4.  CPM Output

The CPM generates tables describing the highway and intersection data used for the evaluation and various reports and plots showing the expected crash frequencies and rates for the project (CPM Evaluation Report). This section describes these reports/graphs, which can be viewed by selecting the evaluation options.

4.1  Highway Data

The CPM generates tables describing the highway and intersection data used for the evaluation. These tables organize data in such a way as to help with interpreting the results of the CPM. The following tables are created by the CPM for the current (before period) and proposed (after period) alignments:

• Highway Segment Data
• Horizontal Curve Data
• Intersection Data
• Segment Traffic Volume
• Intersecting Highway Traffic Volume

4.1.1  Highway Segment Data (Current and Proposed)

The Highway Segment Data Table displays the homogeneous highway segments created by the CPM for the evaluation bounds specified as described in step 2 through step 4 of the Crash Prediction Algorithm. When one of the highway characteristics listed in the columns changes, a new homogeneous segment is created and therefore, a new row is created. Columns in the table are as follows:

• Segment #: Each homogeneous highway segment is assigned a number by the CPM for purposes of identification. Numbers are sequential beginning with 1.
• Station (Start): Beginning station for the homogeneous highway segment in the given row.
• Station (End): End station for the homogeneous highway segment in the given row.
• Length: The length of the homogeneous highway segment in the given row (equal to the difference between the Start Station and End Station).
• Lane Width (Right): The width of the lane to the right of the centerline, in the direction of increasing stations.
• Lane Width (Left): The width of the lane to the left of the centerline, in the direction of increasing stations.
• Shoulder Width (Right): The width of the shoulder to the right of the centerline, in the direction of increasing stations.
• Shoulder Width (Left): The width of the shoulder to the left of the centerline, in the direction of increasing stations.
• Shoulder Type (Right): The type of the shoulder to the right of the centerline, in the direction of increasing stations.
• Shoulder Type (Left): The type of the shoulder to the left of the centerline, in the direction of increasing stations.
• Driveway Density: The number of driveways per length of highway (English: driveways/mi; Metric: driveways/km).
• Roadside Hazard Rating: A seven-point categorical scale from 1 (best) to 7 (worst) to characterize the roadside environment 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.
• Horizontal Curve Number: Each horizontal curve is assigned a number by the CPM for purposes of identification. Numbers are sequential beginning with 1. If the section of highway is a tangent section, this field will display a dash.
• Grade (%): The percent grade on the given highway segment.
• Passing Lane (Right): The presence of a passing lane to the right of the centerline, in the direction of increasing stations. Entry will be "yes" if a passing lane is present and "no" if a passing lane is not present.
• Passing Lane (Left): The presence of a passing lane to the left of the centerline, in the direction of increasing stations. Entry will be "yes" if a passing lane is present and "no" if a passing lane is not present.
• Center TWLTL: The presence of a Two-Way Left-Turn Lane. Entry will be "yes" if a TWLTL lane is present and "no" if a TWLTL lane is not present.

4.1.2  Horizontal Curve Data (Current and Proposed)

The Horizontal Curve Data Table lists the characteristics of the horizontal curves that are within the bounds of the evaluation. There is a separate row for each curve. Interpolated values are shown in a blue font. Columns in the table can be described as follows:

• Horizontal Curve Number: Each curve is assigned a number by the CPM for purposes of identification. Numbers are sequential beginning with 1.
• Station (Start): Beginning station for the curve.
• Station (End): End station for the curve.
• Length of Curve: The length of the curve.
• Radius: The radius of the curve.
• Superelevation (%): The actual superelevation attained on the curve.
• Design Speed: The design speed for the curve.
• Spiral Transition: The presence of a spiral transition. The entry will read "both" if a spiral transition is on both the entry and exit portions of the curve and will read "one" in the rare case where there is a spiral transition on only one side of the curve.

4.1.3  Intersection Data (Current and Proposed)

The Intersection Data Table includes a separate row for each intersection within the evaluation bounds. Columns include:

• Intersection #: Each intersection is assigned a number by the CPM for purposes of identification. Numbers are sequential beginning with 1.
• Title: The title or name of the intersection for the given row.
• Station: The location of the intersection along the highway being evaluated.
• No. of Legs: The number of legs for the intersection for the given row.
• Traffic Control: The type of traffic control for the given intersection.
• Skew Angle (Right): Deviation (in degrees) from a 90 degree angle for intersection legs on the right side of the highway. A positive value indicates a clockwise deviation and a negative value indicates a counter-clockwise deviation.
• Skew Angle (Left): Deviation (in degrees) from a 90 degree angle for intersection legs on the left side of the highway. A positive value indicates a clockwise deviation and a negative value indicates a counter-clockwise deviation.
• Major legs with exclusive LTLs: The number of exclusive left-turn lanes at the intersection.
• Major legs with exclusive RTLs: The number of exclusive right-turn lanes at the intersection.
• Quadrants with Limited ISD: The number of intersection quadrants with limited sight distance. These data are entered via a CPM query dialog box for limited ISD. The query appears only for Evaluations containing intersections with stop-control on the minor road.

4.1.4  Segment Traffic Volume (Current and Proposed)

The Segment Traffic Volume Table provides a row for each segment or groups of segments with the same ADT. Columns include:

• Segment #: Each homogeneous highway segment is assigned a number by the CPM for purposes of identification. Numbers are sequential beginning with 1. The segment numbers correspond to the numbers listed in the Highway Segment Data Table. A new row is begun only when ADT changes, therefore, more than one segment may be listed in a given row.
• Station (Start): Beginning station for the homogeneous highway segment(s) in the given row.
• Station (End): End station for the homogeneous highway segment(s) in the given row.
• Evaluation Period - ADT (v/day): ADTs for each year in the evaluation period.

