Here is some information about the location we discussed where KYTC installed High Friction Surface and received great results. Scott Pedigo, P.E., PTOE Central Office Traffic Operations Oldham County, KY 22, 8/1/07 thru 8/1/09: 59 total crashes (1 Fatal and 23 Injury crashes), 2 crashes since
Total Project Cost = $66,500
Horizontal Curve HFS Installation
Penn DOT High Friction Surface Installation Northampton County SR 611 South of Easton Contact: Stephen Pohowsky Safety Program Specialist – District 5
Mixed 2 part epoxy resin is hand spread with a serrated squeegee.
Skid Testing & Crash Results
Kentucky Compared to Other States Most “Deadly”
Source: Readers Digest, Based on Fatalities Per 100 Million Vehicle Miles Traveled April 2010;
Least “Deadly”
HFS Installation in yellow
Bellevue Washington
Provide Skid Resistant Pavement Surfaces
Installation of a Microtexture surface on a horizontal curve Prior to the installation, this location had 45 accidents.
Installation by hand
Source NCHRP 108
Hamilton County OH
Hamilton County OH
Page 101
Page 107
