Curve Advisory Speed Existing Practices and New Procedures OR

Curve Advisory Speed

Existing Practices and New Procedures

OR OR

CURVE ADVISORY SPEED WORKSHOP

Introduction• Workshop Instructor

– Robert Milstead, PE, PTOE (Brudis & Associates, Inc.)

• Workshop Objectives

– New criteria and tools

• MUTCD 2009 compliance

• New procedures– Limitations of current practices – Uniform and consistent advisory speeds– Horizontal alignment warning signs– Participant discussion and feedback


Agenda

• Background

• MUTCD 2009 Criteria and Guidelines

• Current Practices to Determine Advisory Speed

• New Procedures to Determine Advisory Speed

• Support: Curve Advisory Speed Worksheet

• Confirm Advisory Speed and Curve Sign Selection

• Step by Step Procedures

• Exercises

• Summary


Background

What is an Advisory Speed

– Definition (MUTCD 2009 ): a recommended speed for all vehicles operating on a section of highway and based on the highway design, operating characteristics, and conditions.

– Uniform and consistent advisory speed together with other horizontal alignment warning signs conveys a realistic message establishing driver expectancy and promoting effective roadway operations.

– Typical applications: horizontal curves, highway ramps, and work zones.


Background

Importance of Uniform and Consistent Advisory Speeds

– Safety

– Positive Guidance

– Driver Respect for Signs


Background

Crash Statistics (2008)

– 52% of all fatal crashes involve roadway departures.

– 9,475 (28%) fatal crashes occurred on a curve.

– 85% of the fatal curve crashes involve roadway departures.

“A roadway departure crash is defined as a non-intersection crash which occurs after a vehicle crosses an edge line or a center line, or otherwise leaves the traveled way.”

Source: NHTSA FARS 2008 & FHWA Safety Program


Background

A Nationwide Survey of 344 Practitioners (Lyles and Taylor, 2006)

– Uniformity among curves

• 45% believe that advisory speeds are not uniform throughout their own states.

• Only 58% believe that advisory speeds are consistently estimated.

– Consistency with driver expectation

• 62% believe that advisory speeds are too low.• 3% believe that advisory speeds are too high.


Background

Current Advisory Speeds

–Lack of uniformity among curves

–Commonly more conservative than guidance

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70

Posted Advisory Speed (mph)

10 D

egre

e B

all-

Ban

k B

ased

Ad

viso

ry S

pee

d (

mp

h)

Ball-bank Based (Uniformly Re-established) vs. Posted Advisory Speeds at 65 Curves.


Background

Current Advisory Speeds

– Inconsistent with Driver Expectation

–Majority of drivers exceed advisory speed by 10 mph

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70

Posted Advisory Speed (mph)

Cu

rve

Sp

eed

(m

ph

)

85th% Speeds

Average Speeds

Average and 85th% Speeds vs. Posted Advisory Speeds (data from 65 curves)


MUTCD 2009Section 1A.04 Placement and Operation of Traffic Control Devices

Guidance:

Placement of a traffic control device should be within the road user’s view so that adequate visibility is provided. To aid in conveying the proper meaning, the traffic control device should be appropriately positioned with respect to the location, object, or situation to which it applies. The location and legibility of the traffic control device should be such that a road user has adequate time to make the proper response in both day and night conditions.

Traffic control devices should be placed and operated in a uniform and consistent manner. Unnecessary traffic control devices should be removed. The fact that a device is in good physical condition should not be a basis for deferring needed removal or change.

Source: FHWA MUTCD 2009


MUTCD 2009Section 2C.02 Application of Warning Signs

Standard:

The use of warning signs shall be based on an engineering study or on engineering judgment.

Guidance:

The use of warning signs should be kept to a minimum as the unnecessary use of warning signs tends to breed disrespect for all signs. In situations where the condition or activity is seasonal or temporary, the warning sign should be removed or covered when the condition or activity does not exist.



MUTCD 2009Section 2C.08 Advisory Speed Plaque (W13-1P)

Option:

The Advisory Speed (W13-1P) plaque (see Figure 2C-1) may be used to supplement any warning sign to indicate the advisory speed for a condition .

