Intersection Influence Area

Intersection Influence Area

The influence area (sometimes called functional area) around an intersection is the area that experiences direct and frequent impact from intersection operations. This is measured as a distance from the center of the intersection out along each intersecting leg. UDOT has designated an area of influence around all intersections based on the intersection control type as shown below.

Intersection Type and Influence Area

1. Signal control 300 feet

2. Minor leg stop control 150 feet

3. All way stop control 100 feet

4. Yield control 100 feet

5. Uncontrolled 100 feet

6. Roundabout 300 feet

7. Offset Left-turn (CFI) 400 feet for primary intersection, 300 feet for upstream left-turn signals

8. Median Thru-U Turn 400 feet for primary intersection, 200 feet for downstream u-turn signals

9. R-Cut 400 feet for primary intersection, 300 feet for downstream left-turn signals

10. SPUI 500 feet

11. DDI 400 feet for each intersection

12. Active Transportation Only 100 feet

13. Railroad Crossing 100 feet

Each intersection is unique and actual influence areas may vary. When analyzing a single location site-specific influence areas may be determined. For large-scale or network level analysis these distances should be used for consistency. To simplify analysis UDOT measures the same distance for both entering and exiting traffic on each leg.

Defining and Analyzing Intersection vs. Segment Crashes

When performing network level analysis crashes must be separated and analyzed as “Intersection Crashes” and “Segment Crashes”. This avoids overrepresentation of segments with lots of intersections and allows for correct comparison of crash types.

Intersection Crashes

Analyze as an intersection crash if:

• The crash occurred inside the intersection influence area

AND

• The officer marked the crash as intersection related

All crashes that occur within the influence area of an intersection AND are identified as “intersection related” by the officer's crash report are considered “Intersection Crashes”. Crashes may occur within the intersection but have nothing to do with intersection operations or behavior, so the officer will not indicate them as intersection related. This rule allows those to be excluded.

Segment Crashes

Analyze as a segment crash if:

• The crash occurred outside any intersection influence area

OR

• The crash is inside an intersection influence area but the officer marked it as not intersection related

Segment crashes are any crash that was not related to an intersection or falls completely outside of any intersection influence area. Some crashes may occur outside intersection influence areas but are still indicated as “intersection related” by the officers report. These should still be analyzed with segment crashes because their distance from the intersection suggests that the intersection had very little actual impact.

The final combined graphic below shows the overlap and interaction of the two groups.