In traffic engineering, the K-factor is a critical parameter used in roadway design and traffic analysis. It represents the proportion of Annual Average Daily Traffic (AADT) that occurs during the design hour, and it allows engineers to size and evaluate roadways for their most critical operating conditions.
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What is the K-Factor?
Formally, the K-factor is defined as:
K= DHV /AADT
Where:
- DHV = Design Hourly Volume (vehicles per hour, usually the 30th highest hour of the year)
- AADT = Annual Average Daily Traffic (vehicles per day)
The K-factor therefore expresses the share of total daily traffic that occurs in the chosen design hour.
For example, if a road carries 40,000 vehicles per day (AADT) and the design hour volume is 3,200 vehicles, then:
K= 3,200 / 40,000 = 0.08 (8%)
This means that about 8% of daily traffic occurs during the design hour.
Why the K-Factor Matters
- Design of Roadway Capacity
Highways and arterials must be designed to operate under heavy demand conditions. Using the K-factor, engineers convert AADT into design hour volumes (DHV), which are then compared with the roadway’s capacity. - Traffic Impact Studies
For development-related analyses, K-factors help estimate peak hour traffic from projected daily trips. - Consistency in Planning
Because AADT data is widely available, applying a locally appropriate K-factor ensures consistent, comparable estimates of design-hour conditions.
Typical Ranges of K-Factors
K-factors vary by facility type, location, and trip purpose:
- Urban Freeways: 0.07 – 0.09 (7–9% of AADT in peak hour)
- Rural Highways: 0.12 – 0.18 (12–18%, reflecting more directional commuter flows)
- Major Arterials: 0.08 – 0.12
- Local Streets: often not defined, since they are not designed on K-factor principles.
In general, rural highways have higher K-factors than urban roads because a larger share of daily travel is concentrated in commuting hours, while urban travel tends to be more evenly spread throughout the day.
Directional Factor (D-Factor)
Often used alongside the K-factor is the D-factor, which represents the proportion of DHV traffic moving in the peak direction of travel. Together, K and D allow conversion from daily traffic counts to peak hour, peak direction volumes used in capacity analysis. Try our PHF Calculator
VPHF=AADT×K×D
Where VPHF is the peak hour, peak direction volume.
How Engineers Use the K-Factor
- Capacity analysis: feeding into Highway Capacity Manual (HCM) methodologies.
- Roadway widening/upgrade studies: predicting future design-hour volumes under growth scenarios.
- Traffic forecasting models: converting projected AADT outputs into operational traffic flows.
- Development review: estimating peak hour volumes from trip generation rates.
Sources of K-Factor Data
K-factors are typically derived from permanent traffic count stations and summarized in:
- State DOT traffic count manuals (e.g., FDOT, TxDOT, MTO in Ontario).
- Highway Capacity Manual (HCM) guidance.
- Local jurisdiction planning guidelines.
Where direct data is unavailable, transportation engineers apply regional default values from DOT tables.
Conclusion
The K-factor is a simple but powerful parameter that bridges the gap between daily traffic counts (AADT) and design-hour conditions used in capacity analysis and roadway design. By applying an appropriate K-factor (and D-factor), transportation engineers ensure roadways are designed to handle their most critical traffic conditions safely and efficiently.
