Procedures for Setting

Advisory Speeds on Curves

FHWA Safety Program

http://safety.fhwa.dot.gov FHWA-SA-11-22
Disclaimer

The contents of this handbook reflect the views of the authors, who are 

responsible for the facts and the accuracy of the data published herein. The 

contents do not necessarily reflect the official view or policies of the Federal 

Highway Administration (FHWA). This handbook does not constitute a standard, 

specification, or regulation. It is not intended for construction, bidding, or permit 

purposes. The engineer in charge of the project was Robert Milstead, P.E. #17172 

(Maryland).

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Trade or manufacturers’ names appear herein solely because they are 

considered essential to the object of this handbook.

Acknowledgements

The report that led to the development of this handbook was sponsored by the 

Texas Department of Transportation and the Federal Highway Administration. The 

research report was written by Dr. James Bonneson, Mr. Michael Pratt, Mr. Jeff 

Miles, and Dr. Paul Carlson (1). These researchers are employees with the Texas 

Transportation Institute (TTI).
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FHWA-SA-11-22

Procedures for Setting Advisory Speeds on Curves

Project performed for the Federal Highway Administration

Project Title: Curve Advisory Speed Workshops and Demonstrations

Project Manager: Guan Xu

FHWA Technical Panel: Ed Rice, Richard Knoblauch, Kevin J. Sylvester, Joseph Cheung

Horizontal curves are a necessary component of the highway alignment; however, they tend to be associated with a disproportionate number of severe crashes. Warning signs are intended to improve curve safety by alerting the driver to a change 

in geometry that may not be apparent or expected. However, several research projects conducted in the last 20 years have 

consistently shown that drivers are not responding to curve warning signs or complying with the Advisory Speed plaque. It is 

estimated that half of all speeding related roadway departure crashes occur on curves. One of the reasons that curves are overrepresented in speeding related fatalities is due, in part, to advisory speeds that are not consistent, and therefore, not credible. 

A project by Texas Transportation Institute (TTI), Texas Research Project 0-5439 “Identifying and Testing Effective Advisory 

Speed Setting Procedures”, exemplifies how current procedures for setting advisory speed on curves are not reliable, and has 

developed new criteria and a new procedure that are more consistent with driver expectation. The new procedure involves 

the use of an Excel spreadsheet and a handbook for applying the new criteria.

The procedures described in the handbook are intended to improve consistency in advisory speed signing and, hopefully, 

driver compliance with the advisory speed. The handbook describes; 1) guidelines for determining when an advisory speed is 

needed; 2) criteria for identifying the appropriate advisory speed; 3) an engineering study method for determining the advisory speed; and 4) guidelines for selecting other curve related traffic control devices.

Brudis & Associates, Inc.

Science Applications International Corporation

Texas Transportation Institute

Federal Highway Administration

Office of Safety

1200 New Jersey Ave., SE

Washington, DC 20590

R. Milstead, X. Qin, B. Katz, J. Bonneson, M. Pratt, J. Miles, and P. Carlson

Traffic Control Devices, Warning Signs, Speed Signs, Highway Curves, Speed Measurement, Trucks, Traffic SpeedNo restrictions. 

Unclassified Unclassified 44 N/A

June 2011

2. Government Accession No. 3. Recipient’s Catalog No.

October 2009-June 2011
ii
iii

Table of Co

CHAPTER 1. INTRODUCTION.........................................................................................................1

1.1 Overview..................................................................................................................................................1

1.2 Purpose and Scope................................................................................................................................1

CHAPTER 2. CONVEYING CHANGES IN HORIZONTAL ALIGNMENT .............................................2

2.1 Overview..................................................................................................................................................2

2.2 Horizontal Curve Safety and Operation................................................................................................2

2.3 Warning Signs for Changes in Horizontal Alignment ...........................................................................4

2.4 Curve Advisory Speed Software............................................................................................................6

CHAPTER 3. METHODS FOR ESTABLISHING ADVISORY SPEED....................................................10

3.1 Overview................................................................................................................................................ 10

3.2 Direct Method....................................................................................................................................... 11

3.3 Compass Method................................................................................................................................. 12

3.4 GPS Method.......................................................................................................................................... 18

3.5 Design Method .....................................................................................................................................22

3.6 Ball-Bank Indicator Method.................................................................................................................23

3.7 Accelerometer Method........................................................................................................................25

3.8 Confirming Speed for Conditions........................................................................................................28

3.9 Workshops and Webinars ....................................................................................................................28

3.10 Summary ...............................................................................................................................................29

CHAPTER 4. CURVE SIGNING GUIDELINES .................................................................................31

4.1 Overview................................................................................................................................................ 31

4.2 Guidelines ............................................................................................................................................. 31

REFERENCES ................................................................................................................................35
iv

List of Fi

Figure 1 Effect of Radius, Tangent Speed, and Vehicle Type on Curve Speed.......................................3

Figure 2 Curve Crash Rate as a Function of Radius...................................................................................3

Figure 3 Horizontal Alignment Signs and Plaques (MUTCD 2009 Edition) ...............................................5

Figure 4 Curve Advisory Speed (CAS) Software Analysis Worksheet........................................................7

Figure 5 Location of Critical Portion of Curve........................................................................................... 13

Figure 6 Curve Advisory Speed (CAS) Software Input Data.................................................................... 16

Figure 7 Curve Advisory Speed (CAS) Software Advisory Speed Calculation ...................................... 17

Figure 8 Effect of Lateral Shift on Travel Path Radius................................................................................ 17

Figure 9 GPS Method Equipment Setup in Test Vehicle........................................................................... 19

Figure 10 Texas Roadway Analysis and Measurement Software (TRAMS) Main Panel ..........................20

Figure 11 Import TRAMS Data ......................................................................................................................22

Figure 12 Ball-Bank Indicator .......................................................................................................................24

Figure 13 CurveRite Front Panel...................................................................................................................26

Figure 14 CurveRite Measurement..............................................................................................................27

Figure 15 MUTCD 2009 Edition Criteria for the Selection of Horizontal Alignment Sign...........................32

Figure 16 MUTCD 2009 Edition Criteria for the Typical Spacing of Chevron

Alignment Signs on Horizontal Curves........................................................................................32

Figure 17 MUTCD 2009 Edition Criteria for Advance Placement of Warning Signs .................................33

Figure 18 Curve Advisory Speed (CAS) Software Traffic Control Device Guidance................................34

List of T

Table 1 Comparison of the Six Methods for Establishing Curve Advisory Speeds ...............................30

1

CHAPTER 1  INTR

1.1 Overview

Horizontal curves are a necessary component of the highway alignment; however, they tend to be 

associated with a disproportionate number of severe crashes. Recently, in the United States, about 

33,000 fatalities occur nationwide each year, and about 25 percent of these fatalities occur on 

horizontal curves (2).

Curve warning signs are intended to improve curve safety by alerting the driver to an upcoming 

change in geometry that may not be apparent or expected. One or more of the curve warning signs 

identified in the Manual on Uniform Traffic Control Devices (MUTCD 2009 edition) (3) are typically 

used to notify drivers. Drivers may also be notified of the need to reduce their speed through the use 

of an Advisory Speed plaque. 

Several research projects conducted in the last 20 years have consistently shown that drivers are not 

responding to curve warning signs and are not complying with the Advisory Speed plaque. Evidence 

of this non-responsiveness is supported by the aforementioned curve crash statistics. Chowdhury et 

al. (4) suggest that current practice in the U.S. for setting advisory speeds is contributing to this lack 

of compliance and the poor safety record. They advocate the need for a procedure that can be 

used to: (a) identify when a curve warning sign and advisory speed are needed, and (b) select an 

advisory speed that is consistent with driver expectation. They also recommend the uniform use of 

this procedure on a nationwide basis, such that driver respect for curve warning signs is restored and 

curve safety records are improved.

1.2 Purpose and Scope

The procedures described in this handbook are intended to improve consistency in curve signing 

and, subsequently, driver compliance with the advisory speed. The handbook describes guidelines 

for determining when an advisory speed is needed, criteria for identifying the appropriate advisory 

speed, engineering study methods for determining the advisory speed, and guidelines for selecting 

other curve related traffic control devices.

The handbook is to be used by traffic engineers and technicians who are responsible for evaluating 

and maintaining horizontal curve signing and delineation devices. 

The curve advisory speed and other curve related traffic control devices should be checked 

periodically to ensure that they are appropriate for the prevailing conditions. Changes in the 

regulatory speed limit, curve geometry, or crash history may require an engineering study to 

reevaluate the appropriateness of the existing signs and the possible need for additional signs.
2

CHAPTER 2  CONVEYING CHANGES IN HO

2.1 Overview

This chapter provides a brief overview of topics related to horizontal curve safety, operation, and 

curve warning signs. It consists of three parts. The first part examines the safety and operation of 

horizontal curves. The second part reviews the various warning signs that are used to sign horizontal 

curves. The last part provides an overview of the Curve Advisory Speed (CAS) software that was 

developed to automate the procedures and criteria described in Chapters 3 and 4, respectively.

Additional background information about curve advisory speed is provided in Bonneson et al. 

(5). The information in that report examines the objectives of curve signing and the challenges 

associated with establishing advisory speeds that are uniform among curves and consistent with 

driver expectation. That report also reviews the various criteria that have been used to set advisory 

speeds. 

2.2 Horizontal Curve Safety and Operation

This part of the chapter examines the factors that influence the safety and operation of horizontal 

curves. The focus is on factors related to the curve’s geometric design. The relationship between 

curve design and driver speed choice is described in Section 2.2.1, and the relationship between 

curve design and crash rate is explored in Section 2.2.2. 

2.2.1 Curve Speed

A review of the literature indicates that several variables can have an influence on curve speed. 

These variables include: 

•	Radius,

•	Superelevation,

•	 Tangent Speed,

•	Vehicle Type,

•	Curve Deflection Angle,

•	Curve Length,

•	 Tangent Length,

•	Sight Distance,

Of these variables, research indicates that the first five have the most significant effect on curve 

speed. Using data collected on rural highways in Texas, Bonneson et al. (5) developed a curve 

speed prediction model that includes sensitivity to these variables. The speeds predicted by this 

model are shown in Figure 1. The trends shown indicate that the average truck speed equals about 

97 percent of the average passenger car speed.

•	Grade, 

•	Vertical Curvature,

•	Shoulder Characteristics,

•	Edge Drop-off,

•	Weather,

•	 Lighting, and

•	Roadway surface, type/

condition.
3

Figure 1 – Effect of Radius, Tangent Speed, and Vehicle Type on Curve Speed

The trend lines in Figure 1 indicate that drivers on sharper curves slow from the tangent speed to an 

acceptable curve speed. The amount of speed reduction increases with decreasing radius. For 

curves with a 500 ft radius and a 60 mph tangent speed, the reduction is about 10 mph. In contrast, 

for a 1000 ft radius and 60 mph tangent speed, the reduction is only about 5 mph. 

