Percentile Walkthrough Using Actual Speed Log Data (85th percentile)

This guide will walk you through calculating the percentile from speed bins captured during a given time period

This guide will walk you through calculating the percentile from speed bins captured during a given time period

Speed performance metrics for this deployment are derived from aggregated (binned) vehicle speed data recorded in defined speed ranges over fixed time intervals. The primary evaluation metric, the 85th percentile speed, is calculated using standard grouped-data traffic engineering methods based on cumulative frequency and linear interpolation within the percentile bin. This approach is widely accepted in FHWA and DOT speed-management studies when individual vehicle records are not retained. The bin resolution used provides sufficient accuracy for before-and-after speed analysis and effectiveness reporting, consistent with FHWA guidance and common practice for speed feedback sign evaluations. Raw per-vehicle speed records are not required for FHWA compliance, and the summarized speed distributions provided are adequate to assess changes in operating speeds and driver behavior.

TC Speed connect will automatically compute the 85th, 50th, & 25th percentile speed

Below is an example of how to calculate the 85th percentile manually

Speed Log example sheet

Above is the speed log we will be using for this example

1. Aggregate the Speed Bins (From the Spreadsheet)

Summing all 15-minute intervals across the file yields the following total vehicle counts by speed bin:

Speed Bin (mph)	Count (f)
Under 1	0
1–3	0
4–6	250
7–9	1,071
10–12	1,108
13–15	773
16–17	329
18–20	87
21–23	13
30 and Over	38

2. Total Vehicle Count (N)

3. Cumulative Frequency Table

Speed Bin	Lower Bound (L)	Upper Bound (U)	Width (w)	Count (f)	Cumulative (C)
4–6	4	6	2	250	250
7–9	7	9	2	1,071	1,321
10–12	10	12	2	1,108	2,429
13–15	13	15	2	773	3,202
16–17	16	17	1	329	3,531
18–20	18	20	2	87	3,618
21–23	21	23	2	13	3,631
30+	30	35	5	38	3,669