2.2 Interrupted Flow on Segment Streets

Introduction to Interrupted Flow on Segment Streets

Interrupted flow facilities differ fundamentally from uninterrupted ones due to the presence of external control devices (such as traffic signals, stop signs, and yield signs) that periodically interrupt the traffic stream. These interruptions introduce delay and queueing, which are not present in uninterrupted flow. In transportation engineering, segment street analysis covers facilities that experience these periodic interruptions, including two-lane highways (where passing maneuvers are constrained by opposing traffic) and urban street segments (where overall travel speed is heavily influenced by downstream signal delays).

Two-Lane Highways (HCM Chapter 15)

A two-lane highway is a roadway that has one lane in each direction. On these highways, passing maneuvers must be executed in the lane of opposing traffic when gaps permit. Consequently, the capacity and Level of Service (LOS) of two-lane highways are highly sensitive to directional split, volume, and the percentage of no-passing zones.

Classification of Two-Lane Highways

The Highway Capacity Manual (HCM) classifies two-lane highways into three distinct categories based on their primary function and driver expectations:

1. Class I: Major intercity routes, primary arterials, and daily commuter routes. On these highways, drivers expect to travel at high speeds. Therefore, both Average Travel Speed (ATS) and Percent Time-Spent-Following (PTSF) are used as the primary service measures. The worse-performing of the two metrics determines the overall Level of Service.
2. Class II: Scenic, recreational, or local access routes where drivers are less concerned with travel speed but still experience frustration from being delayed behind slow-moving vehicles. LOS is determined solely by the Percent Time-Spent-Following (PTSF).
3. Class III: Transition segments that pass through moderately developed areas, small towns, or suburban outskirts where speed limits are lower and access points are frequent. LOS is determined by the Percent of Free-Flow Speed (PFFS), calculated as the ratio of average travel speed to free-flow speed (PFFS = ATS / FFS × 100).

Demand Flow Rate Adjustment (v_p)

To analyze a two-lane highway, the raw hourly volume (V) must be adjusted to equivalent passenger-car conditions. The demand flow rate for directional analysis is computed as:

v_p = V / (PHF × f_G × f_HV)

Where:

• V = Hourly traffic volume (veh/h).
• PHF = Peak Hour Factor.
• f_G = Grade adjustment factor, which accounts for the effect of terrain (level, rolling, or specific grades) on two-lane highway operations.
• f_HV = Heavy-vehicle adjustment factor. For two-lane highways, the passenger-car equivalents (E_T and E_R) are different from freeways, reflecting the higher delay caused by heavy vehicles when passing is restricted.

Service Measure Calculations

• Average Travel Speed (ATS): The highway length divided by the average travel time of vehicles. It is estimated using the free-flow speed adjusted for flow rate and no-passing zones:

  ATS = FFS − 0.00776 · v_p − f_np

  Where f_np is the adjustment factor for the percentage of no-passing zones.

• Percent Time-Spent-Following (PTSF): The average percentage of time that vehicles travel in platoons behind slow-moving vehicles. It is estimated as:

  PTSF = BPTSF + f_d/np + f_HV

  Where BPTSF is the base percent time-spent-following, and f_d/np is the adjustment factor for the combined effect of directional split and the percentage of no-passing zones.

Urban Street Segments (HCM Chapter 18)

For urban street segments, travel speed is the primary service measure used to define Level of Service. The analysis evaluates a segment bounded by two signalized intersections, taking into account both midblock running speed and downstream control delay.

Segment Travel Speed (S_T)

The average travel speed of vehicles along an urban street segment is computed using the following equation:

S_T = 3600 × L / (T_R + d)

Where:

• S_T = Travel speed along the segment (mph).
• L = Segment length (miles).
• T_R = Midblock running time of vehicles on the segment (seconds). Running time represents the time required to traverse the segment if there were no intersection delay, and is a function of the segment length and the free-flow speed of the street.
• d = Control delay at the downstream boundary intersection (seconds).

Urban Street Classifications

Urban streets are categorized into four classes based on their design and functional category:

• Class I (Arterial): High-speed arterial streets designed for mobility, with a base free-flow speed (BFFS) of 45 to 55 mph.
• Class II (Arterial): Moderate-speed arterial streets with a BFFS of 35 to 45 mph.
• Class III (Collector/Sub-Arterial): Low-to-moderate speed arterials or collectors with a BFFS of 30 to 35 mph.
• Class IV (Local/Collector): Low-speed local streets or collectors with a BFFS of 25 to 30 mph.

Level of Service (LOS) Determination

The Level of Service of an urban street segment is determined by comparing the travel speed to the free-flow speed, representing the Percent of Free-Flow Speed (PFFS):

PFFS = (S_T / FFS) × 100