What is a speed report?
Roadway speeds reports are presented by roadway classifications and posted speed limit. Reports are presented quarterly and as an annual trend. Speeds are presented using the average 85th percentile speed. This means that if the 85th percentile speed on a 2-lane 2-way rural road with a posted speed limit of 60mph is 64.3mph, 85 percent of the drivers are traveling at or below 64.3mph. Conversely, 15 percent of the drivers are traveling at 64.4mph or higher.
The 85th percentile speed best shows the operational speed at which a majority of drivers travel on a specific roadway type. For the purpose of speed tracking, the 85th percentile is superior to comparing an average speed because average speeds may be skewed by outliers – extremely slow or fast drivers. The average 85th percentile speed for a given reporting period is presented as a snapshot view of roadway speeds. Additionally, roadway speed data are also conservatively filtered to eliminate data collection or recording errors.
How do I read a speed report?
There are a total of 57 automated speed monitoring stations that continuously collect roadway speed data. These sites are located at one of five roadway categories. Speed data are aggregated by roadway category for the purpose of analysis and reporting.
- Urban Freeway
- Rural Freeway
- Rural 2-lane/2-way Highway
- Rural Divided Highway (non-freeway)
- Urban Divided Highway (non-freeway)
Each site collects hourly summary data 24 hours a day; seven days a week; 365 days per year. For any given hour, each site records the number of vehicles traveling within each of the following ranges:
<40 mph but =>45 mph
<45 mph but =>50 mph
<50 mph but =>55 mph
<55 mph but =>60 mph
<60 mph but =>65 mph
<65 mph but =>70 mph
<70 mph but =>75 mph
<75 mph but =>80 mph
<80 mph but =>85 mph
<85 mph but =>100 mph
<100mph but <=110
|Methodology (PDF)||Summaries (PDF)||Speed Trends (PDF)|
|Speed Monitoring Methods||2005-2011 Speed Trends Report|