HCM Chapter 17

Urban Street Reliability and ATDM

Estimate the travel time distribution, travel time index, and reliability rating of a signalized urban street facility over a one-year weekday reporting period with generated weather, demand, and incident scenarios.

Scope. The compute engine is boundary-validated against HCM Chapter 29, Example Problem 4 (Exhibits 29-62 through 29-77), which the page defaults reproduce, along with the Example Problem 5 Strategy 1 and Chapter 37 adaptive signal control directions of effect. Verify results independently before relying on them in engineering work, and please report discrepancies on GitHub.

Facility

The signalized segment chain the reliability run evaluates, drawn upstream to downstream in the subject direction of travel.

Segment 1 · 2,640 ft12,640 ftSegment 2 · 2,640 ft22,640 ftSegment 3 · 2,640 ft32,640 ftSegment 4 · 2,640 ft42,640 ftSegment 5 · 2,640 ft52,640 ftSegment 6 · 2,640 ft62,640 ft

Segment chain, upstream to downstream, separated by its signalized boundary intersections. Widths follow segment length, depth the through-lane count, and the ticks below each link are its subject-side access points. Click a segment to highlight its card. The decks are not colour coded: the reliability method reports one travel time distribution for the whole facility and exposes no per-segment result to tint them with.

Reliability Reporting Period

Weekdays of a full year. Demand ratios follow the HCM defaults for the selected functional class.

h

7 starts the study period at 7 a.m. It is also the hour of the base traffic count.

15-min

12 periods cover a 3-hour study period.

Anchors the calendar. A wrong day puts every Exhibit 17-6 day-of-week demand factor on the wrong date.

%

Passed through to the Chapter 18 segment evaluation of each scenario.

Weather

Monthly normals for the facility's location, January through December. Defaults are the Lincoln, Nebraska record of Exhibit 29-65.

MonthTotal Precipitation (in.)Days with PrecipitationNormal Daily Mean Temperature (F)Precipitation Rate (in./h)
January
February
March
April
May
June
July
August
September
October
November
December

There is no snowfall column. The Chapter 29 procedure decides whether an event falls as rain or as snow from the sampled temperature (Equations 29-3 and 29-4) and sizes the snow depth from the precipitation columns, so a snow climate is entered through the mean temperature and precipitation values above.

Incidents

Crash frequencies drive the incident generator. Per-segment values are entered with each segment below.

crashes/yr

The intersection at the upstream end of the facility.

veh/h

The three seeds fix the weather, demand, and incident streams. Rerunning with the same seeds reproduces the run exactly; any other seeds give an equally valid replication.

ATDM Strategies

Strategies, work zones, and special events are applied to every scenario as input-level adjustments (Chapter 17, Section 4). A blank cell leaves that input alone.

No strategies. The run evaluates the facility as entered.

The green adjustment is added to the coordinated through phase at every boundary signal, which is what Example Problem 5 Strategy 1 does with 5 s of split. The saturation flow adjustment multiplies the boundary saturation flow rate. Adaptive signal control enters here as a saturation flow adjustment of 1.156, the value Chapter 37 implies for its default 13.5% delay reduction target.

Segment 1

Signalized Chapter 18 segment, ordered upstream to downstream.

ft
ln
mph
veh/h

Demand of the base traffic count. Scenario demands are scaled from it.

s
s
veh/h/ln
pts
pts
stops/veh
crashes/yr
crashes/yr
ln

All lanes on the boundary signal's approach, used by the incident generator.

Share of the daily crash count exposed during the study period.

Local adjustment on the crash-to-incident conversion.

Segment 2

Signalized Chapter 18 segment, ordered upstream to downstream.

ft
ln
mph
veh/h

Demand of the base traffic count. Scenario demands are scaled from it.

s
s
veh/h/ln
pts
pts
stops/veh
crashes/yr
crashes/yr
ln

All lanes on the boundary signal's approach, used by the incident generator.

Share of the daily crash count exposed during the study period.

Local adjustment on the crash-to-incident conversion.

Segment 3

Signalized Chapter 18 segment, ordered upstream to downstream.

ft
ln
mph
veh/h

Demand of the base traffic count. Scenario demands are scaled from it.

s
s
veh/h/ln
pts
pts
stops/veh
crashes/yr
crashes/yr
ln

All lanes on the boundary signal's approach, used by the incident generator.

Share of the daily crash count exposed during the study period.

Local adjustment on the crash-to-incident conversion.

Segment 4

Signalized Chapter 18 segment, ordered upstream to downstream.

ft
ln
mph
veh/h

Demand of the base traffic count. Scenario demands are scaled from it.

s
s
veh/h/ln
pts
pts
stops/veh
crashes/yr
crashes/yr
ln

All lanes on the boundary signal's approach, used by the incident generator.

Share of the daily crash count exposed during the study period.

Local adjustment on the crash-to-incident conversion.

Segment 5

Signalized Chapter 18 segment, ordered upstream to downstream.

ft
ln
mph
veh/h

Demand of the base traffic count. Scenario demands are scaled from it.

s
s
veh/h/ln
pts
pts
stops/veh
crashes/yr
crashes/yr
ln

All lanes on the boundary signal's approach, used by the incident generator.

Share of the daily crash count exposed during the study period.

Local adjustment on the crash-to-incident conversion.

Segment 6

Signalized Chapter 18 segment, ordered upstream to downstream.

ft
ln
mph
veh/h

Demand of the base traffic count. Scenario demands are scaled from it.

s
s
veh/h/ln
pts
pts
stops/veh
crashes/yr
crashes/yr
ln

All lanes on the boundary signal's approach, used by the incident generator.

Share of the daily crash count exposed during the study period.

Local adjustment on the crash-to-incident conversion.

The run evaluates roughly three thousand scenarios and can take a few seconds.

Outputs

Results populate after the reliability run completes.

Scenarios Evaluated:
Weather Events Generated:
Incidents Generated:
Oversaturated Scenarios:
Base Free-Flow Travel Time (s):
Mean Travel Time (s):
Mean Travel Time Index:
50th Percentile TTI:
80th Percentile TTI:
95th Percentile TTI (PTI):
Total Vehicle Hours of Delay (veh-h):
Scenarios with Nondry Weather (%):

Urban Street Reliability Rating: