The Highway Capacity Manual classifies roadways based on access control and directional flow. Each facility type has a distinct set of operational characteristics, and the methodology for calculating capacity and LOS is specifically tailored.

Highway capacity analysis is a fundamental tool for planning, designing, and operating transportation facilities. It determines the maximum traffic flow a facility can accommodate and evaluates the quality of traffic flow provided to motorists.

Capacity is not a fixed, theoretical number; it depends heavily on specific "prevailing conditions":

Traffic flow is not uniform throughout the peak hour. There are often shorter, intense bursts of traffic (usually 15-minute intervals) that dictate the true operational stress on the facility. The Peak Hour Factor (PHF) accounts for this peaking phenomenon.

The PHF ranges from 0.25 (highly concentrated traffic in one 15-minute period) to 1.00 (perfectly uniform flow). In capacity analysis, the hourly volume ($V$) is typically divided by the PHF to determine the equivalent peak flow rate ($v_p$) used for analysis.

Unlike capacity (which is the absolute maximum flow before breakdown, or LOS E), a service volume is the maximum flow threshold for a specific, acceptable LOS (e.g., the service volume for LOS C).

While capacity tells us how much traffic a road can handle, Level of Service (LOS) tells us how well the road is handling it from the driver's perspective. It is a qualitative measure describing operational conditions within a traffic stream.

Modern capacity analysis no longer exclusively prioritizes vehicular throughput. The HCM now includes comprehensive methodologies for Multimodal Level of Service (MMLOS), which calculates distinct, separate LOS grades for:

A street might score an LOS B for cars, but an LOS F for pedestrians if it lacks sidewalks or safe crossings. Urban street design requires balancing these competing MMLOS objectives within a limited right-of-way.

A basic freeway segment is a section of a divided highway with full control of access, where the flow is not influenced by merging (on-ramps), diverging (off-ramps), or weaving sections.

For basic freeway segments, LOS is determined strictly by Density.

We must convert the raw hourly volume ($V$, mixed vehicles/hr) into an equivalent flow rate under ideal conditions expressed in passenger cars per hour per lane (pc/h/ln) for the peak 15 minutes.

Where:

Calculate the actual FFS based on geometric characteristics.

Where:

Calculate density using the fundamental equation $k = q/u$, replacing variables with our calculated equivalents.

Where $S$ is the estimated average passenger-car speed (derived from $FFS$ and $v_p$). Once $D$ is calculated, compare it to the HCM density thresholds to determine the final Level of Service (A-F).

For interrupted flow facilities (urban streets and intersections), density and speed are poor indicators of service quality. Instead, the HCM uses Control Delay (d) as the primary measure of effectiveness for LOS at intersections.

Beyond basic freeway segments, engineers must also analyze the critical transition zones where traffic enters, exits, or crosses paths on a freeway. These areas are frequent bottlenecks and sources of capacity degradation.

If field measurement is not possible, FFS is estimated by adjusting the base free-flow speed (BFFS) for factors like lane width ($f_{LW}$), right-shoulder lateral clearance ($f_{LC}$), and interchange density ($f_{ID}$):

The Heavy Vehicle Adjustment Factor ($f_{HV}$) uses Passenger Car Equivalents ($E_T$ for trucks/buses, $E_R$ for RVs) to convert heavy vehicles into an equivalent number of passenger cars:

Where $P_T$ and $P_R$ are the proportions of trucks and RVs in the traffic stream.