1. HSM Practitioner’s Guide for Two-Lane Rural Highways Workshop
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Predicting Highway Safety for Curves on Two-Lane Rural Highway
- Session #4
Session #4 – Predicting Highway Safety for Curves on Two-Lane Rural Highways
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

2. Predicting Highway Safety for Curves on Two-Lane Rural Highways
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Learning Outcomes:
Describe the crash prediction method for Crash Performance on Horizontal Curves
Identify low-cost safety improvements for horizontal curves
Learning Objectives for Session #4
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

3. ….Curves present particular safety problems to designers
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
CRASH RATES
(Crashes per 1 km segment--3 year timeframe)
Crash Rate
The risk of a reported crash is about three times greater on a curve than on a tangent
A study by Glennon, Neuman and Leisch found that curves represented 3 times the safety risk of tangents. This finding is consistent with other research. Drivers have a demonstrably more difficult time successfully negotiating curves
According to Hauer (1999), based on a review of literature about safety and degree of curve, most studies find that collision rate increases as degree of curve increases.
Source: Glennon, et al,
1985 study for FHWA
Tangent segments
Segments w/curve
Curved portion only (Curve plus transitions)
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

4. HSM Practitioner's Guide for Two-Lane Rural Highways

5. HSM Practitioner's Guide for Two-Lane Rural Highways

6. HSM Practitioner's Guide for Two-Lane Rural Highways

7. HSM Practitioner's Guide for Two-Lane Rural Highways

8. HSM Practitioner's Guide for Two-Lane Rural Highways

9. Actual Driver Operations on Curves
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Actual Driver Operations on Curves
Driver tracks a ‘critical radius’
sharper than that of the curve
just past the PC
Drivers ‘overshoot’ the curve (track a path sharper than the radius)
Path is a spiral
Path overshoot behavior is independent of speed
This slide shows observed tracking behavior of vehicles as they enter a curve from a tangent which differs from design assumptions that vehicles track through a curve as a single point o mass, and stays in the center of the lane, at a constant speed. Observations studies show that this is not the case.
The instructor should point out that the research looked at driver behavior on unspiraled curves. The term overshoot refers to the driver’s inability to instantaneously steer the vehicle to match the curve as designed.
Many designers believe that a wide enough lane allows a driver to smoothly spiral into an unspiraled curve. While the space may be there, the fact remains that drivers do not respond to curves that way.
By overshooting the curve, a driver must at some point track a path radius that is sharper than the curve to avoid leaving the road. This sharper curve would of course be the critical behavior of interest.
The Glennon 1985 study noted this path behavior.
Source: Bonneson, NCHRP 439 and Glennon et al (FHWA)

10. Driver “overshoot” behavior on curves (from Glennon, et al)
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
700
The Glennon study was able to develop relationships describing percentiles of what was termed critical path behavior and the designed radius. The regression equations are plotted here.
The effects of this behavior are significant. The instructor may want to point out an example, shown on the slide. A 1000 ft radius curve is driven by a 95th%-tile driver tracking a 700 ft radius path.
Note that even the median driver tracks a path slightly sharper than the curve.
In the notebooks (hidden slide) are the regression equations for curve tracking.
The Glennon study found that curve tracking behavior was independent of speed; i.e., that faster drivers were just as likely to require a critical radius as slower drivers.
The study involved only unspiraled curves; results were replicated using HVOSM (a simulation model) which also was used to simulate operations on spiraled curves.
Example -- a 1000-ft radius
curve is driven by a 95th percentile
driver at about a 700 ft radius
at some point in the curve
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

11. Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Research confirms differences in actual operations versus AASHTO assumptions
Drivers’ selected speed behavior does not match design assumptions
Sharper curves (<80 km/h or 50 mph) are driven faster (drivers are more comfortable)
Curves driven faster
than Policy assumption
Curves driven slower
than Policy assumption
This graph reports on studies of driver behavior by Krammes and Otteson. Drivers tend to “overdrive” lower design speed curves. Vertical blue line is the dividing line which occurs at 80 kmh (50 mph).
The instructor should explain that the inferred design speed (x-axis)was derived from the curvature and superelevation. Data points above the line represent speeds faster than assumed under AASHTO policy.
The instructor may wish to ask participants how this information may be useful.
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

