1. Caldwell and Wilson (1999)
1. Determine primary rating factor for a road section based on traffic volume and user types
2. Primary rating factor is then modified by an adjustment factor accounting for speed, terrain, and heavy vehicles
3. Adjusted rating factors used to prioritize the sections for further safety analysis

2. Caldwell and Wilson (1999) Primary rating factor determined by:

3. Caldwell and Wilson (1999) Adjusted rating factor determined by:

4. Severity Indices
Severity indices serve as indicators of the expected injury consequences of a crash
Many express low confidence in their validity
Two definitions:
(1) proportion of severe injuries experienced in crashes with fixed objects
(2) injury cost for the entire distribution of injuries experienced ~ like an expected value
Hall et al. (1994)
Council and Stewart (1996)

5. Cost-Effectiveness Approaches
Cost-effectiveness utilized as means of comparison
Benefits:
Reduction in the frequency of accidents, or
Reduction in the severity of accidents
Costs:
Societal - injuries and fatalities
Direct - initial, maintenance, repair of accidents

6. AASHTO Roadside Design Guide (1989)
1. Determine:
lateral placement, length, and width of obstacle
encroachment and collision frequency
2. Assign a severity index to the hazard
3. Determine:
initial, average damage, and average maintenance costs as well as other factors for the obstacle
average occupant injury and vehicle damage cost per accident
4. Calculate:
total present worth or
costs incurred by the highway department

7. AASHTO Roadside Design Guide (1989)
(1) Determine the following:
A = lateral placement of the roadside obstacle from EOP (feet)
L = horizontal length of the roadside obstacle (feet)
W = width of the roadside obstacle (feet)

8. AASHTO Roadside Design Guide (1989)
(2) Determine the ADT (vehicle per day)
(3) Determine the encroachment frequency (Ef) (vehicle encroachments per mile per year) using the following figure (other available data may be used in place of figure):

9. Predicting Encroachments

10. AASHTO Roadside Design Guide (1989)
(4) Determine the collision frequency, Cf, from appropriate nomographs:

11. Estimating Collision Frequency

12. Estimating Collision Frequency

13. AASHTO Roadside Design Guide (1989)
(5) Assign a severity index to the obstacle of concern (an extensive list of severity indexes is provided)
(6) Determine the:
initial cost, CI
average damage cost to obstacle per accident, CD (present dollars)
average maintenance cost per year, CM (present dollars)

14. AASHTO Roadside Design Guide (1989)
(7) Determine the:
average occupant injury and vehicle damage cost per accident, COVD (present dollars)

15. Estimating Injury and Vehicle Damage Cost
DOLLAR VALUE OF AN ACCIDENT ($ x 1000)
SEVERITY INDEX
Figure VII-C-6. Average Occupant Injury and Vehicle Damage Costs

16. AASHTO Roadside Design Guide (1989)
(8) Determine the useful life (T) of the obstacle
(9) Determine the economic present worth factors, KT and KJ, for the useful life

17. Present Worth Factors

18. Present Worth Factors (cont.)

19. AASHTO Roadside Design Guide (1989)
(10) Estimate expected salvage, CS value at the end of obstacle’s useful life (future dollars)
(11) Calculate total present worth:
(12) or, costs incurred by the highway department:

20. Mak (1995)
Benefit - cost ratio of alternative 2 compared to alternative 1
B1, C1 = Benefits and cost of alternative 1
B2, C2 = Benefits and cost of alternative 2

21. Predicting Accident Frequency
Accident data-based model
Historical data from reported accidents
Develop multiple-regression models
Input parameters:
Roadway characteristics
Roadside characteristics
Output:
Accident frequency
Mak (1995)

22. Predicting Accident Frequency
Encroachment probability model:
Assumes that accident frequency can be related to encroachment frequency
Assumptions made about the distribution of lateral encroachment distances, speeds and angles, and vehicle sizes
Advantages
Applicable to a wide variety of roadside features
Allows evaluation of multiple performance levels
NOTE
accident data base model based on roadside features that are built to a single performance level; this can evaluate different performance levels, for instance lower cost on a low-volume, low-speed roadway
Mak (1995)

23. Encroachment Probability Model
Sicking and Hayes (1986)

24. Encroachment Probability Model (cont.)
Probability that a vehicle of size W will encroach at speed V and angle  into encroachment range 2, given that an encroachment has occurred
probability that an encroaching vehicle will be of size W
Probability that an encroaching vehicle will be traveling at speed V
Effective vehicle width ( ½ vehicle width + ½ vehicle length) in feet
Sicking and Hayes (1986)

25. Encroachment Probability Model (cont.)
Probability that a vehicle of size W encroaching at speed V and angle  will strike hazard within range 2, given that an encroachment has occurred
Distance from travelway to fixed object (ft)
Probability that the lateral extent of encroachment is greater than or equal to (a + …)
We* cos  (ft)
Sicking and Hayes (1986)