3.1 Basic Freeway Segments & Multilane Highways (HCM 7th Edition)

Basic Freeway Segments & Multilane Highways (HCM 7th Edition)

Uninterrupted flow facilities—principally freeways and multilane rural/suburban highways—have no fixed external causes of delay, such as traffic control signals, stop signs, or railroad grade crossings. Traffic flow characteristics result purely from interactions among vehicles, geometric constraints, environmental conditions, and driver behavior.

Under the Highway Capacity Manual (HCM 7th Edition), operational evaluation focuses on establishing the Level of Service (LOS) by computing the traffic density ($D$) in equivalent passenger cars per mile per lane ($\text{pc/mi/ln}$).

Base (Ideal) Conditions for Uninterrupted Flow

HCM capacity models define base (ideal) operating conditions where geometric and operational parameters impose zero degradation on performance:

• Lane width: Minimum 12 ft (3.6 m).
• Right-side lateral clearance: Minimum 6 ft (1.8 m) between the edge of the travel lane and roadside obstructions or retaining walls.
• Median lateral clearance: Minimum 2 ft (0.6 m) for divided freeways.
• Traffic composition: 100% passenger cars ($0%$ heavy vehicles: trucks, buses, RVs).
• Terrain: Level terrain ($0%$ longitudinal grade).
• Driver population: Regular, familiar commuter drivers ($f_p = 1.00$).
• Interchange spacing: Interchange density of 0.5 interchanges/mile (average 2-mile spacing).
• Environmental / Lighting: Clear weather, dry pavement, and daylight conditions.

When actual field conditions diverge from these base parameters, specific empirical correction factors reduce estimated free-flow speed and adjust volume demands upward.

Free-Flow Speed (FFS) Determination

Free-flow speed is the mean speed of passenger cars measured under low-to-moderate volume conditions ($v_p \le 1,000\text{ pc/h/ln}$) where drivers are unrestrained by upstream or downstream congestion.

1. Basic Freeway Segments

If field measurements of speed are unavailable, FFS is estimated using the HCM regression formula:

$FFS = 75.4 - f_{LW} - f_{RLC} - 3.22 \times TRD^{0.84}$

Where:

• $FFS$ = estimated free-flow speed (mph).
• $f_{LW}$ = adjustment for lane width (mph; $0.0\text{ mph}$ for $\ge 12\text{ ft}$, $1.9\text{ mph}$ for $11\text{ ft}$, $6.6\text{ mph}$ for $10\text{ ft}$).
• $f_{RLC}$ = adjustment for right-shoulder lateral clearance (mph; depends on number of directional lanes $N$ and clear shoulder width; $0.0\text{ mph}$ for $\ge 6\text{ ft}$).
• $TRD$ = Total Ramp Density (ramps/mile), calculated as total on-ramps and off-ramps within a 6-mile segment (3 miles upstream and 3 miles downstream of the midpoint of the study section) divided by 6.0 miles.

2. Multilane Highways

For multilane highways (which may feature at-grade intersections, driveways, or traffic signals spaced $> 2.0\text{ miles}$ apart), FFS is computed from a Base Free-Flow Speed ($BFFS$, typically 60 mph):

$FFS = BFFS - f_{LW} - f_{TLC} - f_M - f_A$

Where:

• $f_{TLC}$ = adjustment for total lateral clearance (sum of right shoulder clearance and left/median clearance, up to 12 ft maximum).
• $f_M$ = adjustment for median type ($0.0\text{ mph}$ for divided highways with physical barrier or raised median; $1.6\text{ mph}$ for undivided highways; $0.0\text{ mph}$ for two-way left-turn lanes [TWLTL]).
• $f_A$ = adjustment for access point density (driveways and minor unsignalized access per mile):

$f_A = 0.25 \times APD$

Where $APD$ is the total number of active access points per mile on the study side of the roadway.

Demand Flow Rate Adjustment ($v_p$)

Raw hourly traffic counts ($V$, veh/h) must be converted into a peak 15-minute equivalent passenger-car flow rate per lane under base conditions ($v_p$, pc/h/ln):

$v_p = \frac{V}{PHF \times N \times f_{HV} \times f_p}$

Where:

• $V$ = peak-hour directional volume (veh/h).
• $PHF$ = Peak Hour Factor $= \frac{V}{4 \times V_{15}}$, where $V_{15}$ is the maximum volume during a single 15-minute interval within the peak hour.
• $N$ = number of travel lanes in the specified direction of travel.
• $f_p$ = driver population adjustment factor ($1.00$ for typical urban commuter streams; $0.85 - 0.97$ for predominantly unfamiliar or recreational weekend traffic).
• $f_{HV}$ = heavy-vehicle adjustment factor.

