Annual Average Daily Traffic (AADT) is one of the most fundamental measures in traffic engineering and transportation planning. It represents the average number of vehicles that travel on a roadway segment each day over the course of an entire year.
Put simply, it is a way to smooth out traffic variations across weekdays, weekends, and seasons, giving planners and engineers a consistent value for roadway design and analysis.
How to Calculate AADT
Average Annual Daily Traffic is not just a simple daily traffic count. Traffic varies by time of day, day of week, and season — so engineers use traffic counts combined with adjustment factors to estimate an annual average.
Step-by-Step Process:
- Collect Traffic Counts
- Short-term counts (e.g., 24-hour or 48-hour) using automatic traffic recorders.
- Permanent count stations also provide continuous data where available.
- Apply Seasonal & Day-of-Week Adjustment Factors
- Traffic on a highway in July may be higher than in January.
- Adjustment factors account for these seasonal and weekly differences.
- Account for Axle Corrections (if applicable)
- Some counters measure axles, which must be converted to vehicles.
- Calculate the Annual Average
- Adjusted volumes are averaged across the year to produce the AADT.
AADT Formula:
AADT=Total annual traffic volume / 365
Why is AADT Important?
- Roadway Design – Selecting lane requirements, pavement thickness, and intersection geometry. It is also used for calculating ESALs. Try using our quick ESAL converter calculator.
- Traffic Impact Studies – Estimating project effects on local roads. What is a traffic impact study anyways?
- Safety Analysis – Comparing crash rates per million vehicle miles traveled.
- Funding & Policy Decisions – Prioritizing roadway investments and maintenance programs. Can be used to calibrate transportation models.
Example Calculation
Suppose a 48-hour traffic count records 22,000 vehicles on a roadway.
- The average daily traffic (ADT) = 11,000 vehicles/day.
- A seasonal factor of 1.10 is applied (to account for higher annual average).
AADT= ADT × Seasonal Factor=11,000 × 1.10 = 12,100
So the Average Annual Daily Traffic is 12,100 vehicles/day.
FAQs About AADT
What is AADT?
AADT stands for Annual Average Daily Traffic, the average daily vehicle count on a road over a year.
How is AADT different from ADT?
ADT is an average for the count period only. AADT adjusts ADT to reflect yearly variations.
Why is AADT important for roadway design?
Because it provides a consistent traffic measure for capacity analysis, safety studies, and infrastructure planning.
What tools are used to calculate AADT?
Automatic traffic recorders, manual counts, and statistical adjustment factors published by state/provincial transportation agencies.
How to calculate ESALs using AADT?
ESALs (Equivalent Single Axle Loads) are calculated by combining AADT with traffic composition and axle load equivalency factors. First, the truck portion of the Average Annual Daily Traffic is determined (using classification counts or truck percentages). Each vehicle type is then converted into ESALs using standard load equivalency factors (LEFs). These daily ESALs are multiplied by 365 and projected over the pavement’s design life, applying traffic growth rates. The result is the cumulative ESALs a pavement must be designed to carry.