The effect of superelevation rate is not shown in Figure 1; however, the model indicates that curve 

speed increases about 1.0 mph for every 2.0 percent increase in superelevation.

2.2.2 Curve Safety

Bonneson et al. (5) examined the relationship between curve radius and crash rate using safety 

relationships documented in the literature (6, 7). These relationships are shown in Figure 2. In this 

figure, crash rate is defined in terms of crashes per million vehicle miles (crashes/mvm). One trend 

line represents the combination of fatal and injury crashes. The other trend line represents the 

combination of fatal, injury, and property-damage-only (PDO) crashes.

The two trend lines in Figure 2 are in fairly good agreement. They indicate that the crash rate 

increases sharply for curves with a radius of less than 1000 ft. They also indicate that more crashes 

on sharper curves result in injury or fatality.

Based on the discussion in this and the previous sections, it is likely that the trends in Figure 2 

are reflecting driver error while entering or traversing a curve. It is possible that some drivers are 

distracted or impaired and do not track the curve. It is also possible that some drivers detect the 

curve but do not correctly judge its sharpness. In both instances, traffic control devices have the 

potential to improve safety by making it easier for drivers to detect the curve and judge its sharpness.

Figure 2 – Curve Crash Rate as a Function of Radius

a. 85th Percentile Speed. b. Average Speed.

20

25

30

35

40

45

50

55

60

0 500 1000 1500

Radius, ft

85th % Curve Speed, mph

Trucks

Passenger Cars

85th % Tangent Speed = 40 mph

50 mph

60 mph 6% Superelevation

20

25

30

35

40

45

50

55

60

0 500 1000 1500

Radius, ft

Average Curve Speed, mph

Trucks

Passenger Cars

85th % Tangent Speed = 40 mph

50 mph

60 mph

6% Superelevation

0.0

1.0

2.0

3.0

4.0

0 500 1000 1500 2000 2500

Radius, ft

Cra

sh R

ate, cra

she

s/mvm

85th% tangent speed = 60 mph

Bonneson et al. (5)

Fitzpatrick et al. (6)

Fatal + Injury + PDO

Fatal + Injury
4

2.3 Warning Signs for Changes in Horizontal Alignment

Most transportation agencies use a variety of traffic control devices to inform road users of a 

change in horizontal alignment. These devices include curve warning signs, delineation devices, 

and pavement markings. The focus of this part of the chapter is on curve warning signs; however, 

conditions where other traffic control devices may be helpful are also identified.

2.3.1 Horizontal Alignment Warning Signs

The MUTCD 2009 edition (3) identifies a variety of warning signs that can be used where the 

horizontal alignment changes in an unexpected or restrictive manner. These signs are shown in 

Figure 3. The recommended signs include:

•	 Turn (W1-1),

•	Curve (W1-2),

•	Combination Horizontal Alignment/Advisory Speed Signs (W1-1a, W1-2a),

•	Reverse Turn (W1-3),

•	Reverse Curve (W1-4),

•	Winding Road (W1-5),

•	One-Direction Large Arrow Sign (W1-6),

•	Chevron Alignment Sign (W1-8),

•	Combination Horizontal Alignment/Intersection Signs (W1-10 Series, W1-10, W1-10a, W1-10b, W1-

10c, W1-10d, and W1-10e)

•	Hairpin Curve (W1-11),

•	270-degree Loop (W1-15),

•	 Truck Rollover (W1-13),

•	Advisory Speed Plaque (W13-1P),

•	Advisory Exit and Ramp Speed Signs (W13-2 and W13-3), and

•	Combination Horizontal Alignment/Advisory Exit and Ramp Speed Signs (W13-6 and W13-7).
5

Figure 3 – Horizontal Alignment Signs and Plaques (MUTCD 2009 Edition) 

Compared to the MUTCD 2003 edition (8), the MUTCD 2009 edition (3) has three changes of the 

sign designation: 1) Combination Horizontal Alignment/Intersection Sign (W1-10) is expanded to 

include an additional series (W1-10a through W1-10e). This change offers more symbol designs to 

approximate the configuration of the intersecting roadway. 2) The Advisory Curve Speed Sign (W13-5 

in MUTCD 2003 edition) is no longer used as a standard sign. 3) Combination Horizontal Alignment/

Advisory Exit and Ramp Speed Signs (W13-6 and W13-7) are added into the horizontal alignment sign 

category.

The previous versions of the MUTCD criterion regarding the use of horizontal alignment warning 

signs can best be described as flexible. It encouraged engineers to base their signing decisions on 

engineering studies and judgment. However, this flexibility had the disadvantage of occasionally 

promoting inconsistencies in the application of traffic control devices. Inconsistent device 

application makes it difficult for drivers to develop reasonable expectancies and, consequently, 

promotes a disrespect of the device and mistrust of its message. The Advisory Speed plaque is 

perhaps one of best examples of the consequences of inconsistent sign usage. Research has found 

it to be among the more disrespected traffic control devices (9).

W1-1 W1-1a W1-2 W1-2a W1-3 W1-4

W1-15 W13-1P W13-2 W13-3

W1-5 W1-6 W1-8 W1-10 W1-10a W1-10b

W1-10c W1-10d W1-10e W1-11 W1-13

Note: Turn arrows and reverse turn arrows may be substituted for the curve arrows and reverse curve arrows on the

 W1-10 series signs where appropriate.

W13-6 W13-7
6

2.3.2 Effectiveness of Horizontal Alignment Warning Signs with Advisory Speeds

Research indicates that the inconsistent use of curve warning signs, especially those with an Advisory 

Speed plaque, may have lessened the average motorists’ respect for the message the signs convey. 

On familiar highways, drivers come to learn that they can comfortably exceed the advisory speed 

for most curves. The concern is that these drivers may occasionally travel on roadways that are 

less familiar to them and where the advisory speed is posted at the maximum safe speed. These 

unfamiliar drivers may find themselves traveling too fast for conditions and experience a crash.

Only one report was found in the literature that documented the effect of horizontal curve signing 

with advisory speeds on safety. This 1968 report was a before-after study by Hammer (10) of the 

installation of warning signs in advance of several curves. He found that the implementation 

of advance horizontal alignment signs reduced crashes by 18 percent. He also found that the 

combined use of advance signing with an Advisory Speed plaque reduced crashes by a total of 22 

percent.

Research by Ritchie (11) examined driver response to the Curve sign and the Advisory Speed plaque. 

He found that average curve speeds exceeded the advisory speed when the advisory speed was 

less than 45 mph. The amount by which the average speed exceeded the advisory speed increased 

with reduced advisory speeds. Thus, for an advisory speed of 40 mph, the average speed exceeded 

the advisory speed by only 2 mph (i.e., the average speed was 42 mph); however, for an advisory 

speed of 20 mph, the average speed exceeded the advisory speed by 10 mph.

The findings of this review are consistent with those noted in Section 2.2.2. Specifically, drivers do 

not appear to be responding to the Advisory Speed plaque by reducing their speed to the advisory 

speed. Hence, measuring the amount of speed reduction may be of limited value in assessing the 

effect this sign has on safety. However, these findings suggest that advance information about an 

upcoming curve, as provided by a curve warning sign, may heighten driver awareness of the curve, 

but it does not cause them to slow considerably. Perhaps this heightened awareness produced the 

safety benefit found by Hammer (10).

2.4 Curve Advisory Speed Software

This part of the chapter provides an overview of the Curve Advisory Speed (CAS) software. 

This spreadsheet was developed to automate the procedures and guidelines described in 

this handbook. The background for the development of the equations in this spreadsheet is 

documented in an earlier research report by Bonneson et al. (5). The current Curve Advisory Speed 

(CAS) software accommodates several methods for establishing advisory speeds and follows the 

curve signing criterion according to the MUTCD 2009 edition (3).

The “Analysis” tab worksheet contains the curve advisory speed calculations. This worksheet is shown 

in Figure 4. Six (6) columns are provided in the worksheet. One column is used for each curve being 

evaluated.
7

Figure 4 – Curve Advisory Speed (CAS) Software Analysis Worksheet

General Information

District: Curve Data Source: Date:

Highway: Analyst:

Input Data

1 2 3 4 5 6

Curve deflection, left or right Right Left Left Left Left Left

Compass heading 1, degrees 251.7 124 254 239 330 189

BBI reading of superelevation, degrees 4.8 5.1 7.4 1.0 1.0 8.3

Right Left Left Right Right Left

0 0 0 0 0 0

Curve length, ft 216 237 118 100 100 110

Compass heading 2, degrees 261 94 244 207 300 158

Survey method (partial or full) Partial Partial Partial Partial Partial Partial

Regulatory speed limit, mph 60 60 60 60 55 60

Estimate of 85th% tangent speed, mph 66 64 66 60 55 60

Alternate Input Data (if data are entered here, they will be used instead of estimates from the data above)

85th% tangent speed, mph 58 66 56

Total curve deflection angle, degrees 90 90 90 96 90 93

Curve deflection angle, degrees 8.3 8.3 8.3 32.0 30.0 31.0

Superelevation rate, percent 5.5 5.5 5.5 -1.6 -1.6 13.0

Curve radius, ft 924 924 924 102 90 203

Max of Speed limit & 85th% speed 60 60 66 60 55 60

Two adjacent reversed curves separated by <= 600' (Yes or No) No No No No No No

Three or more adjacent reversed curves separated by <= 600' (Yes or No) No No No No No No

Intersection occurs within or immediately adjacent to a curve (Yes or No) No No No No No No

Freeway/expressway ramp (Yes or No) No No No No No No

Advisory Speed 

Survey method (partial or full) Partial Partial Partial Partial Partial Partial

Total deflection angle, degrees 28 90 30 96 90 93

Curve deflection angle, degrees 9.3 30.0 10.0 32.0 30.0 31.0

Curve radius, ft 1331 453 676 179 191 203

Degree of curvature, degrees 4.3 12.7 8.5 32.0 30.0 28.2

Curve path radius, ft 1432 463 764 188 201 213

Superelevation rate, percent 7.4 8.0 11.6 -1.6 -1.6 12.9

Average tangent speed, mph 58 51 58 52 48 49

Unrounded advisory speed, mph 57 40 52 26 25 32

Rounded advisory speed, mph 55 40 50 25 25 30

Advisory Speed from other source 50 45 55 25 25 35

Advisory Speed 55 40 50 25 25 30

Traffic Control Device Guidance

Curve Identification Number

Deflection of ball for superelevation reading, left or right

Speed when recording the BBI reading of superelevation, mph

CURVE ADVISORY SPEED WORKSHEET

July 25, 2011

Data Description

Compass

Traffic Control Device Guidance

Curve Warning Signs (for AADT>1000, the following standards shall be used; for AADT<1000, the following standards may be used.)