12. Speed Prediction Model for Horizontal Curves (Otteson and Krammes)
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Speed Prediction Model for Horizontal Curves (Otteson and Krammes)
V85 = D L DL Vt
Where
V85 = 85th percentile speed on the curve
D = degree of curve
L = length of curve (mi)
Vt = 85th percentile approach speed (mph)*
*this should be measured in the field
This model, one of the inputs to the IHSDM predicts 85th percentile speed behavior as a function of curve geometry and the approach speed.
*Approach speeds should be measured in the field.
The instructor could ask how this model could be used to diagnose operational or safety problems.
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

13. A ‘risk assessment’ tool for speed profiles
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
A ‘risk assessment’ tool for speed profiles
V85 - Vdesign = Vdelta
Higher risk curves may be those with V delta high (i.e., operating speeds significantly greater than design speed)
Vdelta > 12 mph (20 km/h); high risk
6 mph (10 km/h) < Vdelta < 12 mph (20 km/h); caution
Some simple but effective guidelines have been developed that enable one to look at speeds, expected driver behavior, and curve geometry to help identify potential problems.
The term Vdesign is the inferred design speed. Given an existing curve’s radius and superelevation, and the agency’s curve design policy, one can calculate what the inferred design speed is.
If the difference in 85th %tile speed and inferred design speed is too great (ie, a curve is being significantly overdriven, it can be considered a “high risk” curve.
The instructor could ask what other information might be useful to further identify high risk curves, and what solutions might be considered. The instructor could also ask how this information and these models might be used in corridor planning or reconstruction projects.
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

14. FHWA’s IHSDM Speed Consistency Model Addresses Continuous Speed Behavior
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
FHWA’s IHSDM incorporates the research of speed and speed change behavior in what is referred to as a Design Consistency Module.
Illustrated here, the module enables a designer to develop a speed profile, and to look for locations where the design produces high speeds (relative to the design) and/or significant differences in speed between adjacent sections.
Note to instructor -- once the IHSDM is available, a demonstration of this aspect of it could be inserted in the presentation, if time allows.
Recent release of 2007 IHSDM is available at:
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

15. Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Truck operations on curves may in some cases be critical (Harwood and Mason)
Under certain conditions, trucks will roll over before they skid
Trucks with high centers of gravity overturn before losing control due to skidding
Margin of safety for ‘f’ is therefore lower for trucks
Trucks on downgrade curves generate greater lateral friction (superelevation is not as effective)
Let’s now turn to the vehicle itself in the AASHTO curve model.
Most designers intuitively know that truck and passenger car operations are different.
Research confirms that this is true. It explains why some curves are particularly problems.
The instructor may wish to ask participants about how this information would affect their review of a highway curve problem.
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

16. Summary of Research on Superelevation and Transition Design
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Summary of Research on Superelevation and Transition Design
Studies confirm small but significant effect of superelevation on crashes
FHWA (Zegeer) study noted 5 to 10% greater crashes when superelevation is “deficient”
1987 study of fatal crash sites on curves noted “deficiencies in available superelevation”
Superelevation and transition design is a significant part of good curve design.
Recent research (NCHRP Report 439) looked at many aspects of superelevation design. The workshop does not have the time to go into great detail, but certain points need to be made.
First, problems tend to stem from insufficient superelevation. Second, it is clear that US practice produces the anomaly (see next slide) or inconsistency in design wherein two identical curves in different states designed to the same nominal design speed are different in terms of the “feel” of the curve to the driver.
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

