2.3 Intersection Capacity and LOS

Introduction to Intersection Capacity

Intersections are the critical bottlenecks in urban street networks. Their operational capacity and Level of Service (LOS) dictate the performance of the entire transportation corridor. Intersection analysis is divided into two primary categories: signalized intersections (where traffic movements are controlled by traffic signals) and unsignalized intersections (including Two-Way Stop-Controlled [TWSC], All-Way Stop-Controlled [AWSC], and roundabouts). The Highway Capacity Manual (HCM) provides the standard analytical procedures for both types, which are heavily tested on the NCEES PE Civil Transportation exam.

Saturation Flow Rate (s)

The saturation flow rate (s) represents the maximum rate of flow that can cross a signalized intersection approach under ideal conditions, assuming the signal is green 100% of the time and a continuous queue of vehicles is present. The standard base saturation flow rate (s_0) is 1,900 passenger cars per hour per lane (pc/h/ln).

In practice, this base rate must be adjusted for local roadway and traffic conditions using the following formula:

s = s_0 × N × f_w × f_HV × f_g × f_p × f_bb × f_a × f_LU × f_LT × f_RT × f_Lpb × f_Rpb

Where:

• N = Number of lanes in the lane group.
• f_w = Lane width adjustment factor. Base is 12 ft. Narrower lanes restrict lateral movement and reduce flow: f_w = 1.0 + (W − 12)/30.
• f_HV = Heavy-vehicle adjustment factor. Heavy vehicles occupy more space and accelerate slower: f_HV = 100 / [100 + P_HV × (E_HV − 1)], where E_HV = 2.0 is the passenger-car equivalent for heavy vehicles.
• f_g = Approach grade adjustment factor. Downhill approaches increase capacity, while uphill grades reduce acceleration: f_g = 1 − 0.005 · %G (where %G is positive for uphill grades and negative for downhill grades).
• f_p = Parking adjustment factor, which accounts for the friction caused by on-street parking maneuvers adjacent to the lane group.
• f_bb = Bus blockage factor, accounting for local buses stopping to receive or discharge passengers.
• f_a = Area type adjustment factor. Central Business Districts (CBD) feature higher pedestrian activity and friction, resulting in a factor of 0.90, whereas non-CBD areas have a factor of 1.00.
• f_LU = Lane utilization factor, which accounts for the unequal distribution of traffic across multiple lanes in a lane group. For a single lane, f_LU = 1.00; for two lanes, it is 0.952; for three lanes, it is 0.908.
• f_LT, f_RT = Adjustment factors for left turns and right turns, which account for conflicts with opposing traffic and pedestrians.

Approach Capacity (c) and Volume-to-Capacity Ratio (X)

The capacity of a lane group represents the maximum hourly volume that can pass through the intersection under prevailing conditions. It is calculated by multiplying the adjusted saturation flow rate by the green ratio:

c = s × (g / C)

Where:

• g = Effective green time for the lane group (seconds).
• C = Traffic signal cycle length (seconds).
• g/C = The green ratio, representing the fraction of the cycle allocated to that movement.

The Volume-to-Capacity ratio (X), also known as the degree of saturation, is computed as:

X = v / c = (V / PHF) / c

Where v is the demand flow rate adjusted for the Peak Hour Factor (PHF). When X < 1.0, the approach is undersaturated, meaning the green time is sufficient to clear the queue in most cycles. When X >= 1.0, the approach is oversaturated, resulting in compounding queues and excessive delays.

Peak Hour Factor (PHF)

The PHF represents the relationship between the hourly volume and the peak flow rate within that hour, accounting for short-term traffic surges. It is calculated as:

PHF = V / (4 × V_15)

Where V is the total hourly volume and V_15 is the maximum 15-minute volume. Demand flow rate is adjusted as v = V / PHF.

Control Delay (d)

Control delay is the primary performance measure used to determine Level of Service (LOS) at intersections. It includes acceleration delay, deceleration delay, and stopped delay. For a signalized intersection lane group, control delay is computed as:

d = d_1 · PF + d_2 + d_3

Where:

• d_1 = Uniform delay (seconds/vehicle). It assumes uniform arrivals and represents the delay if arrivals were perfectly spread out:

  d_1 = [0.5 · C · (1 − g/C)^2] / [1 − min(1,X) · g/C]

• PF = Progression adjustment factor. It accounts for the quality of signal coordination. Good progression (green wave) reduces delay (PF < 1.0). Poor coordination (vehicles arrive on red) increases delay (PF > 1.0). For isolated intersections (random arrivals), PF = 1.0.

• d_2 = Incremental delay, which accounts for random (Poisson) arrivals and cycle failures (oversaturation).

• d_3 = Initial queue delay, which accounts for pre-existing queues at the start of the analysis period.

Level of Service (LOS) Criteria

LOS thresholds for intersections are based on the average control delay per vehicle. Note that unsignalized intersections have lower delay thresholds because drivers have a lower tolerance for delay when they must actively yield or stop without a signal.