Recommended advance placement distance of warning signs (ft.) 100 200 200 325 225 275

Turn (W1-1) Required Required Required

Combination Turn/Advisory Speed (W1-1a) Optional Optional Optional

Curve (W1-2) Recommended Required Required

Combination Curve/Advisory Speed (W1-2a) Optional Optional Optional

Reverse Turn (W1-3)

Reverse Curve (W1-4)

Winding Road (W1-5)

Combination Horizontal Alighment/Intersection

(W1-10 series with turn/reverse turn arrow)

Combination Horizontal Alighment/Intersection

(W1-10 series with curve/reverse curve arrow)

Hairpin Curve (W1-11)

270-degree Loop (W1-15)

Advisory Speed Plaque (W13-1P) Recommended Required Required Required Required Required

Chevrons(W1-8) and/or One Direction Large Arrow (W1-6) Optional Required Required Required Required Required

Spacing of Chevron Alignment Signs (ft.) 160 120 160 40 40 80

Exit Speed (W13-2) and Ramp Speed (W13-3) on exit ramp

Combination Horizontal Alighment/Exit and Ramp Speed (W13-6, W13-7)

Lateral shift in lane, ft 3 Side Friction Demand Increase Thresholds, g

Ratio of average to 85th% speed 0.90 A 0.00 D 0.13

Ratio of truck speed to car speed 0.97 B 0.03 E 0.16

C 0.08

Speed conversion factor (mph to fps): 1.47

BBI reading adjusted for ball deflection: -4.75 -5.1 -7.4 1 1 -8.25

Unrounded advisory speed, mph 57 40 52 26 25 32

Rounded advisory speed, mph 55 40 50 25 25 30

Equivalent ball-bank reading, deg: 5 10 8 16 14 12

Average side friction demand, g 0.08 0.15 0.12 0.26 0.22 0.19

Side friction demand increase (85th %), g 0.00 0.09 0.06 0.20 0.16 0.14

CALIBRATION PARAMETERS

INTERMEDIATE CALCULATIONS

Compass
8

The spreadsheet can be used with six types of input data. One method is based on data obtained 

from a survey of the curve using the Compass Method. This method is described in Chapter 3. The 

drop-down list located in cell F5 is used to specify this method by selecting “Compass,” as shown in 

Figure 4. The data from the Compass Method are entered in the cells that have a light blue shaded 

background in the rows 9 through 21 and are designated “input data” cells. If the 85th percentile 

tangent speed is not known, then this cell should be left blank, and row 25 and the estimate in the 

row 22 (or the speed limit in the row 21, whichever is larger) will be used as the 85th percentile speed. 

The cell in the row 25 (orange shaded) is optional to input the 85th percentile speed of free-flowing 

passenger cars on the tangent prior to the curve. The information in the rows 31 to 34 (light blue 

shaded) are required to account for special roadway configuration. 

The second and third methods of data input are based on describing the curve deflection angle, 

superelevation rate, and radius. These data can be obtained from the GPS Method or the Design 

Method, as described in Chapter 3. The use of either of these methods is specified with the dropdown list located in cell F5 by selecting “GPS” or “Design.” The data from the GPS Method or the 

Design Method are entered in the light blue shaded cells in the row 21 and 26 through 29. If the 85th 

percentile tangent speed is not known, then this cell should be left blank, and the estimate in the 

row 22 (or the speed limit in the row 21, whichever is larger) will be used as the 85th percentile speed. 

The cell in the row 25 (orange shaded) is optional to input the 85th percentile speed of free-flowing 

passenger cars on the tangent prior to the curve. The information in the rows 31 to 34 (light blue 

shaded) are required to account for special roadway configuration.

The fourth, fifth and sixth methods of data input are based on other methods, such as the Direct 

Method, the Ball-Bank Indicator Method, and the Accelerometer Method, as described in Chapter 

3. The use of any of these methods is specified with the drop-down list located in cell F5 by 

selecting “Direct,” “Ball-Bank Indicator,” or “Accelerometer.” The advisory speed established by the 

other method is entered directly in the light blue shaded cells in the row 48. The cell in the row 21 

(regulatory speed limit) is required to apply MUTCD 2009 edition (3) signing criteria. The information 

in the rows 31 to 34 (light blue shaded) are required to account for special roadway configuration. It 

is optional to input the 85th percentile speed of free-flowing passenger cars on the tangent prior to 

the curve in the cell in the 25 (orange shaded). If the 85th percentile speed is not known, then this 

cell should be left blank, and the estimate in the row 22 (or the speed limit in the row 21, whichever is 

larger) will be used as the 85th percentile speed. The information, total curve deflection angle, in the 

cell in the row 26 (light blue shaded) is used for curve warning sign determination process.

The cells in the rows 36 to 46, which do not have background shading, contain equations for the 

first three input methods. The basis for each equation is documented in Bonneson et al. (5). These 

equations document the analysis of advisory speed for each of the six curves. The purple shaded 

cell in the row 49 is the advisory speed established by the applied method.

The purple shaded cells in the rows 54 to 73 of the spreadsheet document the traffic control device 

guidance. The criterion described in Chapter 4 of this manual and the MUTCD 2009 edition (3) are 

used to calculate the information that is summarized in this section of the spreadsheet. If cells are 

blank in this section of the spreadsheet then the traffic control device for that column is not required 

for that curve.
9

The cells in the rows 94 to 107 of the spreadsheet contain many parameters that control the 

computations. The default model used for calculating the advisory speed is based on the average 

truck speed (5). The corresponding formulation is as follows:

Vc,a = ( 15.0Rp (0.112 – 0.00066Vt,a + 0.000091V2

t,a – 0.0108Itk + е/100) )

0.5

≤ Vta 1 + 0.00136Rp

where,

Rp = travel path radius, ft; 

Vc,85 = 85th percentile curve speed, mph;

Vt,85 = 85th percentile tangent speed, mph;

Vc,a = average curve speed, mph;

Vt,a = average tangent speed, mph;

е = superelevation rate, percent; and 

Itk = indicator variable for trucks (= 1.0 if modelis used to predict truck speed; 0.0 

otherwise)

Note that the default model used in the Curve Advisory Speed (CAS) software assumes to use 

the estimated average truck speed to establish the advisory speed for a particular curve. This 

is a conservative approach proposed by TTI to ensure safety. However, there is no consensus 

or agreement among various transportation practitioners on whether to use passenger car vs. 

truck and average speeds vs. 85th percentile speeds as the criteria to establish advisory speeds. 

Therefore, it is suggested, before modifying these parameters in the spreadsheet, to carefully read 

through the report by Bonneson et al. (5) and think about if the default assumption matches your 

engineering experience and judgment.

If other criteria are chosen to establish the advisory speed, the parameters can be modified. For 

example, if the 85th percentile speed is preferred to be used as the advisory speed, the formulation 

shall be changed to the following equation.

If other criteria are chosen to establish the advisory speed, the parameters can be modified. For 

example, if the 85th percentile speed is preferred to be used as the advisory speed, the formulation 

shall be changed to the following equation.

Vc,85 = ( 15.0Rp (0.196 – 0.00106Vt,85 + 0.000073V2

t,85 – 0.0150Itk + е/100) )

0.5

≤ Vt85 1 + 0.00109Rp

The formulation is changed through the modification in the cell in the row 103.

If the passenger car speed is preferred to be used to establish the advisory speed, due to, low 

volume of trucks or low truck crash rate, the value in the cell in the row 97 shall be changed to 1.0 

and Itk in the formulation shall be set to 0.0.
10

CHAPTER 3  METHDS FOR ESTABLISH

3.1 Overview

Six (6) methods for establishing advisory speed are described in this chapter. The advantages and 

disadvantages of each method will also be discussed. The appropriate selection and application of 

the methods shall be considered to provide a uniform and consistent advisory speed among curves.

These methods are the most widely used or newly proposed and promising methods related to curve 

advisory speed determination that are accepted by transportation professionals and researchers.

The methods are all based on the premise that the selection of an advisory speed and the need 

for various curve warning signs should be based on the “critical” portion of the curve. The critical 

portion of the curve is defined as the section that has a radius and superelevation rate that combine 

to yield the largest side friction demand. Through this definition, the procedure is applicable to 

curves that have a constant radius, compound curvature, or spiral transitions.

Any of the following six (6) methods can be used to determine the curve advisory speed if used 

appropriately:

•	Direct Method,

•	 TTI Curve Speed Model - Compass Method,

•	 TTI Curve Speed Model - Global Positioning System (GPS) Method,

•	 TTI Curve Speed Model - Design Method,

•	Ball-Bank Indicator Method, and

•	Accelerometer Method.

The Direct Method is based on field measurements of traffic speeds in the curve. The Compass 

Method is based on a single-pass survey technique using a digital compass, distance measuring 

instrument and ball-bank indicator to estimate the curve radius and deflection angle. The GPS 

Method is also based on a single pass survey using a GPS receiver and software to compute curve 

radius and deflection angle. The Design Method is useful when the radius and deflection angle 

are available from as built plans. The Ball-Bank Indicator Method is based on a set of field driving 

tests to record a ball-bank indicator reading using a ball-bank indicator and a speedometer. The 

Accelerometer Method is based on a set of field driving tests to measure average maximum lateral 

gravitational force using an electronic accelerometer device and a GPS receiver.

The six (6) methods are categorized into two general groups. The Direct Method, the Compass 

Method, the GPS Method, and the Design Method determine advisory speeds based on measured 

operating speeds or estimated operating speeds given curve geometry. The Ball-Bank Indicator 

Method and the Accelerometer Method determine advisory speeds based on lateral acceleration.

Regardless of which method is being used, the procedure for implementing each method consists 

of three steps. During the first step, measurements are taken in the field. During the second step, the 

measurements are used to compute the advisory speed. During the last step, the recommended 

advisory speed is confirmed through a field trial run. The first two steps are described for each 

method in the following sections. The last step is common to each method and will be described in 

the last section of the chapter.

As mentioned, these six (6) methods are the most widely used or newly proposed and promising 

methods. There may be other methods which have been used to determine curve advisory speeds, 

for example, Driver Comfort Speed Method and AASHTO’s Geometric Design Method.
11

The Driver Comfort Speed Method is the oldest empirical method used for determining advisory 

speeds. It was defined in the 1930s as that “which causes an occupant of the vehicle to feel an 

outward pitch” and later refined to be “that speed at which the driver’s judgment recognized 

incipient instability.” The method is very subjective and provides inconsistent results.

The AASHTO’s Geometric Design Method is based on the following equation (12) derived from the 

laws of mechanics and used during the traditional highway design process. The actual value of the 

side friction factor is different for different ranges of advisory speed and there was variation for speed 

and side friction relationship based on the field results (12).

 V2 = 15(0.01е + f)R

where,

V = advisory speed, mph;

е = superelevation, percent;

f = side friction factor; and

R = radius curvature, ft.

This report will not further discuss the Driver Comfort Speed Method, the AASHTO’s Geometric Design 

Method and other non-widely used methods hereafter.

3.2 Direct Method

The Direct Method is based on the field measurement of vehicle speeds on the subject curve. 

There has been widespread discussion about how to choose the advisory speed given direct 

measurements of curve speed distribution and vehicle classification. The disagreement is mainly on 

which speed statistic and which type of vehicle should be used.