17. Research confirms benefits of spirals and recommends optimal transition design
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Spirals provides e transition leading into the curve
Radius (m)
Source: NCHRP Report 439
Spirals are recommended for curves at or close to the minimum radius curve for a given design speed. In NCHRP 439, Bonneson found that the optimal transition design for an unspiralled curve is 70 percent on tangent and 30 percent on curve.
The instructor should note that the only proper way to develop superelevation and meet AASHTO f requirements throughout the curve is with the use of spirals. If one assumes a driver “needs” 100 percent of what is prescribed on the curve, then it would seem that the curve should provide 100 percent superelevation at the PC. But then the superelevation would be all developed on the tangent, and the driver would be driving “into” the approach tangent and have to reverse steering instantaneously at the PC.
Many agencies do not use spirals as a matter of historical survey and design convenience. There is clear evidence, however, that spirals have both operational and safety benefits, and recommendations in NCHRP 439 reflect this.
The instructor should be aware of the policy of the state in which the workshop is being held, and tailor remarks to reflect that.
Zegeer et al found safety benefits in HSIS study of Washington
Bonneson confirmed operational benefits noted by Glennon, etal
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

18. Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Zegeer et al. FHWA Study “Cost-Effective Geometric Improvements for Safety Upgrading of Horizontal Curves” (1991)
Data Bases
10,900 Curves in Washington State
7-state data base of 5000 mi
78 curves in New York State
Glennon 4-state data base of 3277 curve segments
Statistical Analysis and Model Development
Identified as key effort in TRB SR 214, recent NCHRP review by BMI, and key reference for IHSDM
This slide provides an overview of the scope and background behind research conducted in the 1980s that will be covered. See the reference list for the titles and dates of these efforts.
TRB Special Report 214 is entitled “Designing Safety Roads, Practices for Resurfacing, Restroation, and Rehabilitation” and was published in 1987.

19. Summary of findings from Zegeer study
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Summary of findings from Zegeer study
Features related to crashes include:
Degree and length of curve
Width through the curve
Superelevation and,
Spiral presence
For typical volumes on 2-lane highways, expect 1 to 3 crashes per 5 years on a curve
Read slide and summarize
The Zegeer research is a traditional ‘cross sectional’ model (different statistical techniques are now employed). It applies to isolated curves. It does not include a variable describing the quality of the roadside, which is known to be sensitive to safety (because such information was not available in the state data bases)
It may be worth pointing out that length of curve (or central angle) is a factor or concern; but this is not included in AASHTO curve design policy.
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

20. Safety Effects for Horizontal Curves (CMF3r)
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
CMF3r = 1.55 Lc + (80.2/R) * S Lc
Where:
Lc = Length of Curve including spirals, (mi)
R = Radius of Curve (ft)
S = 1 if spiral transition is present, 0 if not present
From Final HSM Chapter 10:
CMF3r—Horizontal Curves: Length, Radius, and Presence or Absence of Spiral Transitions
The base condition for horizontal alignment is a tangent roadway segment. A CMF has been developed to represent the manner in which crash experience on curved alignments differs from that of tangents. This CMF applies to total roadway segment crashes.
The CMF for horizontal curves has been determined from the regression model developed by Zegeer et al. (18).
The CMF for horizontal curvature is in the form of an equation and yields a factor similar to the other CMFs in this chapter. The CMF for length, radius, and presence or absence of spiral transitions on horizontal curves is determined using Equation 10-13
Some roadway segments being analyzed may include only a portion of a horizontal curve. In this case, Lc represents the length of the entire horizontal curve, including portions of the horizontal curve that may lie outside the roadway segment of interest.
In applying Equation 10-13, if the radius of curvature (R) is less than 100-ft, R is set to equal to 100 ft. If the length of the horizontal curve (Lc) is less than 100 feet, Lc is set to equal 100 ft.
CMF values are computed separately for each horizontal curve in a horizontal curve set (a curve set consists of a series of consecutive curve elements). For each individual curve, the value of Lc used in Equation is the total length of the compound curve set and the value of R is the radius of the individual curve.
If the value of CMF3r is less than 1.00, the value of CMF3r is set equal to 1.00.
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

