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

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

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

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)

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

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

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)

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):

Predicting Encroachments

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

Estimating Collision Frequency

Estimating Collision Frequency

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)

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

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

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

Present Worth Factors

Present Worth Factors (cont.)

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:

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

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)

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)

Encroachment Probability Model Sicking and Hayes (1986)

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)

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)