The SPF Manager Segments (Beta) application facilitates creating, managing, evaluating, and updating your agency’s Safety Performance Functions (SPFs) in an easily accessible interface. Agencies that have not yet developed SPFs can use the SPF manager to generate unique SPFs for their roadways based on the Optimal Fit function. Agencies with previously developed SPFs can use the SPF Manager to evaluate the performance of their SPFs for each segment sub-type, and make any modifications as needed to improve their performance. Additionally, all agencies can use the SPF Manager to keep a running history of their SPFs and track how they have changed over time.
The Segment Selector section of SPF Manager allows users to search for and select which segment type they would like to evaluate. All segments are grouped according to the following characteristics: Interstate/Non-Interstate, Urban/Rural, and the number of lanes. Users can use the column on the left to select the roadway characteristics. With each click, the column on the right will display all segment groups that meet the selected grouping criteria. Users can then select the group to evaluate from the second column, and the results will be displayed in the SPF preview on the right side of the page.
(The Segment Selector, with the Major Collector roadway type selected.)
Once the user has selected the Segment Grouping to evaluate, the SPF preview will display the actual Safety Performance Function, and the date on which it was last modified. Administrators will also have the ability to edit the Function and view the history of the SPF. This can be used to evaluate the effectiveness of any updates or changes to the Function.
(The SPF Preview window displaying the results of the Major Collector, Rural, 3 Lane Segment Grouping.)
If no Function is currently defined, administrators will be prompted to set the Function to the Optimal Fit, which is Numetric’s recommended SPF for that Segment Grouping (for more detail on the Optimal Fit feature, see the Optimal Fit section of this article). Alternatively, administrators can edit the Function to enter their own SPF for that Segment Grouping.
(The SPF Preview with no Function displayed.)
When a Segment Grouping with an operational Function is selected, users can see a graph that plots each segment that meets the grouping criteria. Each segment is plotted with the number of Crashes Per Mile on the vertical axis, and the AADT on the horizontal axis. Each point on the graph represents a Segment. The Blue line represents the SPF, as it is currently defined, and the CURE, and ±2 Sigma are indicated by dotted and dashed lines respectively. Additionally, the R-squared value is provided to help users understand the distribution of the segments.
(the SPF Preview graph with the SPF, CURE, ±2 Sigma and R-squared values displayed.)
The Overview window displays three metrics related to the selected Segment Grouping: % of Roadway, % of Crashes, and # of segments.
The % of Roadway metric indicates the percentage of the total statewide roadway that is represented in this Segment Grouping. In the example below, 0.9% of the total statewide roadway system is represented by this Segment Grouping.
The % of Crashes metric indicates the percentage of the total number of statewide crashes represented in this Segment Grouping. In the example below, 0.1% of the number of statewide crashes are represented by this Segment Grouping.
The # of Segments metric indicates the number of segments contained in this Segment Grouping. In the example below, there are 71 segments contained in this Segment Grouping.
(The SPF Overview window with the % of roadway, % of crashes, and # of segments metrics displayed.)
Clicking on any of the points in the SPF Preview chart will highlight that segment in the chart, and open a flyout for the selected segment. This Segment Flyout contains the roadway details, and summary of the selected segment, as well as a Google Streetview thumbnail for the segment.
Additionally, the selected segment (point) will display the Expected Crashes per mile based Empirical Bayes calculations. Additionally, the Adjusted Performance Expectations section in the Segment Flyout will display the expected crashes per mile, and expected crash rate, also based on Empirical Bayes calculations.
(The flyout for a selected Segment.)
Administrators can also use the SPF Manager to edit and revise individual Functions. These edits are tracked and allow for historical analysis - making it easy to see if the revisions to the Function are having the desired outcome.
(The Edit Function window, with the user modifying a portion of the selected function.)
The SPF Manager Segments application provides its users with an optimal fit functionality, which can be used to generate an SPF based on Numetric’s Optimal Fit feature, which minimizes the average error between the annual crash rates for each of the segments and the SPF.
If an SPF is not entered for the selected Segment Grouping, a button will display the option to Set to Optimal Fit. By clicking on this, administrators can set Numetrics Optimal Fit as the Function for that Segment Grouping.
(The Set to Optimal Fit button in the Function Preview window of a given Segment Grouping.)
SPF Manager allows administrators to track and view the history of their SPFs. This allows agencies to track the performance of their SPFs over time, and track historical changes to the function.
(The SPF History window, displaying a historical snapshot of the modifications made to the given function.)