•	Using 85th percentile, average, or median speeds?

•	Based on speeds of passenger cars, trucks, or all vehicles?

The MUTCD 2003 edition (8) recommends the advisory speed be the 85th-percentile speed of freeflowing traffic. It also stated that the 85th-percentile speed on curves approximates a 16-degree 

ball-bank indicator reading and is the speed at which most drivers’ judgment recognizes incipient 

instability along a ramp or curve.

However, the MUTCD 2009 edition (3) no longer provides explicit support to use 85th-percentile freeflowing speed as the advisory speed. The manual mentions that research has shown that drivers 

often exceed existing posted advisory curve speeds by 7 to 10 mph. The MUTCD 2009 edition (3) 

references posted advisory speeds determined by ball-bank values of 16, 14, and 12 degrees to 

address such driver behavior. 

Bonneson et al. (5) recommended that the advisory speed should be based on the average speed 

selected by truck drivers. This speed is 2 or 3 mph below that of passenger car drivers and thereby, 

represents about the 40th percentile car driver. It is rationalized that driver speed choice on sharp 

horizontal curves is largely influenced by safety concerns. Thus, the advisory speed should be 

conservative such that it informs drivers of the speed that is considered appropriate for the unfamiliar 

driver. Given that the speed distribution is approximately normal, the speed most commonly chosen 

by drivers is the average speed.

Using the Direct Method will allow the engineer to set the advisory speed at the speed at which 

drivers are driving the curve and for the type of vehicle that the curve requires. 
12

3.2.1 Conducting Field Measurements

The speed of free-flowing passenger cars should be measured at the middle of the curve in one 

direction of travel. A free-flowing vehicle will be at least three (3) seconds behind the previous 

vehicle. If a radar speed meter is used, then data collection can be discontinued after the speed of 

125 cars has been measured or two hours have elapsed, whichever occurs first. If a traffic counter/

classifier is used, then data collection can be discontinued after the speed of 125 cars has been 

measured or four hours have elapsed, whichever occurs first.

If the roadway is divided or if conditions suggest the need to consider direction of travel, the 

measurements should be repeated for both directions of travel. When two (2) or more curves are 

present, each curve should be surveyed separately in this step; however, if they are separated by a 

tangent section of 600 feet or less, one sign should be selected to apply to all curves. 

The arithmetic average and the 85th percentile of the measured speeds should be computed for 

each direction of travel at each curve studied to determine the unrounded advisory speed.

3.2.2 Determining the Advisory Speed

Depending on the criteria chosen, the unrounded advisory speed is obtained by either calculating 

the 85th percentile passenger car speed or multiplying the average passenger car speed by 0.97 to 

obtain an estimate of the average truck speed. (Bonneson et al. (5)). 

Furthermore, Bonneson et al. (5) proposed a rounding technique to determine the advisory speed. 

First add 1.0 mph to the unrounded advisory speed and then round the sum down to the nearest 5 

mph increment to obtain the advisory speed. This technique yields a conservative estimate of the 

advisory speed by effectively rounding curve speeds that end in 4 or 9 up to the next higher 5 mph 

increment, while rounding all other speeds down. For example, applying this rounding technique to 

a curve speed of 54, 55, 56, 57, or 58 mph yields an advisory speed of 55 mph.

Each curve should be evaluated separately in this step; however, when two (2) or more curves are 

separated by a tangent of 600 feet or less, one (1) sign should apply for all curves. In this case, the 

Advisory Speed plaque should show the value for the curve having the lowest advisory speed in the 

series.

For undivided roadways, if an advisory speed is determined to be needed for one curve travel 

direction but not for the opposite curve travel direction, then only one direction of the curve should 

be posted with the advisory speed.

3.3 Compass Method

The Compass Method is based on a single-pass survey technique using a digital compass, distance 

measuring instrument and ball-bank indicator to estimate the curve geometry. Bonneson et al. (5) 

proposed a speed-prediction model to compute the average truck speed given the curve geometry. 

This speed then becomes the basis for establishing the advisory speed.

The procedure for implementing the Compass Method is based on compass heading and curve 

length measurements taken at the critical portion of the curve or sharpest point of the curve. When 

spiral transitions or compound curves are present, this critical portion of the curve is typically found 

in the middle third of the curve, as shown in Figure 5. If the curve is truly circular for its entire length, 

then measurements made in the middle third will yield the same radius estimate as those made in 

other portions of the curve.
13

Figure 5 – Location of Critical Portion of Curve

The deflection angle associated with the critical portion is referred to as the “partial deflection 

angle.” The curve length associated with the critical portion is referred to as the “partial curve 

length.” 

To ensure reasonable accuracy in the model estimates using this method, the total curve length 

should be 200 ft or more and the partial curve length should be 70 ft or more. Also, the curve 

deflection angle should be 12 degrees or more and the partial curve deflection angle should be 

4 degrees or more. A curve with a deflection angle of less than 12 degrees will rarely justify curve 

warning signs.

The Compass Method allows the engineer to only drive the curve one time; however, this method 

should only be used on very low volume roads because this method requires stopping the vehicle in 

the travel lane.

3.3.1 Conducting Field Measurements

In the first step of the procedure, the technician travels through the subject curve and makes a series 

of measurements. These measurements include:

•	curve deflection in direction of travel (i.e., left or right);

•	heading at the “1/3 point” (i.e., a point that is located along the curve at a distance equal to 1/3 

of curve length and measured from the beginning of the curve);

•	ball-bank reading of curve superelevation rate at the “1/3 point”;

•	length of curve between the “1/3” and “2/3” points;

•	heading at the “2/3 point“; and

•	85th percentile speed (can be estimated using the regulatory speed limit).

These measurements may require two persons in the test vehicle, a driver and a recorder. However, 

with some practice or through the use of a voice recorder, it is possible that the driver can also serve 

as the recorder such that a second person is not needed. The next two subsections describe the 

procedure for making the aforementioned field measurements.

1/3 Curve Length

1/3 Curve Length

1/3 

Curve 

Length

Partial 

Deection Angle

Compass Heading 1

Compass Heading 2

0

0

270

270

180

180

90

90

Partial Deflection Angle = Compass Heading 2 - Compass Heading 1

= 160 - 100

= 60 degrees
14

If the road is divided or if conditions suggest the need for separate consideration of each travel 

direction, the measurements should be repeated for the opposing direction of travel. If multiple 

curves are present, each curve should be evaluated separately in this step; however, when two (2) or 

more curves are separated by a tangent of 600 feet or less, one (1) sign should apply for all curves. 

3.3.1.1 Equipment Setup

The test vehicle will need to be equipped with the following three devices:

•	digital compass,

•	distance-measuring instrument (DMI), and

•	ball-bank indicator (BBI).

The digital compass heading calculation should be based on global positioning system (GPS) 

technology with a position-accuracy of 10 feet or less 95 percent of the time and a position update 

interval of one (1) second or less. It must also have a precision of 1 degree (i.e., provide readings to 

the nearest whole degree).

The compass should be installed in the vehicle in a location that is easily accessed and in the 

recorder’s field of view. The type of mounting apparatus needed may vary; however, the compass 

should be firmly mounted so that it cannot move while the test vehicle is in motion. 

The DMI is used to measure the length of the curve. It should have a precision of 1 ft (i.e., provide 

readings to the nearest whole foot). The DMI can also be used to: (1) locate a specific curve 

(in terms of travel distance from a known reference point), and (2) verify the accuracy of the test 

vehicle’s speedometer. The DMI can be mounted in the vehicle, but should be removable such that 

it can be hand-held during the test run.

The ball-bank indicator must have a precision of at least one (1) degree (i.e., provide readings to the 

nearest whole degree). Indicators with less precision (e.g., 5 degree increments) cannot be used 

with this method. The indicator should be installed along the center of the vehicle in a location that 

is easily accessed and in the recorder’s field of view. The center of the dash is the recommended 

position because it allows the driver to observe both the road and the indicator while traversing the 

curve. The type of mounting apparatus needed may vary; however, the ball-bank indicator should 

be firmly mounted so that it cannot move while the test vehicle is in motion. 

To ensure proper operation of the devices, it is important that the following steps are taken before 

conducting the test runs:

•	Inflate all tires to a pressure that is consistent with the vehicle manufacturer’s specification.

•	Calibrate the test vehicle’s DMI.

•	Calibrate the ball bank indicator.

The instruction manual should be consulted for specific details about the calibration process for the 

DMI and the ball-bank indicator.
15

3.3.1.2 Measurement Procedure

The following task sequence describes the field measurement procedure as it would be used 

to evaluate one direction of travel through the subject curve. Measurement error and possible 

differences in superelevation rate between the two directions of travel typically justify repeating this 

procedure for the opposing direction. Only one test run should be required in each direction.

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

b Stay in the same lane for all measurements. Record the curve deflection (i.e., left or right) 

relative to the direction of travel. This designation indicates which direction the vehicle turns 

as it tracks the curve. A turn to the driver’s right is designated as a right-hand deflection.

c Advance the vehicle to the “1/3 point,” as shown in Figure 5. This point is about one third of 

the way along the curve when measured from the beginning of the curve in the direction 

of travel. It does not need to be precisely located, within a few yards will be acceptable. 

The technician’s best estimate of this point’s location is sufficient. This point is referred to 

hereafter as the point of partial curvature (PPC).

Stop the vehicle and complete the following four tasks 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. 

d Advance the vehicle to the “2/3 point,” as shown in Figure 5. This point is about two thirds of 

the way along the curve. This point is referred to hereafter as the point of partial tangency 

(PPT).

Stop the vehicle and complete the following two tasks while at the PPT:

•	Record the vehicle heading (in degrees). 

•	Press the Display Hold button on the DMI. 

The value shown on the DMI is the partial curve length. With some practice, it may be possible to 

complete the two tasks listed above while the vehicle is moving slowly (i.e., 15 mph or less). However, 

if the measurements are taken while the vehicle is moving, is imperative that they represent the 

heading and length for the same exact point on the roadway. Error will be introduced if the heading 

is noted at one location and then the length is measured at another location.

Safety should be considered before stopping the vehicle in the travel lane on a curve or if moving 

slowly through the curve. Additional maintenance of traffic may be required to perform this step 

safely. 

3.3.2 Determining the Advisory Speed

During this step, the field measurements are used to determine the appropriate advisory speed for 

a specified travel direction through the subject curve. The calculations are then repeated to obtain 

the advisory speed for a different curve or for the opposing direction of travel through the same 

curve.
16

Initially, the data collected are entered in the Analysis worksheet of the Curve Advisory Speed (CAS) 

software. The entry of data for example curve “47R” is shown in Figure 6. The measurements taken 

at this curve are shown in the column headed by the curve’s identification number. The curve 

deflected to the right, relative to the direction of travel during curve measurement. 