Standard:

The use of the Advisory Speed plaque for horizontal curves shall be in accordance with the

information shown in Table 2C-5. The Advisory Speed plaque shall also be used where an engineering study indicates a need to advise road users of the advisory speed for other roadway conditions.


If used, the Advisory Speed plaque shall carry the message XX MPH. The speed displayed shall be a multiple of 5 mph.

Except in emergencies or when the condition is temporary, an Advisory Speed plaque shall not be installed until the advisory speed has been determined by an engineering study.

The Advisory Speed plaque shall only be used to supplement a warning sign and shall not be installed as a separate sign installation.

The advisory speed shall be determined by an engineering study that follows established engineering practices.


MUTCD 2009Support:

Among the established engineering practices that are appropriate for the determination of the recommended advisory speed for a horizontal curve are the following:

A. An accelerometer that provides a direct determination of side friction factors

B. A design speed equation C. A traditional ball-bank indicator using the following criteria:

1. 16 degrees of ball-bank for speeds of 20 mph or less

2. 14 degrees of ball-bank for speeds of 25 to 30 mph

3. 12 degrees of ball-bank for speeds of 35 mph and higher

The 16, 14, and 12 degrees of ball-bank criteria are comparable to the current AASHTO horizontal curve design guidance. Research has shown that drivers often exceed existing posted advisory curve speeds by 7 to 10 mph.



MUTCD 2009Guidance:

The advisory speed should be determined based on free-flowing traffic conditions. Because changes in conditions, such as roadway geometrics, surface characteristics, or sight distance, might affect the advisory speed, each location should be evaluated periodically or when conditions change.



Horizontal Alignment Warning Signs (MUTCD 2003)Horizontal Alignment

Horizontal Alignment + Advisory Speed

Horizontal Alignment + Intersection

One-Direction Large Arrow

Chevron Alignment

Truck Rollover Warning

Advisory Speed Plaque

Advisory Exit/Ramp Speed



Horizontal Alignment Warning Signs (MUTCD 2009)Horizontal Alignment

Combination Horizontal Alignment + Advisory Speed

Horizontal Alignment + Intersection

One-Direction Large Arrow

Chevron Alignment

Truck Rollover Warning

Advisory Speed Plaque

Advisory Exit/Ramp Speed

Horizontal Alignment + Advisory Exit/Ramp Speed



Horizontal Alignment Warning Signs (MUTCD 2003)

• relates sign usage to advisory speed and number of alignment changes

• less guidance



Horizontal Alignment Warning Signs (MUTCD 2009)

• revised significantly

• relates sign usage to the difference between the approach speed and the advisory speed

• hierarchical approach

• only for applications where advisory speed is less than posted



Current Practices to Determine Advisory Speed

• Driver Comfort Speed

• Ball-Bank Indicator

• 85th Percentile Speed

• Geometric Design

• Other Approach


Current Practices>> Driver Comfort Speed• Oldest method used for determining advisory speeds.

• Defined in the 1930s as that “which causes an occupant of the vehicle to feel an outward pitch.”

• Engineers later refined the definition to be “that speed at which the driver’s judgment recognized incipient instability.”

• Drawbacks: most subjective and inconsistent results.


Current Practices>> Ball-Bank Indicator

• A curved level filled with a dampening liquid with a trapped air bubble or “ball”.

• Installed in a test vehicle: multiple test runs at 5-mph increments.

• Indicate the geometric degree of “tip, tilt, or lean” of a curve roadway.

• Advisory speed is the highest test speed that does not exceed threshold.

• Most widely used instrument for determining advisory speeds by 82% of agencies (Lyles and Taylor, 2006).


Current Practices>> Ball-Bank IndicatorVaried Criteria

May lead to different advisory speed recommendations.

SpeedBall-Bank Threshold

2004 AASHTO 2003 MUTCD 2009 MUTCD

≤ 20 mph 14˚

16˚

16˚

25-30 mph 12˚ 14˚

≥ 35 mph 10˚ 12˚

for Truck - - 10˚


Wide variation in reading along curve – Expectation of constant reading along curve– May differ by directions based on superelevation

0

2

4

6

8

10

12

14

0 2 4 6 8 10 12 14

Travel Time Along Curve, s

Ba

ll-B

an

k R

ea

din

g,

de

gre

es

Curve to the Right

Curve to the Left

Expected Relationship

Current Practices>> Ball-Bank Indicator

Data Source: TTI Horizontal Curve Signing Workshop (Nov 2008)

Curve 150 mph test speed

0

2

4

6

8

10

12

14

0 2 4 6 8 10 12 14

Travel Time Along Curve, s

Ba

ll-B

an

k R

ea

din

g,

de

gre

es

Curve to the Right

Curve to the Left

Typical Relationship


Current Practices>> Ball-Bank IndicatorSources of Variability

• Use of different test vehicles• Personnel training• Instrument calibration quality• Speed• Steering corrections

– Tire slip• Pavement variability

– Roughness– Superelevation– Friction supply

Can control

Difficult to control

Cannot control



Current Practices>> 85th Percentile Speed

• Measuring speeds at which all drivers drive along a curve in a free flow condition.

• Set advisory speed at the level which 85% of all drivers drive at, or below.

• Represents the largest number of drivers within a narrow pace of 10 MPH.

• On target to provide the greatest level of safety for the majority.


Current Practices>> 85th Percentile Speed

However, given direct measurements of curve speed distribution and vehicle classification,

– No consensus on how to determine appropriate advisory speeds

– 85th%, average, or median speeds?

– Passenger cars, trucks or all?

– What do you think? …… More discussion later

– MUTCD 2009 no longer provides explicit support

to this method


Current Practices>> Geometric Design

AASHTO calculates an advisory speed by:

V² = 15 (e + f) R

– Where:

• V = Advisory Speed of vehicle in MPH• e = Superelevation in foot per foot of horizontal width• f = Transverse coefficient of friction• R = Radius of curvature in feet

– Source of variability: value of transverse coefficient of friction.


Current Practices>> Other ApproachExample: Electronic Accelerometer

– A gravity-sensitive electronic device

– Measure the lateral forces and accelerations that drivers experience while traversing a highway curve.

– Alternative to the ball-bank indicator. (AASHTO’s 2004 Green Book)


New Procedures to Determine Advisory SpeedRelated TTI Projects (in cooperation with FHWA and the Texas DOT)

– Identifying and Testing Effective Advisory Speed Setting Procedures (Project Director: Marla Jasek)

• Research Report (0-5439-1)• Horizontal Curve Signing Handbook, 1st Edition (0-5439-P1)

– Workshops on Identifying and Testing Advisory Speed Setting Procedures (Project Director: Darren McDaniel)

• Horizontal Curve Signing Handbook, 2nd Edition (5-5439-01-P1)


New Procedures to Determine Advisory Speed

Advisory Speed Criteria - Considerations– Car vs. truck speed

– Average vs. 85th percentile

vs. Ball Bank Indicator reading

– Curves are inherently unsafe, so advisory speed should be conservatively low

– Drivers should feel advisory speed is reasonable

– On sharp curves, 85th percentile driver tends to adopt a speed that may be borderline unsafe

Source: TTI Horizontal Curve Signing Workshop (Nov 2008)



Advisory Speed Criteria - Recommendation

– Average Truck Speed

– Roughly equivalent to 40th percentile car speed

– Truck speed is generally 97% of car speed

– Round to 5-mph increment




• Direct Method

• Compass Method

• GPS Method

• Design Method



Each Method involves the following two steps:

1. Data collection

2. Determination of Advisory Speed

• Direct calculation for Direct Method, or• TTI Curve Speed Prediction Model for Compass, GPS, and

Design Methods.


