What is Traffic Volume Study?
Traffic volume is the number of vehicles crossing a given section or point on a road per unit time. Traffic volume study involves systematically counting and analysing traffic flow to determine the quantity and pattern of traffic on a road network. It is the most fundamental of all traffic studies and forms the basis for all road planning, design, and management decisions.
Traffic volume is expressed in vehicles per hour (veh/hr), vehicles per day, or PCU/hour. It is measured using the traffic survey (census), which can be done manually by trained counters or automatically by electronic counting equipment.
Annual Average Daily Traffic (AADT)
AADT is the average 24-hour traffic volume at a given location calculated over a full 365-day period. It is the most comprehensive traffic volume measure and includes all seasonal variations — summer holiday peaks, monsoon dips, festival surges, and agricultural harvest movements.
AADT is used to: determine the relative importance of routes, justify road development and widening, compute design hourly volume, and calculate pavement structural design loads.
Average Daily Traffic (ADT)
ADT is the average 24-hour volume at a given location for a shorter period — at least 7 days but less than one year. It captures weekly traffic variations (weekend vs weekday differences) but not the full range of seasonal variation. ADT is used for shorter-term planning studies where full AADT data is not available or required.
30th Highest Hourly Volume (Design Hourly Volume)
Of all the 8,760 hours in a year, the 30th highest hourly volume is the volume exceeded in only 29 hours per year. This means it is exceeded just 0.33% of the time (29/8760 × 100).
This volume is adopted as the design hourly volume or design capacity because:
- Designing for the very highest hour (1st or 2nd) would be wasteful — that peak might occur only once or twice a year
- Designing for too low a volume would create frequent congestion
- The 30th highest hour represents a reasonable balance — traffic exceeds it only about 29 times annually
Indian condition: The 30th highest hourly volume = 8 to 10% of AADT. For example, if AADT = 2000 vehicles/day, the design hourly volume = 160 to 200 vehicles/hour.
Expansion Factors
Traffic counts are typically done for short periods (an hour, a day, or a week). To estimate annual traffic volumes, expansion factors are used to scale up the short-count data:
| Factor Type | Formula | Purpose |
|---|---|---|
| Hourly Expansion Factor | Total 24-hr volume / Volume for that hour | Estimate daily volume from 1-hour count |
| Daily Expansion Factor | Avg. weekly volume / Volume for that day | Estimate weekly volume from 1-day count |
| Monthly Expansion Factor | AADT / ADT for that month | Estimate AADT from 1-month count |
Traffic Volume Presentation Methods
- AADT/ADT Tables: Tabular data showing daily traffic volumes by location and time period
- Trend Chart: Volume plotted over years to show growth trends — used for future planning
- Traffic Flow Map: Road network map where line thickness is proportional to traffic volume — gives instant visual overview
- 30th HHV Graph: Hourly volume (% of AADT) plotted against number of hours exceeded per year — determines design volume
Types of Traffic Volume Counts
- Classified Volume Study: Counts of different vehicle types (cars, buses, trucks, motorcycles)
- Directional Study: Distribution of traffic across lanes and directions
- Turning Movement Study: Used in intersection and signal design, channelisation planning
- Pedestrian Traffic Volume: Used for footpath, crosswalk, and pedestrian signal design
Key Summary
- AADT = 365-day average (seasonal variation included)
- ADT = 7+ day average (weekly variation included)
- 30th HHV = design hourly volume = exceeded only 29 hr/year = 8–10% of AADT
- Expansion factors scale short counts to annual estimates
- Traffic notation: 1950 M65 = 1950 vehicles, mixed traffic, 65 km/h design speed