The compass heading at the first “1/3 point” was 251 degrees. A ball-bank indicator reading of 4.0 

degrees was noted at this point. The ball deflected to the right of the “0.0 degrees” tick mark. This 

direction indicates that a positive (i.e., beneficial) superelevation is provided along the curve. The 

vehicle was stopped for these two measurements, so the vehicle speed was input as “0 mph” when 

the ball-bank indicator was read.

A curve length of 201 ft was measured at the “2/3 point.” The compass heading at this point was 281 

degrees. Finally, the regulatory speed limit of 60 mph is entered into the spreadsheet.

Figure 6 – Curve Advisory Speed (CAS) Software Input Data

The speed limit is used to estimate the 85th percentile speed on the highway tangents in the vicinity 

of the curve. If the 85th percentile tangent speed is known, then it can be directly entered in the first 

row of the Alternate Input Data section of the worksheet (i.e., the fifth row from the bottom, in Figure 

6). If a value is entered in the Alternate Input Data section, then it will be used instead of the value 

estimated using the field measurements entered in the Input Data section. This priority is extended to 

the direct entry of 85th percentile tangent speed, curve deflection angle, superelevation rate, curve 

radius, or any combination of these data.
17

The advisory speed is computed using the estimated (or directly input) curve radius, deflection 

angle, and superelevation rate. A curve-speed prediction model is used for this purpose. The 

estimate obtained from this model represents the “unrounded advisory speed” and is shown in the 

second row from the bottom of Figure 7. The advisory speed is computed by first adding 1.0 mph 

to the unrounded speed and then rounding the sum down to the nearest 5 mph increment. The 

rationale for this rounding technique is discussed in the Direct Method Section 3.2.2. The rounded 

advisory speed is shown in the last row of Figure 7.

It should be noted that the computed advisory speed is based on the estimated radius of the 

vehicle’s travel path, as opposed to that of the curve. When traveling through a curve, drivers shift 

their vehicle laterally in the traffic lane, such that the curve is flattened slightly. This behavior allows 

them to limit the speed reduction required by the curve. The difference between the radius of the 

curve and the travel path radius is shown in Figure 8. The estimated path radius for the subject curve 

is listed in the Advisory Speed section of the analysis worksheet, as shown in Figure 8. It will always 

equal or exceed that of the curve radius. The path radius will be notably larger than the curve radius 

on curves with a smaller deflection angle.

Figure 7 – Curve Advisory Speed (CAS) Software Advisory Speed Calculation

Figure 8 – Effect of Lateral Shift on Travel Path Radius

Advisory Speed 

Survey method (partial or full) Partial

Total deflection angle, degrees 90 0 0 0 0 0

Curve deflection angle, degrees 30.0 0.0 0.0 0.0 0.0 0.0

Curve radius, ft 384

Degree of curvature, degrees 14.9

Curve path radius, ft 394

Superelevation rate, percent 6.2

Average tangent speed, mph 55

Unrounded advisory speed, mph 39

Rounded advisory speed, mph 40

Deflection Angle, Ic

Path Radius, R

Curve Radius, R

P
18

In this step, when two or more curves are present, each curve should be evaluated separately; 

however, if they are separated by a tangent section of 600 ft or less, one sign should apply for all 

curves. The Advisory Speed plaque should show the value for the curve that has the lowest advisory 

speed in the series.

For undivided roadways, if an advisory speed is determined to be needed for one curve travel 

direction but not for the opposite curve travel direction, then only one direction of the curve should 

be posted with the advisory speed.

3.4 GPS Method

The GPS Method is based on the field measurement of curve geometry. The geometric data are 

then used with a speed prediction model to compute the average speed of trucks. This speed then 

becomes the basis for establishing the advisory speed.

To ensure reasonable accuracy in the model estimates using this method, the total curve deflection 

angle should be 6 degrees or more. Curves with a smaller deflection angle rarely justify curve 

warning signs or an advisory speed plaque.

The GPS Method allows the engineer to only drive the curve one time; however, this method requires 

driving the curve at a low speed.

3.4.1 Conducting Field Measurements 

3.4.1.1 Equipment

The equipment used includes the following:

•	GPS receiver, 

•	electronic ball bank indicator (optional), and 

•	laptop computer. 

The GPS receiver is used to estimate curve radius and deflection angle. The electronic ball bank 

indicator, which is used to estimate superelevation rate, is optional. If an electronic ball-bank 

indicator is not used, then superelevation rate will need to be estimated using other means. The 

computer is used to run the Texas Roadway Analysis and Measurement Software (TRAMS) program. 

This program is designed to monitor the GPS receiver and the electronic ball bank indicator while 

the test vehicle is driven along the curve. After the curve is traversed, TRAMS calculates curve radius 

and superelevation rate from the data streams. Advisory speed and traffic control device selection 

guidelines can be determined using the radius and superelevation rate estimates with the Curve 

Advisory Speed (CAS) software.

3.4.1.2 Installation

The following activities must be completed the first time TRAMS is installed on the computer. More 

details are provided in the TRAMS Installation Manual and are available from TTI.

•	Install the driver for the GPS receiver.

•	If the electronic ball bank indicator is used, an adapter may be needed to convert its RS 232 

connection into a USB connection. Install the driver for this adapter.

•	Install TRAMS.
19

3.4.1.3 Equipment Setup

The following activities must be completed prior to using the equipment to establish the advisory 

speed for one or more curves.

•	Mount the GPS receiver and electronic ball bank indicator (if used) on the dashboard in a fixed 

position. These devices should not be able to move during the test runs. Figure 9 shows the 

devices mounted on the dashboard and secured with Velcro tape.

a. Equipment Setup. b. Measurement Devices.

Figure 9 – GPS Method Equipment Setup in Test Vehicle

•	If an electronic ball bank indicator is used, activate its auto-leveling feature with the test vehicle 

parked on level pavement determined by survey or a level. Do this under the same vehicle 

loading and tire inflation conditions that will be present during the test runs.

•	With the laptop on, click on the TRAMS icon to launch TRAMS. TRAMS will initially connect with the 

two devices and then present the main panel, as shown in Figure 10.

•	Verify that TRAMS is receiving valid data from the GPS receiver. Information about the status of 

this device is located in the upper right corner of the main panel, as shown in Figure 10. A red 

circle indicates invalid (bad) data. A green circle indicates valid (good) data. 

•	If the electronic ball bank indicator (BBI) is used, verify that TRAMS is receiving valid data from it. 

A red square indicates invalid (bad) data. A green square indicates valid (good) data.

•	If valid data are not being received by one or both of the devices, check the following conditions:

» Are the devices turned on and properly connected to the laptop computer?

» Is the GPS receiver blocked from obtaining good satellite reception? Structures (bridges, 

garage roofs, buildings, etc.) or dense tree coverage may make it difficult to maintain GPS 

reception.

» Has TRAMS been configured with the proper port numbers for the devices? This can usually 

be accomplished by selecting the “Automatic” mode in the Configure Devices panel (from 

the main panel, select File, Configuration Settings, Configure Devices). If used, the electronic 

ball bank indicator (“Rieker Device”) must also be enabled in this panel (i.e., select Enabled 

in the Rieker box).

» If any settings are changed in the Configure Devices panel, the Save Configuration File option 

should be selected to save all settings to file (in which case they will be loaded and used 

each time TRAMS is launched).
20

Before beginning a test run, the curve number and highway name should be entered in their 

respective fields provided on the main panel (see Figure 10). 

If the roadway is divided or if conditions suggest the need for consideration of each direction of 

travel, the measurements should be repeated for the opposing direction of travel. When two (2) or 

more curves are separated by a tangent of 600 ft or less, one sign should apply to all curves even 

though each curve should be surveyed separately in this step.

3.4.1.4 Speed Limit

If the 85th percentile tangent speed is not known, the regulatory speed limit on the roadway where 

the curve is located should be noted. The speed limit can subsequently be used in Curve Advisory 

Speed (CAS) software to estimate the 85th percentile tangent speed.

3.4.1.5 Test Run Speed

The following rules of thumb should be considered when selecting the test run speed: 

•	 The test run speed should be at least 10 mph below the existing curve advisory speed as long as 

the resulting test run speed is at least 15 mph.

•	If superelevation rate is being measured, test runs should be conducted at 45 mph or less, slower 

speeds are desirable because they yield more accurate superelevation estimates.

In general, slower test run speeds improve measurement accuracy by minimizing tire slip and 

allowing the driver to track the curve more accurately.

Figure 10 – Texas Roadway Analysis and Measurement Software 

(TRAMS) Main Panel
21

3.4.1.6 Measurement Procedure

The following task sequence describes the field measurement procedure as it would be used 

to evaluate one direction of travel through the subject curve. Measurement error and possible 

differences in superelevation rate between the two directions of travel typically justify repeating this 

procedure for the opposing direction. Only one test run should be required per direction.

a When the test vehicle is one (1) or two (2) seconds travel time in advance of the beginning 

of the curve, press the space bar or click the large button on the TRAMS main panel. This 

action will start the data collection process. Precise location of the beginning of the curve 

is not required. A reasonable estimate of its location, based on the analyst’s judgment, will 

suffice.

b While driving through the curve, parallel the center of the lane as carefully as possible. This 

process will provide an accurate survey of the intended travel path. The analyst should 

avoid “cutting the corner” of sharp curves. The analyst should also avoid letting the vehicle 

drift to the outside of the lane while traveling along the curve.

c When the test vehicle is one (1) or two (2) seconds travel time beyond the end of the curve, 

press the space bar or click the large button to stop recording curve data. Precise location 

of the end of the curve is not required. A reasonable estimate of its location, based on the 

analyst’s judgment, will suffice.

3.4.1.7 Save the File

When asked whether a curve report file should be saved, “Yes” can be indicated by pressing Enter 

(or clicking on the Yes button). Alternatively, “No” can be indicated if it is believed that the curve was 

not accurately measured during the test run (e.g., the driver did not accurately track the curve, or the 

data recording was not started and stopped at the appropriate times). CAUTION: If the curve has 

the same number as a curve that was previously evaluated, the new file will overwrite the file from the 

previous curve.

3.4.1.8 Optional Check When Superelevation Rate is Measured

At the conclusion of the test run, the 95th percentile error range for superelevation rate is provided 

in the curve report file. It can be checked to confirm that the estimated value is reasonably precise 

based on field observations. If this range exceeds three percent, the test run should be repeated at 

a lower speed. If the aforementioned test run speed rules of thumb (section 3.4.1.5) were followed, 

this check should not be needed.

The curve report file can be accessed from the main panel by selecting File, Open Curve Report, 

and selecting the appropriate “log” file. The file will be named “Curve XX Rpt.Log,” where XX will be 

replaced by the curve number entered on the main panel before the start of the test run. Once the 

file is selected, selecting Open will open the file in a text editor.

3.4.2 Determining the Advisory Speed

Two options are available for determining the advisory speed. One option is based on a review of 

the survey data in the field. The second option is based on a review of the survey data in the office, 

following the survey of all curves of interest.
22

When multiple curves are present in succession, each curve should be evaluated separately; 

however, when two or more curves are separated by a tangent of 600 ft or less, one sign should 

apply for all curves. In this case, the Advisory Speed plaque should show the value for the curve that 

has the lowest advisory speed in the series.