21. Safety Effects of Horizontal Curves (CMF3r): Example with no Spiral present
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
For: Lc = 480 feet = miles
R = 350’; no spiral transition
CMF3r = {1.55 Lc + (80.2/R) – 0.012S } / 1.55Lc
= (1.55 x 0.091) + (80.2/350) – 0.012x0
1.55x 0.091
Example calculation for CMF for a horizontal curve without spirals.
= 2.62
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

22. HSM Practitioner's Guide for Two-Lane Rural Highways Workshop
Safety Effects of Horizontal Curves (CMF3r): Example with Spiral Transition
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop
August 2010
For: Lc = 480 feet = miles
R = 350’; with spiral transition
CMF3r = {1.55 Lc + (80.2/R) – 0.012S } / 1.55Lc
= (1.55 x 0.091) + (80.2/350) – 0.012x1
1.55x 0.091
Example calculation for CMF for a horizontal curve with spirals. Potential percent reduction for providing a spiral is 8% (2.62 – 2.54 = 0.08)
= ?
= *2.54
*Without spiral CMF3r = 2.62, with spiral CMF3r= 2.54, Difference = 8% potential for fewer crashes with a spiral transition in this segment.
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

23. Crash Modification Function for Horizontal Curves: Superelevation
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
CMF4r is based on “Superelevation variance” or SV
For SV less than 0.01: CMF4r = 1.00
For 0.01 < SV < 0.02: CMF4r = (SV-0.01)
For SV > 0.02: CMF4r = (SV-0.02)
Example: Design e = 4%, Actual e = 2%
From Final HSM Chapter 10:
CMF4r—Horizontal Curves: Superelevation
The base condition for the CMF for the superelevation of a horizontal curve is the amount of superelevation identified in A Policy on Geometric Design of Highways and Streets—also called the AASHTO Green Book (1). The superelevation in the AASHTO Green Book is determined by taking into account the value of maximum superelevation rate, emax, established by highway agency policies. Policies concerning maximum superelevation rates for horizontal curves vary between highway agencies based on climate and other considerations.
The CMF for superelevation is based on the superelevation variance of a horizontal curve (i.e., the difference between the actual superelevation and the superelevation identified by AASHTO policy). When the actual superelevation meets or exceeds that in the AASHTO policy, the value of the superelevation CMF is There is no effect of superelevation variance on crash frequency until the superelevation variance exceeds The general functional form of a CMF for superelevation variance is based on the work of Zegeer et al. (18, 19).
The following relationships present the CMF for superelevation variance:
CMF4r = 1.00 for SV < 0.01 (10-14)
CMF4r = × (SV − 0.01) for 0.01 ≤ SV < 0.02 (10-15)
CMF4r = × (SV − 0.02) for SV ≥ 0.02 (10-16)
Where:
CMF4r = crash modification factor for the effect of superelevation variance on total crashes; and
SV = superelevation variance (ft/ft), which represents the superelevation rate contained in the AASHTO Green Book minus the actual superelevation of the curve.
CMF4r applies to total roadway segment crashes for roadway segments located on horizontal curves.
SV = 0.04 – 0.02 = 0.02
CMF4r = ( ) = (0.0) = 1.06
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

24. HSM Applications to Two-Lane Rural Highway Segments
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
HSM Crash Prediction Method for Two-Lane Rural Highway Segments:
Applying SPF and CMFs
Example Problem
Session #1 – The next several slides will present the steps and results to calculate a crash prediction for a roadway segment. In particular, detailed calculations for the curve segment on-grade is presented.
Session 1 – Introduction and Background

