Example data table
These sample values demonstrate typical inputs and computed outputs for a multi‑lane urban corridor.
| ADT (veh/day) | K | D | PHF | Lanes | Growth | Years | PHV (veh/h) | DHV (veh/h) | v15 (veh/h) |
|---|---|---|---|---|---|---|---|---|---|
| 42,000 | 0.095 | 0.55 | 0.92 | 3 | 3.0% | 5 | 3,990 | 2,538 | 690 |
| 18,500 | 0.110 | 0.60 | 0.88 | 2 | 2.0% | 10 | 2,035 | 1,486 | 422 |
| 7,200 | 0.130 | 0.52 | 0.95 | 1 | 0.0% | 0 | 936 | 487 | 128 |
Formula used
This calculator uses common planning relationships:
- Peak hour volume:
PHV = ADT × K × SwhereSis the seasonal/event multiplier. - Directional peak volume:
DPHV = PHV × D. - Growth projection:
G = (1 + g)^{years}, thenDHV = DPHV × G. - Peak 15‑minute flow rate:
v15 = DHV ÷ (4 × PHF). - Time-window rate: The selected window scales rate: 60‑min = 1×, 30‑min = 2×, 15‑min = 4×.
- Equivalent passenger‑car flow:
v_eq = v × (1 − T) + v × T × PCE, whereTis truck share.
How to use this calculator
- Enter ADT for your roadway segment or approach.
- Set K, D, and PHF using local counts or agency guidance.
- Add growth and years if you need a design-year demand.
- Choose lanes in the peak direction for per‑lane demand.
- Adjust truck share and PCE when heavy vehicles matter.
- Press Submit to view results, then export CSV or PDF.
Peak-hour demand drivers
Peak traffic is not the daily average spread evenly. Commuter peaks, school runs, shift changes, and event surges create short windows. Planning starts with ADT, then applies a K‑factor to estimate peak hour volume. Urban arterials often fall near K=0.08–0.12, while recreational corridors can spike on weekends. Seasonal multipliers represent holiday conditions when counts are limited. If peaking is uncertain, run scenarios and compare rates.
Choosing K, D, PHF
K captures concentration, but direction matters. The D‑factor estimates the heavier direction during the peak hour; common commuter patterns range from 0.52 to 0.65, with 0.55 often used for balanced corridors. PHF measures within‑hour peaking using the ratio of hourly volume to four times the highest 15‑minute volume. Values near 0.95 indicate smoother flow; 0.85 suggests sharp surges. Lower PHF increases the calculated 15‑minute demand rate.
Growth and design year
For design checks, today’s peak may not govern. A 3% annual growth over five years increases demand by about 16%. The calculator applies compound growth, so longer horizons magnify differences between 1% and 4% assumptions. Use land‑use plans, permits, and regional forecasts to select a defensible rate, then test low, base, and high scenarios. Reporting a range improves transparency and review speed.
Heavy vehicles and equivalents
Trucks and buses consume more capacity than passenger cars, especially on grades or in dense weaving. Passenger‑car equivalents translate heavy vehicles into an equivalent flow rate. With 10% trucks and PCE=2.0, effective demand rises about 10%. On steep upgrades, PCE values of 3–4 are common, and the same truck share can raise effective flow by 20–30%. Use the equivalent rate when comparing against lane capacity.
Using outputs for decisions
Peak hour volume supports corridor screening, while the selected window rate informs operational design. Per‑lane demand highlights whether lanes, timing, or access management are warranted. The confidence band uses variability and percentile to express risk; higher percentiles produce conservative sizing for uncertain growth or event traffic. Export CSV and PDF to document assumptions and results consistently across alternatives and phases.