For undivided roadways, if an advisory speed is determined to be needed for one curve travel 

direction but not for the opposite curve travel direction, then only one direction of the curve should 

be posted with the advisory speed.

Option 1: In Field Determination

This option is performed in the field. The 

data from the most recent test run are 

exported directly to CAS software. This 

action is accomplished from the main 

panel by selecting File, Export to CAS 

software. At this point, CAS software will 

load. The analyst will need to click on 

the “Import TRAMS Data” button in CAS 

software to import the test run data into 

CAS software. Figure 11 shows the Import TRAMS Data screen. The analyst will also need to enter the 

85th percentile speed for the curve. Alternatively, the analyst can enter the speed limit and let CAS 

software compute an estimate of the 85th percentile speed.

The imported data from TRAMS are always placed in the same CAS column (i.e., column F). 

Therefore, if the analyst wants to save any data in this column, then he or she should copy and paste 

the data to another column in CAS software (or another spreadsheet) and save the file. 

Option 2: In Office Determination

This option is performed back in the office. The curve report file for each curve is opened in text 

editor and printed by selecting File, Print. There is one report for each unique curve number entered 

in TRAMS. The data on the report can then be typed into CAS software and the appropriate advisory 

speed can be determined. Instructions for opening a curve report file were provided in Section 

3.4.1.8.

3.5 Design Method

The Design Method is based on the use of curve geometry data obtained from files or as built plans. 

This method is suitable for evaluating newly constructed or reconstructed curves because the 

needed data are available from the construction plans. However, data for the Design Method may 

not be readily available for most curves.

3.5.1 Obtaining Curve Geometry

The appropriate files must be consulted to obtain the radius, deflection angle, and superelevation 

rate for the curve. If the curve is circular, the “total curve deflection angle” is equivalent to the “curve 

deflection angle,” as used in CAS software. The total curve deflection angle equals the deflection 

angle of the two intersecting tangents. 

If spiral transition curves are included in the design, the radius and superelevation rate data for the 

central circular curve should be obtained. The total curve deflection angle is the same as defined 

in the previous paragraph. The curve deflection angle represents the deflection angle of the central 

circular curve, defined previously as the partial deflection angle. 

Figure 11 – Import TRAMS Data
23

If compound curvature is used in the design, the radius and superelevation rate data for the sharpest 

component curve should be obtained. The total curve deflection angle is the same as defined 

in the first paragraph. The curve deflection angle represents the deflection angle of the sharpest 

component curve.

If the road is divided or if conditions suggest the need for separate consideration of each curve travel 

direction, the aforementioned data for both directions of travel should be obtained. Although data 

for each curve should be obtained separately in this step, when two or more curves are separated by 

a tangent of 600 ft or less, one sign should apply for all curves. 

3.5.2 Determining the Advisory Speed

The data obtained are entered in CAS software in the section titled Alternate Input Data. Note: 

the drop-down list at the top of the spreadsheet should be set to “Known curve geometry”. If a 

reasonable estimate of the 85th percentile tangent speed is not available, the speed limit can be 

used in CAS software to estimate the 85th percentile tangent speed. 

Each curve should be evaluated separately in this step, but when two or more curves are separated 

by a tangent of 600 ft or less, one sign should apply for all curves. In this case, the Advisory Speed 

plaque should show the lowest advisory speed for the series of curves.

For undivided roadways, if an advisory speed is determined to be needed for one curve travel 

direction but not for the opposite curve travel direction, then only one direction of the curve should 

be posted with the advisory speed.

3.6 Ball-Bank Indicator Method

The Ball-Bank Indicator Method is based on a set of field driving tests to record ball-bank indicator 

reading using a ball-bank indicator and a speedometer. There are varied criteria for establishing the 

curve advisory speed based on ball-bank indicator readings.

The AASHTO’s Geometric Design of Highways and Streets (2004) states that curve speeds that do not 

cause “driver discomfort” corresponding to ball-bank readings of 14-degree for speeds of 20 mph 

or less, 12-degree for speeds of 25 to 30 mph, and 10-degree for speeds of 35 mph or more. It notes 

that these readings are consistent with side friction factors of 0.21, 0.18, and 0.15, respectively.

The MUTCD 2003 edition indicates that the advisory speed may be the speed corresponding to a 

16-degree ball-bank indicator reading. However, the MUTCD 2009 edition (3) modified the criteria as 

following:

•	16 degrees of ball-bank for speeds of 20 mph or less,

•	14 degrees of ball-bank for speeds of 25 to 30 mph, and

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

The manual mentions that research has shown that drivers often exceed existing posted advisory 

curve speeds by 7 to 10 mph. The MUTCD 2009 edition (3) references posted advisory speeds 

determined by ball-bank values of 16, 14, and 12 degrees to address such driver behavior.

Ball-Bank Indicator Method determines the advisory speeds in the field using a vehicle equipped 

with a ball-bank indicator and an accurate speedometer. The speedometer should be check using 

a calibrated radar gun or other method. The simplicity of construction and operation of this device 
24

has led to its widespread acceptance as 

a guide to determine advisory speeds for 

changes in horizontal alignment. Figure 

12 shows a typical ball-bank indicator.

The ball-bank indicator consists of a 

curved glass tube which is filled with 

a liquid. A weighted ball floats in the 

glass tube. The ball-bank indicator is 

mounted in a vehicle, and as the vehicle 

travels around a curve, the ball floats 

outward in the curved glass tube. The 

movement of the ball is measured in 

degrees of deflection, and this reading is indicative of the combined effect of superelevation, lateral 

(centripetal) acceleration, and vehicle body roll. The amount of body roll varies somewhat for 

different types of vehicles, and may affect the ball-bank reading by up to one degree, but generally 

is insignificant if a standard passenger car is used for the test. Therefore, when using this technique, 

it is best to use a typical passenger car rather than a pickup truck, van, or sports utility vehicle. Also, 

the ball-bank indicator test is normally a two-person operation, one person to drive and the other to 

record curve data and the ball-bank readings, especially if advisory speeds are being determined 

for a series of curves.

The Ball-Bank Indicator Method requires two people in the vehicle and multiple runs through the 

curve to get the correct advisory speed. In addition, reading the ball-bank indicator to determine 

the maximum degree of lean can be subjective.

3.6.1 Obtaining Field Measurements

3.6.1.1 Equipment Setup

To ensure proper results, it is critical that the following steps be taken before starting test runs with the 

ball-bank indicator: 

•	inflate all tires to uniform pressure as recommended by the vehicle manufacturer,

•	calibrate the test vehicle’s speedometer, and

•	 zero the ball-bank indicator .

The vehicle speedometer is calibrated to ensure proper and consistent test results. This can be 

done by checking the vehicle speed with a radar or laser speed meter, or by timing the vehicle 

over a measured distance (such as milepost spacing). Alternatively, a moving radar unit can be 

used to measure speed while conducting the ball-bank test runs rather than relying on the vehicle’s 

speedometer.

The ball-bank indicator must be mounted in the vehicle so that it displays a 0-degree reading when 

the vehicle is stopped on a level surface. The positioning of the ball-bank indicator is checked 

before starting any test. This can be done by stopping the car so that its wheels straddle the 

centerline of a two-lane highway on a tangent alignment. In this position, the vehicle is essentially 

level, and the ball-bank indicator gives a reading of 0-degree. It is essential that the driver and 

recorder be in the same position in the vehicle when the ball-bank indicator is set to a 0-degree 

reading as they will be when the test runs are made because a shift in the load in the vehicle can 

affect the ball-bank indicator reading.

Figure 12 – Ball-Bank Indicator
25

3.6.1.2 Measurement Procedure

Starting with a relatively low speed, the vehicle is driven through the curve at a constant speed 

following the curve alignment as closely as possible, and the reading on the ball-bank indicator 

is noted. On each test run, the driver must reach the test speed at a distance of at least 1⁄4 mile in 

advance of the beginning of the curve, and maintain the same speed throughout the length of the 

curve. The path of the car must be maintained as nearly as possible in the center of the innermost 

lane (the lane closest to the inside edge of the curve) in the direction of travel. If there is more than 

one lane in the direction of travel, and these lanes have differing superelevation rates, the lane with 

the lowest amount of superelevation should be used. Because it is often difficult to drive the exact 

radius of the curve and keep the vehicle at a constant speed, it is suggested that at least three test 

runs in each direction be made to more accurately determine the ball-bank reading for any given 

speed. On each test run, the recorder carefully observes the position of the ball throughout the 

length of the curve and records the deflection reading that occurs when the vehicle is as nearly as 

possible driving the exact radius of the curve.

If the reading on the ball-bank indicator for a test run does not exceed the criteria stated in the 

MUTCD 2009 edition (3) (see Section 3.6, above), then the speed of the vehicle is increased by 5 mph 

and the test is repeated. The vehicle speed is repeatedly increased in 5 mph increments until the 

ball-bank indicator reading exceeds the acceptable maximum.

3.6.2 Determining the Advisory Speed

The curve advisory speed is set at the highest test speed that does not result in a ball-bank indicator 

reading greater than an acceptable level.

For example, a series of test runs for a curve were started at 25 mph, with ball-bank indicator reading 

of about 6-degree. This is well below the suggested criteria of 14-degree for a speed of 25 mph. The 

speeds of the test runs were increased in five mile an hour increments until the speed of 35 mph gave 

readings of 10-degree to 12-degree. These are the highest readings attained without exceeding the 

suggested criteria of 12-degree for a speed of 35 mph or more, because the speed of 40 mph gave 

the readings of 13-degree to 15-degree. Therefore, this field measurement would result in posting an 

advisory speed of 35 mph for this curve.

3.7 Accelerometer Method

An accelerometer is an electronic device which can measure the lateral (centripetal) acceleration 

experienced by a vehicle as it travels around a curve. The Accelerometer Method can be used as 

an alternative to the Ball-Bank Indicator Method to establish the advisory speed (AASHTO’s 2004 

Green Book (12), and MUTCD 2009 edition (3)). 

The accelerometer is mounted on a level surface in a standard passenger vehicle such as on the 

windshield or along the floor centerline. An example of a commercially available unit is CurveRite 

1200 shown in Figure 13. The CurveRite unit is equipped with a high-grade accelerometer and GPS 

that measures the lateral gravitational force that the vehicle encounters and the speed in which the 

average maximum lateral gravitational force was encountered.

Figure 13 – CurveRite Front Panel
26

Similar to the ball-bank indicator study procedure, the vehicle is driven through a curve at a constant 

speed following the radius of the curve as closely as possible. The advisory speed of the curve 

is set at the highest speed that can be driven without exceeding a specified, comfortable lateral 

acceleration. The accelerometer does not require a second person to act as recorder because the 

data are stored for later recording, or the data can be transferred to a computer back at the office.