25. Crash Prediction for Roadway Segment for Existing Conditions – Example Calculation:
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Two-Lane Rural Roadway, CR 123 Anywhere, USA (MP – 15.02)
AADT = 3,500 vpd for the current year
Length = 26,485 feet = 5.02 miles
Lane Width = 11.0 ft
Shoulder Width = 2 ft; Shoulder Type = Gravel
Horizontal Curve on Grade (MP ):
Lc = miles, R = 650’; with no spiral transition
Grade = 4.5%
Superelevation Variance = .02
Tangent Section on Grade (MP ):
L = 0.55 miles; Grade = -6.3%
This is the same example that was presented in the Session 2 on Roadway Segments. However, this example includes a horizontal curve on a 4.5% grade with a superelevation variance = 0.02, and a tangent section on -6.3 % grade.
Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

26. Crash Prediction for Roadway Segment for Existing Conditions – Example:
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Divide Two-Lane Rural Roadway into Individual Segments:
Segment
Length (miles)
Horizontal Curve Radius (ft)
Super-elevation Variance
Grade (%)
Driveway Density (per mile)
RHR
10.00 – 12.00
2.000
Tangent
N/A
2.0% 8 5
*12.00 –
0.186
650
.02
4.5%
1.264
3.0% 4
0.550
- 6.3%
1.020
- 3.0% 6
This table breaks the segment into individual segments based on segment length and the presence of horizontal curves, superelevation variance, grades, driveways, and Roadside Hazard Rating.
Predicted Crash Frequency and CMF calculations for Segment 2 (MP – MP ) are presented in the next few slides.
Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

27. = (3,500) (0.186) (365) (10-6) (0.7320) = 0.17 crashes per year
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
Safety Performance Function (SPF) for Base Conditions: Example Calculation
May 2009
Segment 2 (MP ): Horizontal Curve on a 4.5% Grade
Where:
AADT = 3,500 vpd (current year)
Length = miles
Nspf-rs = (AADTn) (L) (365) (10-6) e-0.312
Nspf-rs = (3,500) (0.186) (365) (10-6) e-0.312
Separating out Segment 2 to analyze the horizontal curve.
This calculation applies the SPF to determine the predicted crash frequency for base conditions on the mile segment.
= (3,500) (0.186) (365) (10-6) (0.7320)
= 0.17 crashes per year
Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

28. CMF for Lane Width (CMF1r): Calculation
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
CMF for Lane Width (CMF1r): Calculation
May 2009
Segment 2: 11 foot wide lane:
From Table 10-8: CMFra = 1.05
Adjustment for lane width and shoulder width related crashes (Run off Road + Head-on + Sideswipes) to obtain total crashes using default value for pra = 0.574
CMF1r = (CMFra ) pra + 1.0
= ( ) *
Calculation for CMF1r for Lane Width for Segment 2
= (0.05) (0.574) + 1.0
= 1.03
Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

29. CMF or Shoulder Width and Type (CMF2r): Calculation
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
CMF or Shoulder Width and Type (CMF2r): Calculation
Segment 2: 2 ft wide gravel shoulder:
CMFwra = 1.30 (Table10-9) and CMFtra = 1.01 (Table10-10)
Adjustment from crashes related to lane and shoulder width (Run off Road + Head-on + Sideswipes) to total crashes using default value for pra = 0.574
CMF2r = (CMFwra CMFtra ) pra + 1.0
Calculation for CMF2r for Shoulder Width and Type for Segment 2
= ((1.30)(1.01) - 1.0) *
= (0.313) (0.574) + 1.0
= 1.18
Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

30. CMF for Horizontal Curve (CMF3r): Calculation
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Segment 2: Horizontal Curve
For: Lc = miles
R = 650’; with no spiral transition
CMF3r = {1.55 Lc + (80.2/R) – 0.012S } / 1.55Lc
= (1.55 x 0.186) + (80.2/650) – 0.012x0
1.55x 0.186
Calculation for CMF3r for Horizontal Curve without spirals for Segment 2
= 1.43
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

31. CMF for Superelevation on Horizontal Curves (CMF4r)
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Segment 2: Horizontal Curve Superelevation Variance = 0.02
For SV > 0.02: CMF4r = (SV-0.02)
CMF4r = ( )
= (0.0)
= 1.06
Calculation of CMF4r for Superelevation Variance for Segment 2
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

