Calculator Inputs
Use the fields below to estimate directional capacity, lane service flow, operating pressure, and reserve margin.
Example Data Table
These examples show how changing geometry, demand, and heavy vehicles can shift capacity use and service quality.
| Scenario | Volume (veh/h) | Lanes | Lane Width (m) | Heavy Vehicles (%) | PHF | Terrain | Typical Outcome |
|---|---|---|---|---|---|---|---|
| Urban commuter segment | 4,800 | 3 | 3.65 | 8 | 0.95 | Level | Moderate v/c and stable flow |
| Freight corridor peak | 6,400 | 3 | 3.30 | 18 | 0.88 | Rolling | Higher pressure and weaker reserve |
| Mountain approach | 3,900 | 2 | 3.20 | 15 | 0.90 | Mountainous | Heavy vehicle effect becomes significant |
Formula Used
fHV = 1 / [1 + PT(ET - 1) + PR(ER - 1)]PT = heavy vehicle share, PR = recreational vehicle share, ET and ER depend on terrain.
Cadj,ln = Cbase × fLW × fSH × fSpeedThis calculator uses a screening base value of 2200 pc/h/ln, then applies lane, shoulder, and speed factors.
Ceff = Cadj,ln × N × PHF × fp × fHVN = lane count, PHF = peak hour factor, fp = driver population factor.
Demandeq = V / (PHF × fp × fHV)vpl = Demandeq / NThis converts the entered traffic stream into passenger-car equivalent flow for lane-based comparison.
v/c = vpl / Cadj,lnDensity ≈ vpl / SpeedLOS is estimated from approximate density bands and saturation level.
How to Use This Calculator
- Enter the directional peak hour traffic volume.
- Set the number of lanes serving that direction.
- Provide lane width, shoulder width, and free flow speed.
- Input PHF, driver population factor, and heavy vehicle percentages.
- Choose the terrain class that best matches the segment.
- Click Analyze Capacity to calculate capacity, reserve margin, lane flow rate, and LOS.
- Use the chart to see how demand growth affects saturation.
- Download the results as CSV or PDF for reporting.
Frequently Asked Questions
1) What does highway capacity analysis measure?
It estimates how well a road segment can serve traffic demand. The analysis compares entered volume against adjusted capacity, then reports operating pressure, reserve margin, estimated density, and approximate level of service.
2) Why is the volume to capacity ratio important?
The v/c ratio shows how close the segment is to saturation. Values well below 1.00 usually indicate stable performance. Values near or above 1.00 suggest congestion risk, unstable flow, and limited recovery after small disturbances.
3) How do heavy vehicles affect capacity?
Heavy vehicles occupy more road space and accelerate more slowly. The calculator converts their influence into passenger-car equivalents through the heavy vehicle factor, which lowers effective capacity when truck shares increase.
4) What is the role of peak hour factor?
PHF reflects how sharply demand spikes inside the hour. Lower PHF means demand is more concentrated in shorter intervals, which raises service flow pressure and reduces the effective directional capacity.
5) Why do lane width and shoulder width matter?
Narrow lanes and limited shoulders reduce driver comfort and operating efficiency. This screening model applies simple adjustment factors so geometry can reduce the capacity available for the same traffic demand.
6) Is the LOS result exact?
No. The LOS result here is a planning-level estimate based on simplified density screening. It is useful for quick comparison, but formal project work should use the exact method required by your adopted highway analysis manual.
7) When should I increase lane count?
Consider added lanes when v/c stays high during critical periods, reserve capacity becomes small or negative, and operational goals are not met. This tool also shows a rounded-up lane requirement for quick early-stage scoping.
8) Can I use this for reports and presentations?
Yes, for preliminary studies, concept screening, and internal comparisons. The built-in CSV and PDF options help document assumptions and outputs, but final report values should still be checked against your governing procedures.