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Traffic Statistics Background


The categories in Traffic Statistics are defined as follows.


Category Definition
Length of segment (miles) Length of highway/freeway segment.
Average highway speed Weighted average of the design speed within a highway section. (Design speed is a speed selected to establish specific minimum geometric design elements for a particular section of highway.) On non-engineered roads the average highway speed has been estimated.
Accident rate times statewide average Accident rate comparison or times the statewide average. The ratio of a segment accident rate to the statewide average accident rate for this type of facility in comparable terrain. Higher numbers indicate more dangerous routes.
Accidents per mile Accidents per year divided by the segment length in miles.
Fatal accidents per mile Fatal accidents per mile per year for last three calendar year average.
Fatal accidents and injuries per mile Fatal accidents and injuries per mile per year for last three calendar year average.
Length of segment urban area - miles Length of highway/freeway segment in urban area.
Present design hour traffic volume Present design hour traffic volume. Higher numbers indicate higher design capacity.
Operating Speed A computed value based on the Volume to Capacity ratio (defined below) and the average highway speed. It represents the present operating speed during the present design hour volume of traffic on existing highway geometrics. For segments of highway controlled by traffic signals, an “S” replaces the operating speed and generally represents speeds of 15 to 30 MPH.
Ratio of volume to capacity A ratio of traffic that the highway can carry to traffic that wants to use the highway. A value of 100 means the highway can provide desirable traffic service. A value of 200 means that traffic can double before service drops below desirable levels. A value of 50 means the highway is carrying twice the design amount of traffic volume but at a lower level of service than desired.Also called capacity adequacy.
Statewide accident rate for comparable road type Statwide average accident rate for the same type of facility in comparable terrain. The Statewide Accident Rate generally conforms to the Basic Expected Accident Rate for the Rate Groups as annually published by the Caltrans Traffic Operations Program in “Accident Data on California State Highways.”
Accident rate per 1,000,000 vehicle miles Accidents per million vehicle-miles averaged for the last three calendar years.
Fatality rate per 100,000,000 vehicle miles Fatal accidents per 100 million vehicle miles averaged for the last three calendar years.
Accident cost per mile per year (thousands of dollars) Accident cost per mile per year (in 1,000's of dollars) considers direct out-of-pocket costs such as property damage, medical treatment, and future lost earnings, but does not attempt to value the loss of life or limb.
Grade Grade line. Flat = 0-3% grade; rolling = 3-6%; moderate = 6% grade or more for less than one-half of the segment; steep = 6% grade or more for more than one-half of the segment
Terrain Adjacent topology as it affects construction costs (flat; rolling; mountainous). Flat reflects minor grading; rolling reflects moderate grading; mountainous reflects heavy grading as economic considerations.
Lanes Number of lanes, both directions.
Highway type Facility types are conventional, expressway, freeway, no existing State highway, undivided, divided