4.1.5  Intersecting Highway Traffic Volume (Current and Proposed)

The Intersecting Highway Traffic Volume Table provides a separate row for each intersection. Columns include:

• Intersection # - Each intersection is assigned a number by the CPM for purposes of identification. Numbers are sequential beginning with 1. The intersection number corresponds to the number in the Intersection Data Table.
• Station - The location of the intersection along the highway being evaluated.
• Intersecting Highway (Name) - The name of the intersection for the given row.
• Intersecting Highway (Major/Minor)
• Year - The year to which the ADT data in the given row applies.
• Leg (MJ1) - Major road, leg 1 (lower station).
• Leg (MJ2) - Major road, leg 2 (higher station).
• Leg (MN1) - Minor road, leg 1 (to right in direction of increasing station).
• Leg (MN2) - Minor road, leg 2 (to left in direction of increasing station).

4.2  Evaluation Report

The CPM Evaluation Report displays CPM results in tabular and graphical format and is accessed through the "Show Report" option. CPM graphs can be viewed, edited, and printed separately by selecting the "Show Graph" option. The Evaluation Report displays project, highway, and evaluation information at the top of the report, followed by the highway data described in Section 4.1, Highway Data. The evaluation results are then presented as described below.

4.2.1  Expected Crash Frequencies and Rates (for entire project within evaluation bounds)

This report summarizes the expected crash frequencies and rates for the entire project within the specified evaluation bounds. Crash severity breakdown is based on the default crash severity distribution. The values of this distribution can be viewed in Section 9., Default Tables.

4.2.2  Expected Crash Type Distributions

This report summarizes the expected crash type distributions for the project within the specified evaluation bounds. Crash type breakdown is based on the default crash type distribution. The values of this distribution can be viewed in Section 9., Default Tables.

4.2.3  Reporting Results by Homogeneous Analysis Section

The remaining output reports present the results by homogeneous analysis section, (homogeneous highway segment and intersection) as created in step 2 through step 4 of the Crash Prediction Algorithm.

4.2.3.1  Caution on 'Microanalysis' Use of CPM Results

The user has the option of generating (and viewing in the CPM Evaluation Report) several tables and graphs that summarize predicted crash rates and frequencies for the analysis section, including:

• A summary of predicted crash rates and frequencies for the entire analysis section
• Predicted crash rates and frequencies for individual CPM homogeneous highway segments
• Predicted crash rates and frequencies by horizontal design element (e.g., tangents and curves)

The user should apply caution in interpreting results for individual highway segments. The CPM algorithm is primarily intended for project-level crash predictions, rather than for microanalysis (i.e., predictions for small highway segments). Predictions for individual segments or short sections of highway will have a higher degree of uncertainty than predictions for longer highway sections. Therefore, the user should have much more confidence in the summary of predicted crashes for the entire analysis section, than in the predictions for individual homogeneous highway segments.

Crash predictions for individual highway segments are made available to the user to illustrate how the total was derived for the entire analysis section.

The user may compare relative levels of safety of individual highway segments (e.g., Curve A has a relatively higher number of predicted crashes than Curve B), but should understand that the absolute number of predicted crashes for a small segment will have a higher degree of uncertainty compared to the prediction for an extended length of highway.

4.2.3.2  Expected Crash Frequencies and Rates (by highway segment and intersection)

This report shows the expected safety performance of individual homogeneous highway segments and intersections in tabular form. Intersection entries include the intersection name or the name of the cross-road and are highlighted in yellow to make it easier to identify intersection locations.

4.2.3.3  Expected Crash Frequencies and Rates by Horizontal Design Element

This report shows the expected safety performance of individual homogeneous highway segments and intersections in tabular form. Intersection entries include the intersection name or the name of the cross-road and are highlighted in yellow to make it easier to identify intersection locations.

4.2.3.4  Plot of Expected Crash Rates by Highway Segment and Plot of Expected Crash Rates Plotted Using a Sliding Scale

This report includes two superimposed plots:

• A histogram of the expected crash rates (expressed in crashes/mi/year or crashes/km/year) by highway segment that were tabulated in the Expected Crash Frequencies and Rates (by homogeneous highway segment and intersection) Report
• A sliding scale plot of the expected crash rates (expressed in crashes/mi/year or crashes/km/year) for segments of a given fixed length at a given increment

The sliding scale plot has the effect of "smoothing" the data from the plot of expected crash rates by highway segment providing a better understanding of the effects of adjacent features rather than individual isolated features. Refer to Caution on Microanalysis Use of CPM Results for interpreting results from the Expected Crash Rates by Highway Segment Plot.

Three parameters are needed for the sliding scale analysis: the smoothing window or fixed segment length (W), the smoothing factor (1/n), and the moving interval or increment (I). The moving interval is equal to the smoothing window times the smoothing factor (I=W/n), and n is an odd number for the purposes of our evaluation.

The procedure for producing the sliding scale plot is as follows: (1) starting from the expected crash rate for each homogeneous highway segment, calculate the average crash rate within the window of length W, (2) plot this average at the mid-point of the smoothing window, (3) advance the window by the moving interval (I) and repeat the calculation.

4.2.3.5  Plot of Expected Crash Rates by Horizontal Design Elements

This plot is a histogram of the expected crash rates (expressed in crashes/mi/year or crashes/km/year) by horizontal design element that were tabulated in the Expected Crash Frequencies and Rates by Horizontal Design Element Report (see Section 4.2.3.3, Expected Crash Frequencies and Rates by Horizontal Design Element).

4.2.3.6  Plot of Expected Crash Frequencies by Intersection

This report is a plot of the expected crash frequencies (expressed in crashes/year) by intersection that were tabulated in the Expected Crash Frequencies and Rates (by homogeneous highway segment and intersection) Report.