Saturation Flow Rate Calculation

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Saturation Flow Rate Calculator

Standard is 1,900 pcphpl.
All other areas (1.00) CBD – Central Business District (0.90)

Results

Adjusted Saturation Flow Rate (s):
0 vph

Adjustment Factors Calculated:

  • Lane Width Factor (fw):
  • Heavy Vehicle Factor (fHV):
  • Grade Factor (fg):

Understanding Saturation Flow Rate

In traffic engineering, the Saturation Flow Rate (s) represents the maximum hourly rate at which vehicles can cross a stop line at a signalized intersection, assuming the signal is always green. It is a critical parameter used in signal timing design and capacity analysis according to the Highway Capacity Manual (HCM).

The Calculation Formula

This calculator utilizes the simplified HCM method to adjust the base saturation flow rate based on prevailing site conditions:

s = s0 × N × fw × fHV × fg × fa

Key Adjustment Factors Explained

  • Base Saturation Rate (s0): Usually set at 1,900 passenger cars per hour per lane (pcphpl).
  • Lane Width Factor (fw): Narrower lanes reduce flow. Standard width is 12ft. If width is less, flow decreases.
  • Heavy Vehicle Factor (fHV): Trucks and buses occupy more space and take longer to accelerate. The formula accounts for the passenger car equivalent (ET) usually taken as 2.0.
  • Grade Factor (fg): Uphill grades slow down vehicles (especially trucks), reducing the flow rate, while downhill grades may slightly increase it.
  • Area Type Factor (fa): Central Business Districts (CBD) typically have more pedestrian activity and distractions, leading to a 10% reduction in saturation flow.

Practical Example

Suppose you have a 2-lane approach in a non-CBD area with 11-foot lanes, 5% heavy vehicles, and a 2% uphill grade. Using a base rate of 1,900:

  1. fw: 1 + (11 – 12)/30 = 0.967
  2. fHV: 100 / (100 + 5(2.0 – 1)) = 0.952
  3. fg: 1 – (2 / 200) = 0.99
  4. Total s: 1900 × 2 × 0.967 × 0.952 × 0.99 ≈ 3,460 vph
function calculateSaturationFlow() { // Get Input Values var s0 = parseFloat(document.getElementById("baseS0").value); var N = parseFloat(document.getElementById("numLanes").value); var W = parseFloat(document.getElementById("laneWidth").value); var HV = parseFloat(document.getElementById("hvPerc").value); var G = parseFloat(document.getElementById("gradePerc").value); var fa = parseFloat(document.getElementById("areaType").value); // Validate inputs if (isNaN(s0) || isNaN(N) || isNaN(W) || isNaN(HV) || isNaN(G)) { alert("Please enter valid numbers in all fields."); return; } // 1. Lane Width Adjustment Factor (fw) // HCM formula: fw = 1 + (W – 12)/30 var fw = 1 + (W – 12) / 30; if (fw < 0.5) fw = 0.5; // Safety floor // 2. Heavy Vehicle Adjustment Factor (fHV) // fHV = 100 / [100 + %HV(ET – 1)], where ET = 2.0 var fhv = 100 / (100 + HV * (2.0 – 1)); // 3. Grade Adjustment Factor (fg) // fg = 1 – (%G / 200) var fg = 1 – (G / 200); // Calculate final Saturation Flow Rate (s) var resultS = s0 * N * fw * fhv * fg * fa; // Output results document.getElementById("sValue").innerText = Math.round(resultS).toLocaleString(); document.getElementById("fwVal").innerText = fw.toFixed(3); document.getElementById("fhvVal").innerText = fhv.toFixed(3); document.getElementById("fgVal").innerText = fg.toFixed(3); // Show result container document.getElementById("flowResult").style.display = "block"; }

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