New Procedures>> Direct Calculation• Data Collection

– Measure speed of 125 free-flow passenger cars

• Stop after 2 hours (radar)• Stop after 4 hours (classifier)

– Compute Average and 85th percentile Speeds

• Determination of Advisory Speed

– Average truck speed = 0.97 x average passenger car speed

– Add one and drop down

e.g. 34, 35, 36, 37, 38 → 35 mph; 39, 40, 41, 42, 43 → 40 mphSource: TTI Horizontal Curve Signing Workshop (Nov 2008)


New Procedures>> TTI Curve Speed Prediction Model

Compass Method GPS Method Design Method–Compass–Ball-Bank Indicator–Distance-Measuring

Instrument

–GPS Receiver–Electronic Ball-Bank Indicator–Laptop Computer with

TRAMS*

–Plans of newly constructed or reconstructed curves

Data Collection:

* TRAMS: Texas Roadway Analysis and Measurement Software


New Procedures>> TTI Curve Speed Prediction Model

Determination of Advisory Speed

Advisory speed is a function of

• curve deflection angle;• superelevation rate;• curve length;• 85th% speed (observed or estimated).

Source: Horizontal Curve Signing Handbook (TTI, Oct 2008)

Curve Geometry


Curve Advisory Speed Worksheet• An Excel® Spreadsheet based on and enhanced from Texas Curve

Advisory Speed Software

• Calculates Advisory Speed for

– Compass Method

– GPS Method

– Design Method

• Provides Curve Signing Guidelines for

– TTI methods

– Other sources (Accelerometer, AASHTO geometric design equation, Ball-bank indicator, 85th% speed, driver comfort speed…)

TTI Curve Speed Predication Model


Curve Advisory Speed Worksheet

Analysis

– General Info.

– Input Data

– Alternate Input Data

– Advisory Speed

– Traffic Control Device Guidance



Analysis

– General Info.

– Input Data


– Advisory Speed


for Compass Method

for GPS Method or Design Method

for Advisory Speed from Other Source



Analysis

– General Info.

– Input Data


– Advisory Speed


Survey of Curve (Compass Method)

Speed Limit



Analysis

– General Info.

– Input Data


– Advisory Speed


Curve geometry (GPS & Design Method)

Curve surrounding features

Tangent speed (optional data)



Analysis

– General Info.

– Input Data


– Advisory Speed


Advisory Speed by TTI model

Advisory Speed from other sources

Advisory Speed Selection



Analysis

– General Info.

– Input Data


– Advisory Speed


Warning signs

(follow MUTCD 2009 guidelines)


Confirm Advisory Speed and Curve Sign Selection

Adjust the advisory speed or modify the warning sign layout based on consideration of:

1. driver approach sight distance to the beginning of the curve;

2. visibility around the curve;

3. position of the most critical curve in a sequence of closely-spaced curves.




4. unexpected geometric features within the curve;

• presence of an intersection• presence of a sharp crest curve in the middle of the horizontal

curve• sharp curves with changing radius (including curves with spiral

transitions)• sharp curves after a long tangent section• broken-back curves



Confirm Advisory Speed and Curve Sign SelectionApproach Sight Distance




Approach Sight Distance




Visibility Around Curve




Intersections or Driveways in Curve




3 curves in series

Proximity to Other Curves: may influence curve speed




Check Data Consistency– Consistent between opposing directions?

• Superelevation– Differ by 0 - 4%

• Radius– Within 10% in opposing directions

• Deflection angle– Within 2 degrees in opposing directions

• Smooth curve tracking is important– Consistent with judgment?

• 85th percentile tangent speed

or




Example:

– Radius and deflection angle are consistent

– Superelevation differs by 7%

– Consider re-driving the curve

Direction Left Right

Radius (ft) 1040 1105

Deflection angle (deg) 38 38

Superelevation (%) 2 9



Step by Step Procedure

• GPS Method

• Compass Method


Step by Step Procedure>>GPS Method• Equipment Setup

• Measurement Procedure

• Texas Roadway Analysis and Measurement Software (TRAMS) Program



Step by Step Procedure>>GPS Method

Equipment Setup

Laptop computer

Electronic ball-bank indicator

GPS receiver




Equipment Setup




Measurement Procedure

1. 1-2 seconds before start of curve• Press space bar to start

recording

2. Drive through one curve

3. 1-2 seconds after end of curve• Press space bar to stop

recording





– Track the centerline carefully

– Choose slow but reasonable speed

• Rule of thumb: 10 mph below existing advisory speed• No less than 15 mph• No greater than 45 mph if superelevation is being measured




Device status

Control button Curve &

highway designation

Curve data

Texas Roadway Analysis and Measurement Software (TRAMS) Program Interface



Step by Step Procedure>>GPS MethodTRAMS Curve Report File




TRAMS Curve Report File



Step by Step Procedure>>GPS MethodAdvisory Speed and Curve Signs


Step by Step Procedure>>Compass Method

Travel through the curve and collect data:





1. Record the regulatory speed limit and the curve advisory speed.

2. Stay in the same lane for all measurements. Record the curve deflection (left or right) relative to the direction of travel.





