Figure 6. CPM Intersection Skew Angle Definition. In the direction of increasing stations, a positive skew represents a clockwise rotation and a negative skew represents a coutter-clockwise rortation.
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After Period
Also called the analysis period, this is the period for which the crash frequency is being predicted. The minimum length of after-period is one year; there are no partial year predictions. The after period cannot overlap the before period.
AMF (Accident Modification Factor)
AMFs are multiplicative factors used to adjust the expected base crash frequency for the effect of individual geometric design and traffic control features. Each AMF is formulated so that the nominal or base condition is represented by an AMF of 1.00. Conditions associated with higher crash experience than the nominal or base condition will have AMFs greater than 1.00 and conditions associated with lower crash experience than the nominal or base condition will have AMFs less than 1.00. The AMFs are applied in step 9 of the Crash Prediction Algorithm and are described in Section 6., AMFs for Highway Segments and Section 7., AMFs for Intersections.
Approach (Leg) Classification
Approach (leg) classification is an intersection geometric design variable with permitted values of major leg or minor leg. The values enumerate whether the leg being analyzed is the major or minor leg at a given intersection. The IHSDM automatically designates the legs of the same highway as major or minor based on the traffic control, volume, or simply the highway being evaluated or Intersecting Highway. The user has also the ability to overwrite these designations. These designations are used in calculation of the expected number of crashes at intersections.
Base Model
The base model for highway segments is the best available regression model for predicting the total crash frequency of a homogeneous highway segment on a rural two-lane highway. These models are available in the Report No. FHWA-RD-99-207. The base model predicts the total expected crash frequency on the highway segment during a specified time period as a function of the highway segment's traffic volume, geometry, and traffic control. The specific regression model used by the CPM is presented in step 7 of the Crash Prediction Algorithm.
Separate base models have been developed for three-leg intersections with minor-road STOP control, four-leg intersections with minor-road STOP control, and four-leg signalized intersections. The specific regression models used by the CPM for intersections are presented in step 7 of the Crash Prediction Algorithm.
Before Period
The period for which historical crash data are applied to the model. As is the case with the after or analysis period, only full calendar years are permitted. The before period cannot overlap with the after period.
Calibration Factor
The calibration factors are applied to the base models in the crash prediction algorithm to allow highway agencies to tailor the safety prediction to their local conditions. By default all calibration factors are set to 1.00. The procedure for developing calibration factors is documented in Appendix C of Report No. FHWA-RD-99-207, Prediction of the Expected Safety Performance of Rural Two-Lane Highways.
Homogeneous Analysis Section
A homogeneous highway segment or intersection.
Homogeneous highway Segment
A homogeneous highway segment is a segment whose geometry, traffic volume, and traffic control are all constant throughout the entire length of the segment. The minimum length for a homogeneous highway segment is one meter. There is no maximum length for a homogeneous highway segment.
Intersection Skew Angle
The intersection skew angle is the deviation (in degrees) for the intersecting leg. The skew angle is reported for the left and right legs of the intersecting highway. The left/right orientation is relative to increasing station numbers on the highway being evaluated.
Figure 6.shows a four-legged intersection. The highway being evaluated is shown running vertically in the figure, the intersecting highway is shown running horizontally. The station numbers on the highway being evaluated increase when moving from the lower leg to the upper leg. The station numbers on the intersecting highway increase when moving from the left leg to the right leg.
Figure 6. CPM Intersection Skew Angle Definition. In the direction of increasing stations, a positive skew represents a clockwise rotation and a negative skew represents a coutter-clockwise rortation.
The figure shows a intersection with zero degrees intersection skew on both the left and right intersecting highway legs. On the right side leg, as the orientation of the leg is moved counter clockwise (up), the skew angle decreases from zero (a negative value in degrees). Likewise, as the orientation of the right side leg is moved clockwise (down), the skew angle increases from zero (a positive angle in degrees). On the left side leg, as the orientation of the leg is moved clockwise (up), the skew angle increases from zero degrees (a postive value); as the orientation of the leg is moved counter clockwise (down), the skew angle decreases from zero degrees (a negative value).
Limited Intersection Sight Distance
Intersection sight distance in a quadrant is considered limited if the available sight distance is less than the sight distance specified by AASHTO policy for a design speed of 10 mph (16.093 km/h) less than the major leg design speed. For the purpose of the CPM this sight distance is calculated based on a required gap of 7.5 sec. This distance is measured along the major leg lane from the intersection of the center of the minor leg lane and the center of the major leg lane beside the quadrant.
Major Leg
An intersection leg would be designated as a Major Leg if one of the following is true:
Minor Leg
An intersection leg would be designated as a Minor Leg if it is not designated as Major Leg.
Quadrant
Quadrant is a part of roadside that falls between adjacent legs of different intersecting highways. There are two quadrants at a three-leg intersection and four quadrants at a four-leg intersection. The limitation of the intersection sight distance for minor legs on minor-stop intersections created by each quadrant could be due to the vertical and horizontal curvature and are determined by the user.
Roadside Hazard Rating
A seven-point categorical scale from 1 (best) to 7 (worst) to characterize crash potential for roadside designs on two-lane highways. The Roadside Hazard Rating is defined in Appendix D of Report No. FHWA-RD-99-207, Prediction of the Expected Safety Performance of Rural Two-Lane Highways. For more information on roadside hazard rating, refer to Roadside Hazard Ratings used by IHSDM.
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