Heavy Vehicle Adjustment Factor ($f_{HV}$)

Heavy vehicles occupy more physical space and exhibit inferior acceleration/deceleration performance compared to passenger cars. The adjustment factor is:

$f_{HV} = \frac{1}{1 + P_T(E_T - 1) + P_R(E_R - 1)}$

Where:

• $P_T, P_R$ = proportion of trucks/buses and recreational vehicles (RVs) in the traffic stream, expressed as decimals.
• $E_T, E_R$ = passenger car equivalents for trucks/buses and RVs for the given terrain profile.

Note for Specific Grades: For extended steep grades ($> 3%$ grade over $> 0.5\text{ miles}$), the HCM provides specific $E_T$ lookup tables categorized by percent grade, length of grade, and truck weight-to-power ratio.

Speed-Flow Curves, Density, and Level of Service Thresholds

Speed-Flow Relationship & Breakpoint

Under HCM 7th Edition formulations, vehicle operating speed remains constant at the Free-Flow Speed ($FFS$) up to a breakpoint demand ($BP$):

$BP = 1000 + 40 \times (75 - FFS) \quad [\text{for } 55 \le FFS \le 75\text{ mph}]$

• For $FFS = 75\text{ mph}$, $BP = 1,000\text{ pc/h/ln}$.
• For $FFS = 70\text{ mph}$, $BP = 1,200\text{ pc/h/ln}$.
• For $FFS = 65\text{ mph}$, $BP = 1,400\text{ pc/h/ln}$.
• For $FFS = 55\text{ mph}$, $BP = 1,800\text{ pc/h/ln}$.

When $v_p \le BP$, the space mean speed $S = FFS$. When $BP < v_p \le C$, speed decreases non-linearly toward capacity ($C$). At capacity ($v_p = C$), the density reaches exactly $45.0\text{ pc/mi/ln}$ for all freeway free-flow speed curves.

Directional Lane Capacities

• $FFS = 75\text{ mph} \implies C = 2,400\text{ pc/h/ln}$
• $FFS = 70\text{ mph} \implies C = 2,400\text{ pc/h/ln}$
• $FFS = 65\text{ mph} \implies C = 2,350\text{ pc/h/ln}$
• $FFS = 60\text{ mph} \implies C = 2,300\text{ pc/h/ln}$
• $FFS = 55\text{ mph} \implies C = 2,250\text{ pc/h/ln}$

Density Calculation

Density ($D$) defines the degree of proximity between vehicles and serves as the primary operational measure of effectiveness:

$D = \frac{v_p}{S} \quad (\text{pc/mi/ln})$

Level of Service (LOS) Criteria (HCM 7th Edition)

Step-by-Step Worked Engineering Example

Problem Statement

A suburban 6-lane freeway (3 lanes in each direction, $N=3$) carries a directional peak-hour volume of $V = 4,860\text{ veh/h}$. Field survey data indicates:

• Peak Hour Factor ($PHF$) = $0.90$
• Traffic composition: $8%$ heavy trucks ($P_T = 0.08$), $0%$ RVs
• Terrain: Rolling terrain ($E_T = 3.0$)
• Driver population: Daily commuters ($f_p = 1.00$)
• Geometric attributes: 12-ft lanes, 6-ft right shoulder, $TRD = 1.5\text{ ramps/mile}$

Step 1: Determine Free-Flow Speed ($FFS$)

$FFS = 75.4 - f_{LW} - f_{RLC} - 3.22 \times TRD^{0.84}$ Since lanes are 12 ft ($f_{LW} = 0.0$) and right shoulder is 6 ft ($f_{RLC} = 0.0$): $FFS = 75.4 - 0.0 - 0.0 - 3.22 \times (1.5)^{0.84} = 75.4 - 3.22 \times 1.405 = 75.4 - 4.52 = 70.88 \approx 70.9\text{ mph}$

Step 2: Compute Heavy Vehicle Factor ($f_{HV}$)

$f_{HV} = \frac{1}{1 + 0.08(3.0 - 1)} = \frac{1}{1 + 0.16} = \frac{1}{1.16} = 0.8621$

Step 3: Calculate Peak 15-min Demand Flow Rate ($v_p$)

$v_p = \frac{4,860}{0.90 \times 3 \times 0.8621 \times 1.00} = \frac{4,860}{2.3277} = 2,088\text{ pc/h/ln}$

Step 4: Check Breakpoint and Determine Speed ($S$)

For $FFS \approx 70.9\text{ mph}$, the breakpoint is: $BP = 1000 + 40 \times (75 - 70.88) = 1000 + 40 \times 4.12 = 1,165\text{ pc/h/ln}$ Since $v_p = 2,088 > 1,165\text{ pc/h/ln}$, speed is slightly reduced below FFS. Evaluating along the 70 mph curve gives space mean speed $S \approx 68.2\text{ mph}$.

Step 5: Compute Density ($D$) and Level of Service

$D = \frac{v_p}{S} = \frac{2,088}{68.2} = 30.6\text{ pc/mi/ln}$

Comparing against HCM Level of Service density thresholds:

• $26.0 < D \le 35.0\text{ pc/mi/ln} \implies$ Level of Service D (LOS D).