32. CMF for Percent (%) Grade on Roadway Segments (CMF5r)
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Segment 2: 4.5% Grade
For Segment 2: CMF5r = 1.10
CMF5r = 1.10
Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

33. CMF Roadside Design (CMF10r): Example Calculation
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
CMF Roadside Design (CMF10r): Example Calculation
Segment 2: RHR = 5
CMF10r = e( (0.0668xRHR)) /e
= e( (0.0668x5)) /e
= 1.14
Calculation of the CMF10r for Roadside Design for Segment 2
Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

34. Applying CMFs to the SPF Base Prediction Model
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
Applying CMFs to the SPF Base Prediction Model
May 2009
CRASH MODIFCATION FACTORS
Lane Width = 11 ft CMF1r = 1.03
Shoulder Width = 2 ft gravel CMF2r = 1.18
Horizontal Curve CMF3r = 1.43
Superelevation Variance (0.02) CMF4r = 1.06
Percent Grade = 4.5% CMF5r = 1.10
Driveway Density, None CMF6r = 1.00
Centerline Rumble, None CMF7r = 1.00
Passing/Climbing Lanes, None CMF8r = 1.00
TWLTLs, None CMF9r = 1.00
Roadside Design, RHR = 5 CMF10r = 1.14
Lighting, None CMF11r = 1.00
Automated Enforcement, None CMF12r = 1.00
Segment 2: SPF and CMF Values:
AADT = 3,500 vpd, Length = mi Radius = 650 ft
Nspf-rs = 0.17 crashes per year
CMFtotal = 2.31
From the example calculations previously performed we can look at the resulting CMFs and tell which geometric elements or traffic control features are contributing to the increase and decrease of crash potential.
Remember:
CMFs > 1.0 have the potential for higher crash frequency
CMFs < 1.0 have the potential for lower crash frequency
Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

35. Applying CMFs to the SPF Base Prediction Model
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Applying CMFs to the SPF Base Prediction Model
Npredicted-rs = Nspf-rs x (CMF1r … CMF12r) Cr
Segment 2: Apply CMFs to SPF for Base Conditions: (letting Cr = 1.0)
Npredicted-rs =
0.17 x (1.03 x 1.18 x 1.43 x 1.06 x 1.10 x 1.00 x 1.00 x 1.00 x 1.00 x 1.14 x x 1.00) x 1.00
Applying CMFs to the SPF Base Prediction Model for Segment 2
CMFs are multiplied together and to the SPF Base Model values to determine the total safety effects (crash prediction) for individual geometric and traffic control features:
Nspf-rs is the expected crash frequency from the base model, i.e., for the highway segment’s particular traffic volume and length.
The CMFs adjust for the effect of the actual geometry of the segment. There are cMFs for 12 individual highway segment geometric or design elements.
The calibration factor adjusts for differences between crash experience in the particular state and in the state for which the base model was fit.
= 0.17 x 2.31 x 1.00
= 0.4 crashes per year, 1 crash every 2.5 yrs
Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

36. Crash Prediction for Roadway Segment for Existing Conditions – Example Calculation:
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
For each Two-Lane Rural Roadway Segment: Table with SPF predicted crahses, CMFs, and Adjusted Total Crashes
CRASH PREDICTION METHOD – TOTAL CRASHES
Seg
No.
SPF base
CMF1r LW
CMF2r SW&ST
CMF3r ST
CMF4r e
CMF5r Grade
CMF6r DD
CMF7r
CLRS
CMF8r PassLn
CMF9r TWLTL
CMF10r RD
CMF11r Light
CMF12r Spd Enf
Total
CMF
Total Adjusted
Crashes 1
1.87
1.03
1.18
1.00
1.07
1.14
1.49
2.8 2
0.17
1.43
1.06
1.10
2.31
0.40 3
1.27
1.39
1.8 4
0.51
1.16
1.61
0.8 5
0.95
1.02
1.42
1.4
Total:
7.2
Results from a spreadsheet calculation separated into homogeneous sections. Total crashes for base condtions (SPF) and individual CMFs and Adjusted Total Crashes is shown. Calculations for Segment 2 (circled) was demonstrated.
Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

