HCM Chapter 11

Freeway Reliability Analysis

Estimate the travel time reliability of a freeway facility across a whole-year reporting period on Monday through Friday. This beta models demand variability by month and weekday plus randomly generated incidents, and optionally severe weather and a local demand multiplier table. Work zones and special events are not included.

Scope. The engine reproduces the scenario generation and the central reliability measures of the published HCM worked example within a few percent (median and mean TTI, misery index). Tail measures depend on the published engine's Monte Carlo stream and are not reproduced, the 95th percentile TTI runs up to about 20% high, because those measures depend on which scenarios the random draw pairs a severe incident with. Work zones and special events are not modeled on this page. Verify results independently before relying on them in engineering work, and please report discrepancies on GitHub.

Facility

The Chapter 10 seed dataset that every scenario is built from.

mi/h
%
veh/h

Comma-separated list, one value per 15-min analysis period. The list length sets the study period.

Segments

Ordered upstream to downstream. The facility must begin and end with a basic segment. Choosing Weaving opens that segment's own details card below the table.

Segment 1 · Basic1BasicSegment 2 · Merge2MergeSegment 3 · Basic3Basic

Seed facility, upstream to downstream. Widths follow segment length. Reliability results describe the whole facility across scenarios, so segments are not LOS-colored here.

#TypeLength (ft)LanesOn-Ramp Demand (veh/h)Off-Ramp Demand (veh/h)Ramp FFS (mi/h)Accel Lane (ft)Decel Lane (ft)
1
2
3

Scenario Generation

The reliability reporting period covers all 12 months on weekdays with HCM default demand ratios.

4 replications across 12 months and 5 weekdays yields 240 scenarios.

Keep the same seed to reproduce a scenario set.

per 100M VMT

The HCM national default is 4.9.

mi/h
Weather Events Not modeled

HCM Chapter 11 Step B-6. Timewise probability of each severe weather type by month (Equation 25-75) with its mean event duration, which the generator rounds to the nearest 15-min analysis period. Capacity and speed adjustments come from the Exhibit 11-20 and 11-21 defaults at the facility free-flow speed and are not edited here. Opening the panel seeds Example Problem 7's own weather, so nothing is applied blank.

Demand Multipliers Exhibit 11-18 defaults

A local demand multiplier table replacing the Exhibit 11-18 national ratios (Equation 25-72). Only each cell's ratio to the seed date's own multiplier is used, so any consistent base works. Weekend columns stay at zero while the reporting period is weekdays only. Opening the panel seeds Example Problem 7's Exhibit 25-100 table.

Facility Parameters Engine defaults

The four parameters the reliability run forwards to its internal Chapter 10 facility. Leave any of them blank to keep the engine default.

pc/mi/ln

Blank uses 190 pc/mi/ln.

%

Blank uses 7%.

/mi

Blank uses 1.0 ramps/mi.

/mi

Used by weaving segments. Blank falls back to the total ramp density, which is 1.0/mi where that is also blank.

The HCM reports Example Problem 7's summary measures probability-weighted over the observations and defines the reliability rating on VMT.

Outputs

Results populate after pressing Calculate. Every scenario is evaluated with the Chapter 10 core methodology, so the run can take a few seconds.

Scenarios Evaluated:
Travel Time Observations:
Free-Flow Travel Time (min):
Mean TTI:
50th Percentile TTI:
80th Percentile TTI:
95th Percentile TTI (PTI):
Misery Index:
Semi-Standard Deviation:
Expected Vehicle Hours of Delay (veh-h):
Share of Travel Below 45 mi/h (%):

Reliability Rating: