CE 578 Highway Traffic
Operations
Freeways Merge Influence Areas
3/3/06
Michael Dixon
1
Objectives
•
•
•
3/3/06
Estimate flow entering influence area
Estimate capacity and determining LOS
Special cases (next class)
Michael Dixon
2
Flow Entering Influence Area (V12)
•
•
V12 = PFM * VF
4-lane freeways (PFM = 1.0)
V12 = VF
•
•
6-lane freewaysmore complex
8-lane and 10-lane freeways
LA
PFM_8  0.2178  0.000125 VR  0.01115
SFR
3/3/06
Michael Dixon
3
Flow Entering Influence Area (V12):
Choosing the PFM Equation
•
•
•
•
3/3/06
four-lane: PFM = 1.0
eight-lane: only one choice
ten-lane: only one choice
six-lane: See Exhibit 25-6
Michael Dixon
4
Flow Entering Influence Area (V12):
Choosing the PFM Equation
3/3/06
Michael Dixon
5
Flow Entering Influence Area (V12):
Choosing the PFM Equation (cont.)
• Only an issue for six-lane freeway sections
• On-ramps do not affect V12
• If only on-ramps are involved Eq. 1
• If an off-ramp is involved calculate LEQ
• if LEQ < Lup and/or Ldown then use Eq. 1
3/3/06
Michael Dixon
6
Steps for Evaluating a Merge Area
• Calculate the adjusted volumes (VR, VF, VD)
• Step 1: Evaluate the equilibrium distance
• Step 2: Calculate PFM
• Step 3: Calculate V12
• Step 4: Calculate total flow entering merge area
(VR12)
3/3/06
Michael Dixon
7
Steps for Evaluating a Merge Area
(cont.)
• Step 5: Capacity for downstream freeway
segment (c1)
• Step 6: Capacity for merge influence area (only
if v/c for step 5 is less than 1)(Exhibit 25-7)
• Step 7: Determine the LOS of the influence area
(only if v/c for step 5 is less than 1)
3/3/06
Michael Dixon
8
3/3/06
Michael Dixon
9
Six-Lane Example: Conditions
Example of the analysis of an on-ramp to a six-lane freeway with a downstream off-ramp:
Ldown  1300
LA  300
cdownstream  3 2300
VF  5000
cdownstream  6900
VR  400
FFSfreeway  60
VD  500
RFFS  40
Ideal freeway conditions
3/3/06
Michael Dixon
10
Six-Lane Example: Step 1
Step 1: Evaluate equilibrium distance
VD
LEQ 
0.1096  0.000107 LA
LEQ  3529
LEQ is greater than Ldown so use Equation 3
3/3/06
Michael Dixon
11
Six-Lane Example: Step 2 and 3
Step 2: Calculate PFM
VD
PFM_3  0.5487  0.2628
Ldown
PFM_3  0.65
Step 3: Calculate V12
V12  VF PFM_3
3/3/06
V12  3249
Michael Dixon
12
Six-Lane Example: Step 4 and 5
Step 4: Calculate total flow entering the ramp influence area
VR12  V12  VR
VR12  3649
Step 5: Determine the capacity for the downstream freeway segment
v  VF  VR
v_over_c_ratio downstream 
v  5400
v
cdownstream
v_over_c_ratio downstream  0.78
downstream capacity is sufficient so we should continue to look at the influence area
3/3/06
Michael Dixon
13
Six-Lane Example: Step 6 and 7
Step 6: Determine the capacity for the merge influence area
cmerge  4600
v_over_c_ratio merge 
VR12
v_over_c_ratio merge  0.79
cmerge
Step 7: Determine the LOS of the merge area
Note that conditions are undersaturated on the freeway mainline so we can use Equation 25-5
to estimate density
DR  5.475  0.00734 VR  0.0078 V12  0.00627 LA
DR  31.87
LOS = E
3/3/06
Michael Dixon
14