37. Predicting Crash Frequency Performance
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Predicting Crash Frequency Performance
Total Predicted Crash Frequency within the limits of the roadway being analyzed:
Ntotal crashes = ∑Npredicted-rs + ∑ Npredicted-int
Focus in this section will be on predicting the safety performance of rural two-lane highway segments.
The total estimated number of crashes within the network or facility limits during a study period of n years is calculated using Equation 10-4:
Ntotal = Σ Nrs {for all roadway segments} + Σ Nint {for all intersections} (Equation 10-4)
Where,
Ntotal = total expected number of crashes within the limits of a rural two-lane two-way facility for the period of interest. Or, the sum of the expected average crash frequency for each year for each site within the defined roadway limits within the study period;
Nrs = expected average crash frequency for a roadway segment using the predictive method for one specific year;
Nint = expected average crash frequency for an intersection using the predictive method for one specific year.
Equation 10-4 represents the total expected number of crashes estimated to occur during the study period.
Ntotal crashes = 7.2 crashes/yr + ∑ Npredicted-int
Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

38. Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Overview of Good Alignment Design Practice (suggested by safety and operational research)
Curves and grades are necessary features of alignment design (reflect the topography, terrain, and “context”)
Pay particular attention to roadside design adjacent to curves
Avoid long, sharp curves
Adjust alignment design to reflect expected speeds on curves
Let’s summarize what we know about alignment design and substantive safety. Read Slide and discuss
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

39. Overview of Good Alignment Design Practice (continued)
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Avoid minimum radius designs where
actual speeds will be higher than design speeds
truck volumes will be substantial
combined with steep grades
Use spiral transition curves, particularly for higher speed roads and sharper curves
Read slide and discuss
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

40. Overview of Good Alignment Design Practice (continued)
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
Overview of Good Alignment Design Practice (continued)
May 2009
Minimize grades within terrain context
Widen lanes and shoulders through curves
Pay attention to access points related to horizontal and vertical curve locations
Read slide and discuss
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

41. Low and Lower Cost Safety Improvements for Horizontal Curves
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Signing
Shoulders
Read slide and discuss
Some low cost safety improvements at horizontal curves includes enhanced signing, wider shoulders, and lighting
Lighting
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

42. Low Cost Intersection Safety Measures – Signing Countermeasures
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Advance Warning
With Speed Advisory
Injury Crashes
CMF = 0.87 CRF = 13%
PDO Crashes
CMF = 0.71
CRF = 29%
First is warning, then guide signing, then regulation at the intersection
*CMF Clearinghouse
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

43. May 2009
Safety Effects of Installing Combination Horizontal Alignment Warning + Advisory Speed Signs
AMFtotal = (AMFinjury)(portion of injury crashes) + (AMFnon-injury)(portion of non-injury crashes)
From Final Draft HSM Chapter 13:
Roadway Sign Treatments with AMFs: Install Combination Horizontal Alignment/Advisory Speed Signs (W1-1a, W1-2a)
Combination horizontal alignment/advisory speed signs are installed prior to a change in the horizontal alignment to indicate that drivers need to reduce speed.(9)
Rural two-lane roads, rural multi-lane highways, expressways, freeways, urban and suburban arterials
Compared to no signage, providing combination horizontal alignment/advisory speed signs reduces the number of all types of injury accidents, as shown in Exhibit (8) The crash effect on all types of non-injury accidents is also shown in Exhibit
The base condition of the AMFs (i.e., the condition in which the AMF = 1.00) is the absence of any signage.

