3.2 Freeway Weaving, Merging, and Diverging Segments

Freeway Weaving, Merging, and Diverging Segments

Freeway merge, diverge, and weaving areas represent points of intensive traffic stream turbulence. Drivers must execute mandatory lateral lane changes within restricted longitudinal distances to enter, exit, or transition across travel lanes, generating intense vehicular conflicts and localized speed reductions.

Freeway Weaving Segments

A weaving segment is formed when a merge area is closely followed by a diverge area, joined by one or more continuous auxiliary lanes, requiring entering and exiting vehicles to cross each other's paths.

Weaving Length Parameters

• Short Length ($L_S$): The distance between the end of the merge gore (where the barrier curb or 2-ft solid white line ends) and the beginning of the diverge gore (where the 2-ft solid white line starts).
• Base Length ($L_B$): The total distance between merge and diverge gore points measured from the merge point of the entering roadway to the split point of the exiting roadway.
• Maximum Weaving Length ($L_{MAX}$): The maximum physical distance over which weaving maneuvers create systemic turbulence. If $L_S > L_{MAX}$, weaving interactions dissipate, and the segment must be analyzed as two independent entities: an isolated merge followed by an isolated diverge!

$L_{MAX} = \left[ 5,738 \times (1 + VR)^{1.6} \right] - \left[ 2,383 \times N_{WL} \right]$

Where:

• $VR$ = Volume Ratio $= \frac{V_W}{V}$ (weaving volume divided by total volume).
• $V_W$ = total weaving demand flow rate $= V_{on\text{-}to\text{-}freeway} + V_{freeway\text{-}to\text{-}off}$ (pc/h).
• $V$ = total volume entering the weaving segment (pc/h).
• $N_{WL}$ = number of lanes from which weaving maneuvers can be made with $\le 1$ lane change (typically 2 or 3).

Weaving Configurations & Lane Changing

Historically classified into Types A, B, and C, modern HCM formulations parameterize weaving segments by the minimum number of mandatory lane changes ($LC_{MIN}$):

• Type A (Ramp-Weave): Every weaving vehicle must execute at least one lane change ($LC_{MIN} \ge 1$). This is the standard configuration where an on-ramp and off-ramp are connected by a single auxiliary lane.
• Type B (Major Weave with Lane Drop/Option): One weaving movement can be completed without making a lane change ($LC_{MIN} = 0$), while the opposing movement requires at most one lane change ($LC_{MIN} = 1$).
• Type C (Major Weave without Continuous Lane): One weaving movement requires zero lane changes, but the opposing movement requires two or more lane changes ($LC_{MIN} \ge 2$).

Lane-Changing Models ($LC_{ALL}$)

Total lane changes within the weaving zone ($LC_{ALL}$) comprise weaving lane changes ($LC_W$) and non-weaving lane changes ($LC_{NW}$):

$LC_{ALL} = LC_W + LC_{NW}$

Where:

• $LC_W = L_{MIN} + 0.39 \left[ (L_S - 300)^{0.5} \times N_{WL}^2 \times (1 + ID)^{0.8} \right]$
• $ID$ = Interchange Density (interchanges/mile).

Weaving Level of Service (LOS) Density Thresholds

Freeway Merging and Diverging Segments

Merge and diverge segments are evaluated across an empirically defined influence area.

Definition of the 1,500-ft Influence Area

• Merge Influence Area: Extends 1,500 ft (450 m) downstream from the physical merge gore point. It includes Lanes 1 and 2 of the freeway mainline plus the acceleration lane ($L_A$).
• Diverge Influence Area: Extends 1,500 ft (450 m) upstream from the physical diverge gore point. It includes Lanes 1 and 2 of the freeway mainline plus the deceleration lane ($L_D$).

Capacity Checks for Merge / Diverge Operations

Before calculating density and LOS, three capacity boundaries must be checked to confirm demand does not exceed capacity ($v/c \le 1.00$):

1. Total Downstream Freeway Capacity: Total flow downstream of merge ($v_F + v_R$) cannot exceed mainline capacity ($N \times C_{lane}$, e.g., $3 \times 2,400 = 7,200\text{ pc/h}$). If exceeded $\implies$ LOS F.
2. Ramp Roadway Capacity: On-ramp demand ($v_R$) or off-ramp demand ($v_D$) cannot exceed ramp roadway capacity ($2,200\text{ pc/h}$ for single-lane ramp at $\ge 50\text{ mph}$ FFS; $4,400\text{ pc/h}$ for 2-lane ramp). If exceeded $\implies$ LOS F.
3. Maximum Flow Entering Influence Area: Total flow entering Lanes 1 and 2 plus the ramp cannot exceed 4,600 pc/h:
   • For Merge: $v_{12} + v_R \le 4,600\text{ pc/h}$
   • For Diverge: $v_{12} \le 4,400\text{ pc/h}$ (approaching diverge gore in Lanes 1 & 2)

Demand in Freeway Lanes 1 and 2 ($v_{12}$)

Mainline vehicles in the rightmost two lanes ($v_{12}$) immediately upstream of the merge are estimated using the proportion factor $P_{FM}$:

$v_{12} = v_F \times P_{FM}$

Where $P_{FM}$ depends on freeway lane count ($N$), total freeway flow ($v_F$), ramp flow ($v_R$), upstream/downstream ramp spacing, and acceleration length ($L_A$).

Density Estimation and Merge/Diverge LOS Criteria

Merge Influence Area Density ($D_R$)

Density within the 1,500-ft merge influence area is calculated from the empirical regression model:

$D_R = 5.475 + 0.00734 \, v_R + 0.0078 \, v_{12} - 0.00627 \, L_A$

Where:

• $D_R$ = density in the merge influence area (pc/mi/ln).
• $v_R$ = ramp demand flow rate (pc/h).
• $v_{12}$ = freeway demand flow rate in Lanes 1 and 2 upstream of merge (pc/h).
• $L_A$ = length of acceleration lane (ft).

Diverge Influence Area Density ($D_D$)

$D_D = 4.25 + 0.0086 \, v_{12} - 0.0065 \, L_D$

Where:

• $D_D$ = density in the diverge influence area (pc/mi/ln).
• $v_{12}$ = flow in Lanes 1 and 2 upstream of diverge $= v_D + (v_F - v_D) P_{FD}$ (pc/h).
• $L_D$ = length of deceleration lane (ft).

Level of Service Thresholds for Ramp Influence Areas

Notice that ramp merge/diverge areas reach capacity (LOS E/F boundary) at 40.0 pc/mi/ln, which is lower than basic freeway segments (45.0 pc/mi/ln) due to the heavy turbulence of merge/diverge maneuvers!