3. Advance the vehicle to the “1/3 point” (does not need to be precisely located). It is the point of partial curvature (PPC).

Stop the vehicle and complete the following while at the PPC:• Record the vehicle heading (in degrees).• Press the Reset button on the DMI to zero the reading.• Record the ball-bank indicator reading (in degrees).• Record whether the ball has rotated to the left or right of the “0.0

degree” reading.





4. Advance the vehicle to the “2/3 point” (does not need to be precisely located). It is the point of partial tangency (PPT).

Stop the vehicle and complete the following while at the PPT:• Record the vehicle heading (in degrees).• Press the Display Hold button on the DMI.

5. Record the value shown on the DMI which is the partial curve length.



Step by Step Procedure>>Compass MethodAdvisory Speed and Curve Signs


Step by Step Procedure>>AccelerometerAdvisory Speed and Curve Signs


Exercise 1Objective: determine needed devices and advisory speed



Exercise 1• Speed limit: 60 mph

• Advisory speed: 45 mph



Exercise 1

Known Data:

• Regulatory speed limit: 60 mph

• Total deflection angle: 51 degrees

• Curve deflection angle: 17 degrees

• Superelevation rate: 6.5 percent

• Curve radius: 715 ft


Exercise 1Questions:

• What is the advisory speed?

• What warning signs are needed?


Exercise 2Objectives: determine needed devices and advisory speed



Exercise 2

• Speed limit: 55 mph




Exercise 2Known Data:


• Total deflection angle: 83.9 degrees

• Curve deflection angle: 6.2 degrees


• Curve radius: 278 ft






Exercise 3Objectives: determine needed devices and advisory speed



Exercise 3





Exercise 3Known Data:





• Radius: 924 ft






Exercise 3Discussion:

• Southbound approach has 5.5 percent downhill grade

• Review of crash data reveals crash problem on this approach (run-off-road near PC)

• May consider adding delineators



Exercise 4Objectives: sensitivity analysis of guidance from Exercise 3



Exercise 4





Exercise 4Initial Data:





• Radius: 924 ft



Exercise 4For total deflection angles of 80, 90, 100 deg






Exercise 4If the superelevation rate is 4, 5.5, 7 percent






Exercise 4If curve radius is 874, 924, 974 ft






Exercise 4For 85th% tangent speed of 58 and 66 mph






Summary• Importance of advisory speed and curve signing: SAFETY

• Advisory speed should be consistent with driver expectation.

• Devices should be uniform among curves of similar geometry, character, and road conditions.

• Current practices: lack of uniformity and inconsistent

• Proposed procedures: four new procedures to establish uniform and consistent advisory speed and curve signing. Comparable results among methods.

• Proposed procedures and software are uncomplicated and easy to implement, but still under further improvement.


For More Information• Development of Guidelines for Establishing Effective Curve Advisory

Speeds

– http://tti.tamu.edu/publications/catalog/record_detail.htm?id=29831

• Horizontal Curve Signing Handbook, 2nd Edition

– http://tcd.tamu.edu/Documents/5-5439-01-P1.pdf

• Texas Curve Advisory Speed Software

– http://tcd.tamu.edu/Documents/TCAS(19)p.xls

• The 2009 Edition of the MUTCD

– http://mutcd.fhwa.dot.gov/kno_2009.htm


• Contact

Robert J. Milstead, PE, PTOEVice President

Brudis & Associates, Inc.9240 Rumsey Road, Suite C

Columbia, MD 21045Email: [email protected]

Phone: (410) 884-3607 ext. 224Fax: (410) 884-3609

The End

• Questions and Comments?