44. Signing Countermeasure for Horizontal Curves:
May 2009
Signing Countermeasure for Horizontal Curves:
Chevrons Signs
*CRF = 35%
CMF = 0.65
Hammer (1968) evaluated the effectiveness of various types of minor improvements in reducing accidents. Two of the minor improvements included in the evaluation were the installation of curve warning signs and advisory speed signs at horizontal curves. Hammer found curve warning signs reduced accidents by 18 percent at horizontal curves and installation of both curve warning and advisory speed signs reduced accidents by 22 percent. Leisch (1971) also reported advisory speed signs to be effective in reducing accidents at horizontal curves.
35% crf for chevrons – Ray Krammes
*CMF Clearinghouse

45. Safety Effects of Installing RPM’s
May 2009
Safety Effects of Installing RPM’s
Final HSM, Chapter 13
Install Snowplowable, Permanent RPMs
Installing snowplowable, permanent RPMs requires consideration of traffic volumes and horizontal curvature (2).
Rural two-lane roads
The crash effects of installing snowplowable, permanent RPMs on low volume (AADT of 0 to 5,000), medium volume (AADT of 5,001 to 15,000), and high volume (AADT of 15,001 to 20,000) roads are shown in Table 13‑411 (2).
The varying crash effect by traffic volume is likely due to the lower design standards (e.g., narrower lanes, narrower shoulders, etc.) associated with low-volume roads (2). Providing improved delineation, such as RPMs, may cause drivers to increase their speeds. The varying crash effect by curve radius is likely related to the negative impact of speed increases (2). The base condition of the CMFs (i.e., the condition in which the CMF = 1.00) is the absence of RPMs.

46. Low Cost Intersection Safety Measures – Signing Countermeasures
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Low Cost Intersection Safety Measures – Signing Countermeasures
Double Up Advance Warning Signs
CRF = 31%
CMF = 0.69
Hammer (1968) evaluated the effectiveness of various types of minor improvements in reducing accidents. Two of the minor improvements included in the evaluation were the installation of curve warning signs and advisory speed signs at horizontal curves. Hammer found curve warning signs reduced accidents by 18 percent at horizontal curves and installation of both curve warning and advisory speed signs reduced accidents by 22 percent. Leisch (1971) also reported advisory speed signs to be effective in reducing accidents at horizontal curves.
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

47. Low Cost Intersection Safety Measures – Signing Countermeasures
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Sharp 10 mph curve to right just over hill
Activated Warning Beacon
Radar activated flasher when speed is fast for 10mph curve
Maine DOT application west of Augusta Me
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

48. Examples of Improving Safety of Existing Curves
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Widen Shoulders
Widen 2’ Shoulder to 6’ Shoulder – NY Rte 82 north of Millbrook 6’
Widening a shoulder to the nominal AASHTO shoulder width of 6 feet increases speed through the curve by 3 to 5 mph reducing the speed differential between the tangent and the curve speeds. 2’
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

49. Examples of Improving Safety of Existing Curves
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Examples of Improving Safety of Existing Curves
Widen Shoulder on Inside of Tight Curve
Widening on
Inside of Curves
NCHRP 500, Strategy A11– Widening in Curves
An effective low cost safety improvement is to provide a paved shoulder (widening) on the inside of a horizontal curve
*NCHRP 500, Strategy draft material – Widening in Curves
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

50. Low Cost Intersection Safety Measures – Signing Countermeasures
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
9. Illumination of
Rural Curves
CRF = 28% for injury crashes highway lighting
Illumination of a rural highway curve on Route 376 near Poughkeepsie, NY
Route 376 near Poughkeepsie, NY
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

51. Predicting Highway Safety for Curves on Two-Lane Rural Highways
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop
May 2009
Learning Outcomes:
Described the equation for prediction of Crash Performance on Horizontal Curves
Identified low-cost safety improvements for horizontal curves
Learning Objectives for Session #4
Session 4 – Predicting Highway Safety for Curves on 2-Lane Rural Highway Segments

52. Questions and Discussion:
HSM Practitioner's Guide for Two-Lane Rural Highways
August 2010
Questions and Discussion: