Overview
Percentage filters let you focus on the most critical roadways in your network by filtering based on where they rank. Instead of setting an absolute threshold like "crash rate above 5," you can find the top 10% highest crash rates in your state or other geographic area.
How The Percent Filter Works
Percentage filters work differently than other filters and needs four inputs
What Metric? Pick a column (crash rate, fatalities, severity index, etc.)
What Operator? Pick an Operator from the list
What Percent? Top or bottom percent
Of What? Choose a comparison area. The comparison area defines what you're measuring against. If you pick "State," you're finding the top X% statewide. If you pick "County," you're finding the top X% countywide.
Keep in mind that the selection will not filter the area that is displayed in the screening
When you add a percentage filter, all your other filters work normally by narrowing the results you see
State comparison area
If you set the comparison area to "State," the percentage calculation ignores any geographic filters in your filter bar. It looks at all roadways statewide.
But your geo filters still determine what locations you see in the results.
Example:
Percentage filter: Fatal Crashes ≥ 85% (State)
Road type filter: Functional Class = Rural Two-Lane
Location filter: District 3 (geo)
This finds rural two-lane highways in District 3 that have fatal crash counts in the top 15% of all rural two-lane highways statewide.
The state comparison uses ALL rural two-lane highways statewide (because of your road type filter), but you only see the ones in District 3.
How to use the filter
Click the filter bar and select a numeric column (crash rate, fatalities, EPDO, etc.)
In the operator field, toggle to enable percentage mode
Choose your operator (top %, bottom %, or range)
Enter your percentage value or range
Select your comparison area from the dropdown (State, County, City, etc.)
Click "Apply"
Analysis Example
Example 1: Statewide High-Risk Rural Roads in Your County
Goal: Find rural highways in your county that are dangerous compared to all rural highways statewide.
Filters:
Crash Rate ≥ 85% (State)
Functional Class = Rural Two-Lane
County = Your County
Result: Rural two-lane highways in your county with crash rates in the top 15% of all rural two-lane highways statewide.
Example 2: County High Risk Rural Roads
Goal: Find rural highways in your county that are dangerous compared to all rural highways in your county.
Filters:
Crash Rate ≥ 85% (Your County)
Functional Class = Rural Two-Lane
County = Your County
Result: Rural two-lane highways in your county with crash rates in the top 15% of all rural two-lane highways statewide.
Edge Cases
Empty Results
If your filters create an empty comparison pool, the percentage filter returns no results. This can happen when you combine filters that exclude everything.
Compare one area’s results to another area (This doesn’t have any safety value. It is an example of how the filters work)
Example:
Crash Rate ≥ 90% (County A)
County = County B
You see all County B roadways that have a crash rate that is 10% or higher than the 90th percential of crash rates of County B
All Filters Affect the Comparison Pool
Every filter in the filter bar narrows the comparison pool calculation, not just geographic filters.
If you filter to only show rural two-lane highways, the percentage calculation only considers rural two-lane highways when computing the percentile. This lets you do things like "top 10% of rural roads" or "top 10% of high-speed roads" by adding the appropriate filters first.
Tied Values at the Boundary
If multiple locations have the exact same value at the percentage cutoff, all of them are included.
Example: You filter for top 10% crash rate. The 90th percentile value is 8.5 crashes/mile. Three locations have exactly 8.5 crashes/mile. All three are included in the results.
Use Cases
HSIP Applications
Rank locations statewide and find the highest-risk sites in your jurisdiction for funding applications. Show that your projects target locations in the top 10% statewide, not just locally high-risk.
Vision Zero Priority Identification
Identify the worst 5% of locations in your city for immediate intervention. Revisit quarterly to track progress as you implement countermeasures.
Loss Alternative
Mathematically, LOSS IV is roughly equal to greater-than 80% SPF for the State
Equity Analysis
Compare crash rates within specific neighborhoods to citywide or statewide rates to identify underserved areas where crashes are disproportionately high.