Traffic Factors
Factors are used to adjust short-term traffic counts. They are used to estimate "average" conditions and to account for variability in the traffic stream. Short-term counts are typically collected for 48-hour periods with counters that record hourly data.
Factors are calculated from continuous counters or Automatic Traffic Recorders (ATRs). The Office of Transportation Data has increased the number of ATR sites to more than 3 times the amount than in the past. There are currently 307 ATR sites located on various roads throughout the State and we plan to increase this number to 354 ATR sites.
As a result of the increased number of ATR sites, it has been possible to provide additional Traffic Factor Categories that are more specific to each Functional Class. We have improved our overall system analysis of Georgia roadways and traffic patterns. For example, the new Traffic Factor Categories differentiate between Urban Major Arterials in Atlanta and Urban Major Arterials Not in Atlanta.
Factor Groups
The Office of Transportation Data plans to review our factoring approach every year based upon a variety of factors: additional ATR sites that are installed, traffic trends/patterns, high growth areas, truck traffic patterns, etc. The continuous counters or Automatic Traffic Recorders (ATRs) are currently grouped into 16 traffic factor groups:
| Factor Group |
Descriptions |
| Rural - Minor Arterial |
Routes which provide service to corridors with trip lengths and travel densities greater than that of collectors or local roads (i.e. Appling County SR-15) |
| Rural - Major Arterial |
Routes which have a trip length and travel density indicative of substantial state wide travel (i.e. Bartow County SR-20) |
| Rural - I-75 |
I -75 Outside of an Urban Area |
| Rural - I-85 |
I-85 Outside of an Urban Area |
| Rural - Interstates |
Interstates that are accessed only using ramps and are outside of an Urban Area (i.e. I-16 in Bryan County) |
| Small Urban - Local Collector |
Local roads or collectors in a small urban area which provide low levels of travel mobility (i.e. a subdivision road or intra-county travel corridor) |
| Small Urban - Arterial |
Routes of moderate to high lengths and travel that are within a small urban area (i.e. Bulloch County SR-67 Bypass) |
| Small Urban/Urban - Freeways Interstate (Not Atlanta) |
Freeways, Expressways and Interstates which are restricted access roadways and not in Atlanta (i.e. I-95) |
| Urban - Local Collector |
Local roads or collectors in an urban area which provide low levels of travel mobility (i.e. a subdivision road or intra-county travel corridor) |
| Urban - Minor Arterial (Not Atlanta) |
Routes that serve the major centers of activity with the highest traffic volumes and the longest trip lengths not in Atlanta (i.e. Catoosa SR-146) |
| Urban - Major Arterial (Not Atlanta) |
Routes that serve the major centers of activity with the highest traffic volumes and the longest trip lengths not in Atlanta (i.e. Catoosa SR-146) |
| Urban - Minor Arterial (Atlanta) |
Routes which provide trips of moderate length linking principal arterials in Atlanta (i.e. Fulton County SR-3) |
| Urban - Major Arterial (Atlanta) |
Routes that serve the major centers of activity with the highest traffic volumes and the longest trip lengths in Atlanta (i.e. Cobb County SR-5) |
| Urban - Freeways Interstate (Atlanta) |
Freeways, Expressways and Interstates which are restricted access roadways and are in Atlanta (i.e. I-75, I-85, SR-400, I-985) |
| Urban - I-285 (Atlanta) |
I-285 Loop |
Annual Average Daily Traffic
The Annual Average Daily Traffic (AADT) for a portable count is calculated as follows:
AADT = ∑ 24-hour Short Term Volume * Daily Factor * Monthly Factor * Axle Factor
Monthly Adjustment Factors
Monthly Adjustment Factors are applied to counts taken with pneumatic tube counters (portable counters) that do not cover periods long enough to account for traffic variation from month to month.
Daily Adjustment Factors
Daily Adjustment Factors are applied to counts taken with portable traffic counters that do not account for differences in traffic for all days of the week.
Axle Adjustment Factors
Axle Adjustment Factors are applied to counts taken with portable traffic counters which register two axle impacts as one vehicle. It is used to account for vehicles with more than two axles, typically trucks with three or more axles, in the traffic stream on a particular type of road.
Federal Highway Administration Traffic Monitoring Guide
For a more detailed discussion of factors, please refer to the Federal Highway Administration (FHWA) Traffic Monitoring Guide: http://www.fhwa.dot.gov/ohim/tmguide/.
Disclaimer: The Georgia Department of Transportation makes no representation or warranties, implied or expressed, concerning the accuracy, completeness, reliability, or suitability for any particular purpose of this information and data contained on this Web Site.