Under normal conditions, the suggested advisory speed for comfort in a passenger vehicle is when 

the average lateral gravitational force displayed on the device is 0.28 g (ft/sec2), Brudis & Associates, 

Inc. (13). A measurement of 0.26 g (ft/sec2) to 0.30 g (ft/sec2) is considered an acceptable range 

for establishing advisory speeds. The operator should not conduct a test run through any curve in 

excess of 0.40 g (ft/sec2).

The Accelerometer Method requires only one person in the vehicle and multiple runs through 

the curve to get the correct advisory speed. The accelerometer provides the maximum lateral 

gravitational force and the speed at which that force occurred for each run.

3.7.1 Obtaining Field Measurements

After a thorough site assessment, the operator may begin conducting tests at the curve site. The 

operator should note any unique characteristics of the curve such as intersections, adequate sight 

distances etc. In order to reflect the worst condition of the curve, test runs should be made on the 

inside of the curve or the travel lane with the shortest radius. It is important to maintain a smooth, 

uniform path throughout the curve; any sudden movement will cause an inaccurate measurement. 

It is important to remember to drive at a constant speed, stay on a steady path and maintain a 

constant speed based on the speedometer within the vehicle (note: Cruise control cannot be used 

during the test run to avoid fluctuation due to a change in vertical grade). The predetermined speed 

must be held constant, if constant speed is not maintained, repeat the run and record both results.

3.7.1.1 Equipment Setup

Select either a compact, mid-size, or sports utility vehicle in good working condition to prepare for the 

installation of the meter. The suspension of the vehicle should not have been modified. The vehicle 

must sit level upon its suspension and the tires should be evenly inflated. Before test runs, secure 

accelerometer, connect GPS, prepare power connection and calibrate the CurveRite according to 

the instructional manual.

3.7.1.2 Measurement Procedure

To obtain adequate measurements, the operator must drive a CurveRite equipped test vehicle 

through the same curve. After each pass the operator will increase the test vehicle speed by five 

mph increments. The CurveRite consistently records the average maximum lateral gravitational force 

(g-force) of each pass through the curve. The initial test vehicle speed selected should be 5 to 10 

mph below the existing posted advisory speed. The speed of the vehicle is then increased in 5 mph 
27

increments in order to obtain a lateral g-force measurement above the recommended value of

0.28 g. The vehicle operator should stop increasing curve speeds when the test measurements 

exceed 0.40 g. Test runs should be conducted by following the steps:

•	Start from a location ahead of the curve site to allow acceleration to determined test speed 

before beginning to turn through the curve (see Figure 14).

» Push the START button just before entering the curve at its tangent.

− The display will show “logging data” after pressing start.

» Push the STOP button after exiting the curve at its tangents.

− All data recording will then stop and the display will show “processing data”.

» The average maximum lateral g-force of the curve will then be displayed.

•	 The operator should proceed to a safe stopping location to record the results of the previous test 

run.

•	Record CurveRite reading.

•	After each run of the same curve (with the vehicle stopped), the operator must push the CLR 

button to reset the CurveRite meter.

•	Pushing the CLR button does not remove the accelerometer calibration or vehicle classifications 

from memory.

•	Repeat test run at 5 mph increments.

3.7.2 Determining the Advisory Speed

The curve advisory speed is set at the highest test speed that does not result in CurveRite reading 

greater than 0.28 g (ft/sec2).

For example, a series of test runs for a curve were started at 25 mph, then 30 mph, 35 mph, and 40 

mph, with CurveRite reading of 0.22 g, 0.26 g, 0.28 g, and 0.29 g, respectively. 35 mph is the highest 

testing speed without exceeding the suggested criteria of 0.28 g. Therefore, this field measurement 

would result in posting an advisory speed of 35 mph for this curve.

Stop

Tangent

Tangent

Curve

Test Site

Start

Figure 14 – CurveRite Measurement
28

3.8 Confirming Speed for Conditions

This procedure is to evaluate the appropriateness of the advisory speed determined. The field 

evaluation of curve conditions includes consideration of the following factors:

•	approach sight distance at the beginning of the curve,

•	visibility around the curve,

•	unexpected geometric features within the curve, and

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

The unexpected geometric features noted in the third bullet may include: 

•	presence of an intersection,

•	presence of a sharp crest vertical 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, and

•	broken-back curves.

A test run shall be conducted through the curve while traveling at the advisory speed determined in 

the previous description for each method. The engineer may choose to adjust the advisory speed or 

modify the warning sign layout based on consideration of the aforementioned factors. 

For undivided roadways, if an advisory speed is determined to be needed for one curve travel 

direction but not for the opposite curve travel direction, then only one direction of the curve should 

be posted with the advisory speed.

3.9 Workshops and Webinars

A series of four workshops and one webinar were held across the country on establishing advisory 

curve speeds. The workshops were held in Maine, Virginia, Illinois and California. Discussions and 

comments that came up during the workshops included the following:

•	Most attendees did not think that the existing advisory speeds are set at the correct speed.

•	Several attendees felt that the advisory speed should be based on truck speed or should have a 

dual posting.

•	Several attendees use the 85th percentile speed to determine the advisory speed.

•	Most attendees do not think that advisory speeds should be set conservatively low. However, 

some attendees did think they should be conservatively low due to liability.

•	Most, if not all, attendees would not use the Compass Method to determine the advisory speed. 

This is mostly due to the fact that this method requires that a vehicle must stop in the middle of 

the road to record data.

•	Some attendees would use the GPS or Accelerometer Method except they lack the equipment.

•	Most attendees use a ball-bank indicator or the electronic ball-bank indicator. Some use the 

Geometric Design Method or the 85th Percentile Speed Method.

•	Most attendees like the Curve Advisory Speed Worksheet which would allow for consistent 

advisory speeds.
29

•	Some attendees were concerned about how too many requirements translate into liability.

•	During the presentation it was pointed out that the reverse curve sign was curving to the left 

and should be curving to the right (Slide 50). However, the MUTCD 2009 edition (3) says it is the 

direction of the first curve on the sign and the first curve on the road. This was agreed to, and 

it was also pointed out that if it is confusing to traffic engineers, it is probably confusing to the 

general public.

3.10 Summary

In this chapter, the six (6) methods for establishing advisory curve speeds were introduced; and 

methodological procedures, including required equipment, software, installation and setup, and 

testing steps in the field were described. All methods can be used with the Curve Advisory Speed 

(CAS) software. The advisory speed is then determined based on the field measurements or known 

information and adjusted based on field confirmation and judgment.

The Direct Method is based on the field measurement of curve speeds under free-flow conditions. 

The devices used to record speeds are usually a radar speed meter and/or a traffic counter/

classifier, both of which are commonly available devices to most traffic agencies. However, the traffic 

agencies do not have unanimous agreement regarding how best to determine the advisory speed 

given the distribution of curve speed and vehicle classification. The disagreement involves whether 

to use 85th percentile, average, or median speeds, as well as whether to use speeds of passenger 

cars, trucks, or all vehicles. Setting advisory speeds at 85th percentile speeds has been an accepted 

and common practice in the past, while also supported by the MUTCD 2003 edition, but is no longer 

a preferred approach in the MUTCD 2009 edition (3). Bonneson et al. (5) recommended using 

average truck speeds to determine advisory speeds.

The Compass Method is based on a single-pass survey technique using a digital compass, 

distance measuring instrument and ball-bank indicator to estimate the curve radius and deflection 

angle. The GPS Method is also based on a single pass survey using a GPS receiver and software 

to compute curve radius and deflection angle. The Design Method is useful when the radius 

and deflection angle are available from as built plans. The three methods were based on the TTI 

proposed curve speed prediction model which correlates the curve speed with the curve geometry. 

An Excel spreadsheet program Curve Advisory Speed (CAS) software was developed by TTI and 

hereafter enhanced to provide a means to calculate advisory speeds given curve geometry. The 

default model in the program is setting advisory speeds at average truck speed. The parameters in 

the program can be customized to setting advisory speeds at 85th percentile speeds of trucks and/

or passenger cars.

The Ball-Bank Indicator Method is based on field driving tests to record ball-bank indicator readings 

using a ball-bank indicator and a speedometer. The simplicity of construction and operation of this 

device has led to its widespread acceptance as a guide to determine advisory speeds for changes 

in horizontal alignment; however, there are varied criteria for establishing the curve advisory speed 

based on ball-bank indicator readings. The AASHTO Green Book 2004 (12) recommends setting 

advisory speeds based on 14-degree for speeds of 20 mph or less, 12-degree for speeds of 25 to 30 

mph, and 10-degree for speeds of 35 mph or more. The MUTCD 2009 edition (3) recommends the 

criteria of 16-degree, 14-degree, and 12-degree for the same range of speeds as in the AASHTO’s. 

The manual mentions that research has shown that drivers often exceed existing posted advisory 

curve speeds by 7 to 10 mph. The MUTCD 2009 edition (3) references posted advisory speeds 

determined by ball-bank values of 16, 14, and 12 degrees to address such driver behavior.

The Accelerometer Method is based on field driving tests that measure average lateral gravitational 

force using an electronic accelerometer device and a speedometer. It is an alternative to the 

Ball-Bank Indicator Method that is used to establish the advisory speed. CurveRite is a currently 

commercially available unit that can collect the required measurements. Unlike the Ball-Bank 
30

Indicator Method which needs two (2) people to collect the data, CurveRite requires only one (1) 

person to run the tests. Under normal conditions, the suggested advisory speed to maintain comfort 

in a passenger vehicle is when the average lateral gravitational force displayed on the device is 0.28 

g (ft/sec2).

The following table (Table 1) summarizes the advantages and disadvantages for the six (6) methods 

discussed in this report.

Table 1 Comparison of the Six Methods for Establishing Curve Advisory Speeds

Method Advantages Disadvantages

Direct •	Device is commonly available;

•	Suggest advisory speeds through 

speed study.

•	May be subjective to specific traffic 

flow in the field collection;

•	Need to determine advisory 

speed criteria (85th%tile, average, 

passenger car or truck...).

Compass •	Only need single test run;

•	Devices are easily obtained.

•	Need to determine advisory 

speed criteria (85th%tile, average, 

passenger car or truck...);

•	Only to be used on very low volume 

roads due to frequent stopping.

GPS •	Only need single test run;

•	Devices combine GPS technology 

and provide more accuracy in 

measurements.

•	Need to determine advisory 

speed criteria (85th%tile, average, 

passenger car or truck...);

•	Method requires TRAMS software, 

GPS device, a laptop and an 

electronic ball-bank indicator;

•	Requires driving the curve at a low 

speed to achieve high accuracy.

Design •	No field test running is needed;

•	Suitable for newly constructed or 

reconstructed curves.

•	Need to determine advisory 

speed criteria (85th%tile, average, 

passenger car or truck...);

•	Geometry data may not be readily 

available for most existing curves in 

service for many years.

BBI •	Device is commonly available;

•	Easy to operate

•	Needs two (2) people during test 

runs;

•	Varied criteria between AASHTO and 

MUTCD;

•	Reading ball-bank indicators to 

determine the maximum degree of 

lean can be subjective.

