How to calculate AADT from a short-duration traffic count
You've got a 48-hour tube count on a corridor segment and a capacity study due Friday. The raw count gives you volume for Tuesday and Wednesday in March. What you need is Annual Average Daily Traffic, the number that goes into the signal warrant analysis or the functional classification update. The gap between those two numbers is where most of the factoring headaches live.
Why a short count isn't AADT
A 24- or 48-hour count captures whatever traffic happened to be on the road during that window. Tuesday in March isn't August Saturday on a beach-access route, and it isn't the Wednesday after a holiday either. AADT smooths all of that into a single annualized average. To get there from a short count, you borrow factors from your state DOT's continuous count stations, the permanent recorders that run all year and already know what a typical day looks like at that time of year, on that type of road.
FHWA's Traffic Monitoring Guide lays out the standard approach: continuous stations get grouped by seasonal pattern (commuter, recreational, mixed), and your short-count site inherits the day-of-week and seasonal factors from whichever group it resembles.
The factors you're actually multiplying
Day-of-week factor. Converts your sample day's volume to an average weekday, or average day if you counted through the weekend, for that month. A Tuesday count on a commuter corridor usually needs a small downward adjustment. A Saturday count on the same road needs a much bigger one, in the other direction.
Seasonal adjustment factor. Converts the monthly average to an annual one. This is the factor that bites people on recreational or seasonal routes, where a March count can sit noticeably below the summer peak. Pull the factor for the month you counted, matched to the factor group for your site, and multiply.
Axle correction factor, if you ran tube counters. Tubes count axles, not vehicles, and trucks throw that off. If your count location doesn't have its own axle-correction data, use the state's standard factor for that functional class before you touch the raw number at all.
Multiply raw volume by the day-of-week factor, then the seasonal factor, then the axle correction factor if it applies, and you've got your estimated AADT.
Where this breaks down
Factor groups assume your link behaves like the continuous stations in its group. On most links that's a reasonable bet. On a link near a seasonal destination, a new development that opened after the base year, or a detour pattern from nearby construction, it isn't, and no factor table rescues a count that was never representative to begin with. The TMG says as much: factoring improves a representative short count, it doesn't fix an unrepresentative one.
That's also where the short-duration problem shows up before factoring even enters the picture. A lot of the links that need an AADT estimate for a capacity study or a signal-timing retime are the ones nobody has ever put a counter on: minor arterials, collector roads feeding a new interchange, segments where closing a lane to lay tubes for two days isn't worth the traffic control cost. Vehicle Counting turns a single scheduled satellite or aerial pass over the corridor into a vehicle tally by link and direction, no tube to lay, no lane closure, which at least gives you a count to factor on a link that would otherwise wait for the next full program cycle.
If you're staring at a link with no counting history and a deadline, a single-pass count might be the faster way to get a number into that factor equation.