Inputs
Formula used
This calculator uses a planning estimate based on a speed-adjusted base capacity and multiplicative reduction factors:
- Per-lane base capacity: cpl = 1900 + 8(FFS − 60), bounded to 1600–2400 pc/h/ln.
- Length factor: fL = 0.60 + 0.40(1 − e−L/300), bounded to 0.60–1.00.
- Weaving intensity factor: fW = 1 − 0.30·Pw, bounded to 0.70–1.00.
- Heavy vehicle factor: fHV = 1 / (1 + PT(ET − 1)).
- Total capacity: C = cpl · N · fL · fW · fHV · fp.
- Demand ratio: v/c = V / C, used for service bands.
How to use this calculator
- Enter the number of continuous lanes through the weaving section.
- Measure weaving length between merge and diverge influence points.
- Provide free-flow speed from posted or observed off-peak conditions.
- Enter demand volume in passenger-cars per hour for the peak period.
- Estimate weaving proportion and heavy vehicle share for the same period.
- Adjust truck equivalency and driver factor for local conditions.
- Click Calculate to view capacity, v/c, and service level.
- Use CSV or PDF to attach results to your documentation.
Example data table
| Scenario | Lanes | Length (m) | FFS (km/h) | Demand (pc/h) | Weaving (%) | HV (%) | Capacity (pc/h) | v/c | LOS |
|---|---|---|---|---|---|---|---|---|---|
| S1 | 3 | 450 | 95 | 4,200 | 25 | 8 | 5,101 | 0.82 | D |
| S2 | 2 | 250 | 80 | 3,600 | 35 | 12 | 2,530 | 1.42 | F |
| S3 | 4 | 600 | 105 | 5,200 | 18 | 6 | 7,873 | 0.66 | C |
Weaving demand and operational meaning
Weaving capacity describes how much traffic a merge–diverge segment can carry while drivers still find acceptable gaps. The calculator converts your peak demand into a demand-to-capacity ratio, helping you anticipate turbulence, lane changing pressure, and queue risk during the design hour. Use consistent units, and prefer peak 15-minute counts expanded to an hourly rate for comparability.
Key geometric drivers for capacity
Lanes and weaving length dominate the estimate. More continuous lanes increase usable maneuvering space, while longer weaving length reduces forced lane changes per unit distance. Short sections amplify conflicts, so the length factor intentionally penalizes lengths below typical interchange spacing. When ramps are closely spaced, consider auxiliary lanes and clear lane balance to reduce last-second merges.
Speed, mix, and equivalency adjustments
Free-flow speed influences the base per-lane capacity because higher speeds often reflect better alignment and higher driver expectancy. Heavy vehicles are converted using an equivalency value, so trucks and buses consume more “passenger-car” space. The driver factor lets you reflect local discipline, signing clarity, and work-zone constraints. On grades, increase the truck equivalency and validate with observed truck speeds and platooning.
Scenario testing for design decisions
Use the example table approach to build three scenarios: existing conditions, opening year, and a stressed case with higher weaving percentage. Compare v/c to common planning targets such as 0.85 for stable operations and 0.93 for near-capacity monitoring. If v/c remains high, test added lane continuity, collector–distributor lanes, ramp metering, or longer spacing. Sensitivity runs also reveal whether demand growth or weaving share drives the bottleneck.
Documenting outputs for reviews
Export CSV for quick checking and sharing, and export PDF for reports and submissions. Record assumptions beside each run: demand source, conversion to passenger cars, chosen truck equivalency, and the basis for weaving percentage. For final design, align results with your jurisdiction’s method and field observations. Include a short narrative on risk: if v/c exceeds 1.00, identify mitigation and operational strategies. Track crashes and lane changes too.
FAQs
What is weaving percentage in this tool?
It is the share of section demand that performs lane changes to reach an exit or merge. Higher values mean more conflicts, so the calculator applies a reduction factor to represent reduced usable capacity.
How should I choose weaving length?
Use the effective distance between the merge influence point and the diverge influence point along the mainline. If signs or markings shift the maneuver earlier, use the shorter effective length to stay conservative.
When do I adjust truck equivalency?
Increase it on upgrades, tight curves, narrow lanes, or work zones where trucks lose speed and create platoons. Use local studies if available; otherwise run sensitivity cases from 1.8 to 2.5 to bracket outcomes.
What v/c target is recommended for planning?
Many teams use 0.85 as a stable target for peak operations and resilience. Values near 0.93 suggest monitoring and refinement. Values above 1.00 indicate over-capacity conditions and likely queuing.
Does the service level replace a full analysis?
No. It is a screening indicator derived from v/c bands. For final design, apply your jurisdiction’s accepted method, confirm lane balance and ramp geometry, and validate assumptions with counts, speeds, and observations.
Why do my results change after small input edits?
Weaving performance is sensitive to length, lanes, and weaving share. Small changes can shift reduction factors and the resulting capacity. Use scenario sets and document assumptions so reviewers can follow the logic.