Accelerometer •	Only need one (1) person to run 

tests;

•	Device is easy to operate.

•	Multiple test runs may be needed.

No matter which method is chosen for establishing the advisory speed, the enhanced Excel 

spreadsheet program Curve Advisory Speed (CAS) software also has the capability to recommend 

the appropriate warning signs in advance of and/or at curves according to the MUTCD 2009 edition 

(3) criteria, given the advisory speed, the posted speed limit along the tangent section and other 

geometric features.
31

CHAPTER 4  CURVE SI

4.1 Overview

This chapter describes guidelines for the signing of horizontal curves. These guidelines were derived 

from the MUTCD 2009 edition (3) and are intended to complement the procedures for establishing 

the advisory speed that are described in Chapter 3. Together, the procedure and guidelines provide 

a rational basis for establishing uniform signing for horizontal curves. 

4.2 Guidelines

Guidelines for selecting curve related traffic control devices are described in this section and are 

based on the MUTCD 2009 edition (3). They consist of recommended combinations of traffic control 

devices associated with the difference between speed limits and advisory curve speeds. The 

guidelines were developed to reflect a balance of the following goals:

•	Promote the uniform and consistent use of traffic control devices;

•	Base guidance for these devices on difference between speed limit and advisory speed;

•	Avoid overuse and misuse of devices; and

•	 Limit the number of devices used at a given curve.

A table in the MUTCD 2003 edition that related sign usage to the advisory speed and number of 

alignment changes has been replaced by a completely new table that relates sign usage to the 

difference between the speed limit and the advisory speed.

As shown in the Figure 15 of this report, the criteria in the MUTCD 2009 edition (3) states: “In advance 

of horizontal curves on freeways, on expressways, and on roadways with more than 1,000 AADT that 

are functionally classified as arterials or collectors, horizontal alignment warning signs shall be used 

in accordance with Table 2C-5 based on the speed differential between the roadway’s posted or 

statutory speed limit or 85th-percentile speed, whichever is higher, or the prevailing speed on the 

approach to the curve, and the horizontal curve’s advisory speed...Horizontal Alignment Warning 

signs may also be used on other roadways or on arterial and collector roadways with less than 1,000 

AADT based on engineering judgment.”

Application of Table 2C-5. Horizontal Alignment Sign Selection (MUTCD 2009 edition (3)) (see Figure 

15 of this report) requires knowledge of the 85th percentile tangent speed for passenger cars. This 

speed can be obtained from a survey of speeds on a tangent section of highway in the vicinity of the 

curve. The location at which tangent speed data are collected should be sufficiently distant from the 

curve that the curve does not influence the observed speeds. If the 85th percentile tangent speed is 

not available, an equation is provided in the Curve Advisory Speed (CAS) software for estimating this 

speed.
32

Table 2C-5. Horizontal Alignment Sign Selection

Type of Horizontal 

Alignment Sign

Difference Between Speed Limit and Advisory Speed

5 mph 10 mph 15 mph 20 mph 25 mph or 

more

Turn (W1-1), Curve (W1-

2), Reverse Turn (W1-3), 

Reverse Curve (W1-4), 

Winding Road (W1-5), and 

Combination Horizontal 

Alignment/Intersection 

(W10-1) 

(see Section 2C.07 to 

determine which sign to use)

Recommended Required Required Required Required

Advisory Speed Plaque 

(W13-1P) Recommended Required Required Required Required

Chevrons (W1-8) and/or One 

Direction Large Arrow (W1-6) Optional Recommended Required Required Required

Exit Speed (W13-2) and 

Ramp Speed (W13-3) on 

exit ramp

Optional Optional Recommended Required Required

Note: Required means that the sign and/or plaque shall be used, recommended means that the sign and/or plaque 

should be used, and optional means that the sign and/or plaque may be used.

See Section 2C.06 for roadways with less than 1,000 ADT.

Figure 15 – MUTCD 2009 Edition Criteria for the Selection of Horizontal Alignment Sign

There is a new table (see Figure 16) added to the MUTCD 2009 edition (3), “Table 2C-6. Typical 

Spacing of Chevron Alignment Signs on Horizontal Curves.”

Figure 16 – MUTCD 2009 Edition Criteria for the Typical Spacing of Chevron Alignment Signs on Horizontal Curves

To determine the location for installing advance horizontal alignment warning signs, “Table 2C4. Guidelines for Advance Placement of Warning Signs” in the MUTCD 2009 edition (3) shall be 

consulted (Refer to Figure 17).

Table 2C-6. Typical Spacing of Chevron 

Alignment Signs on Horizontal Curves

Advisory Speed 

15 mph or less

20 to 30 mph

35 to 45 mph

50 to 60 mph

More than 60 mph

Note: The relationship between the curve radius and the advisory speed shown in this 

table should not be used to determine the advisory speed.

Sign Spacing

40 feet

80 feet

120 feet

160 feet

200 feet

Curve Radius

Less than 200 feet

200 to 400 feet

401 to 700 feet

701 to 1,250 feet

More than 1,250 feet
33

Figure 17 – MUTCD 2009 Edition Criteria for Advance Placement of Warning Signs

These guidelines have been incorporated into the enhanced CAS software and the 

recommendation of both sign type and location are automated in the spreadsheet. Figure 18 is the 

screenshot of the CAS software traffic control device guidance.

Table 2C-4. Guidelines for Advance Placement of Warning Signs

Posted 

or 85thPercentile 

Speed

Advance Placement Distance 1

Condition A: 

Speed reduction 

and lane 

changing in

heavy traffic2

Condition B: Deceleration to the listed advisory speed (mph) for the condition

03 104 204 304 404 504 604 704

20 mph 225 ft 100 ft 6 N/A5

25 mph 325 ft 100 ft 6 N/A5 N/A5

30 mph 460 ft 100 ft 6 N/A5 N/A5

35 mph 565 ft 100 ft 6 N/A5 N/A5 N/A5

40 mph 670 ft 125 ft 100 ft 6 100 ft 6 N/A5

45 mph 775 ft 175 ft 125 ft 100 ft6 100 ft6 N/A5

50 mph 885 ft 250 ft 200 ft 175 ft 125 ft 100 ft 6

55 mph 990 ft 325 ft 275 ft 225 ft 200 ft 125 ft N/A5

60 mph 1,100 ft 400 ft 350 ft 325 ft 275 ft 200 ft 100 ft 6

65 mph 1,200 ft 475 ft 450 ft 400 ft 350 ft 275 ft 200 ft 100 ft 6

— — — — — —

— — — — —

— — — — —

— — — —

— — — —

— — —

— — —

— —

— —

—

70 mph 1,250 ft 550 ft 525 ft 500 ft 450 ft 375 ft 275 ft 150 ft —

75 mph 1,350 ft 650 ft 625 ft 600 ft 550 ft 475 ft 375 ft 250 ft 100 ft6

1 The distances are adjusted for a sign legibility distance of 180 feet for Condition A. The distances for Condition B have been adjusted for sign legibiltiy

distance of 250 feet, which is appropriate for an alignment warning symbol sign. For Conditions A and B, warning signs with less than 6-inch legend or

more than four words, a minimum of 100 feet should be added to the advance placement distance to provide adequate legibility of the warning sign.

2 Typical conditions are locations where the road user must use extra time to adjust speed and change lanes in heavy traffic because of a complex

driving situation. Typical signs are Merge and Right Lane Ends. The distances are determined by providing the driver a PRT of 14.0 to 14.5 seconds

for vehicle maneuvers (2005 AASHTO Policy, Exhibit 3-3, Decision Sight Distance, Avoidance Maneuver E) minus the legibility distance of 180 feet

the appropriate sign.

3 Typical condition is the warning of a potential stop situation. Typical signs are Stop Ahead, Yield Ahead, Signal Ahead, and Intersection Warning signs. 

The distances are based on the 2005 AASHTO Policy, Exhibit 3-1, Stopping Sight Distance, providing a PRT of 2.5 seconds, a deceleration rate of

11.2 feet/second2, minus the sign legibility distance of 180 feet.

4 Typical conditions are locations where the road user must decrease speed to maneuver through the warned condition. Typical signs are Turn, Curve,

Reverse Turn, or Reverse Curve. The distance is determined by providing a 2.5 second PRT, a vehicle deceleration rate of 10 feet/second2, minus the

sign legibility distance of 250 feet.

5 No suggested distances are provided for these speeds, as the placement location is dependent on site conditions and other signing. An alignment

warning sign may be placed anywhere from the point of curvature up to 100 feet in advance of the curve. However, the alignment warning sign should

be installed in advance of the curve and at least 100 feet from any other signs.

6 The minimum advance placement distance is listed as 100 feet to provide adequate spacing between signs.
34

Figure 18 – Curve Advisory Speed (CAS) Software Traffic Control Device Guidance

Once the advisory speed and the roadway’s posted or statutory speed limit or 85th-percentile speed, 

whichever is higher, are determined, the appropriate and consistent traffic control devices for the 

signing of horizontal curves can be determined.

These guidelines were derived from the MUTCD 2009 edition (3) and are intended to complement the 

procedures for setting advisory speeds on curves. Together, the procedure and guidelines provide a 

rational basis for establishing uniform signing for horizontal curves. The curve warning sign guidelines 

have been incorporated into the Curve Advisory Speed (CAS) software and the sign suggestion 

is automatically displayed once the advisory speed value is determined and the other relevant 

geometry information is entered.
35

REF

1. Bonneson, J, M. Pratt, J. Miles, and P. Carlson. “Procedures for Setting Curve Advisory Speeds.” FHWA/TX-09/5-5439-

01-P1. Federal Highway Administration, U.S. Department of Transportation, Washington, D.C., and Texas Department of 

Transportation, Austin, Texas, 2009.

2. Torbic, D.J., D.W. Harwood, D.K. Gilmore, R. Pfefer, T.R. Neuman, K.L. Slack, and K.K. Hardy. “NCHRP Report 500: Guidance 

for Implemention of the AASHTO Strategic Highway Safety Plan - Volume 7: A Guide for Reducing Collisions on Horizontal 

Curves”. Transportation Research Board, Washington, D.C., 2004. 

3. Manual on Uniform Traffic Control Devices. Federal Highway Administration, U.S. Department of Transportation, Washington, 

D.C., 2009.

4. Chowdhury, M., D. Warren, H. Bissell, and S. Taori. “Are the Criteria for Setting Advisory Speeds on Curves Still Relevant?” ITE 

Journal. Institute of Transportation Engineers, Washington, D.C., February 1998, pp. 32-45.

5. Bonneson, J, M. Pratt, J. Miles, and P. Carlson. “Development of Guidelines for Establishing Effective Curve Advisory Speeds”. 

FHWA/TX-07/0-5439 1. Texas Department of Transportation, Austin, Texas, 2007.

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For More Information:

http://website.com

FHWA, Office of Safety

Guan Xu

guan.xu@dot.gov

202-366-5892

FHWA-SA-11-22