10.3.4.1 Service Volumes

FHWA has created “rule-of-thumb” intersection capacities that are used to determine if left-turn lane configuration at a signalized intersection is sufficient. See for thresholds based on various left-turn treatment scenarios.
Table 10-2: Operational Analysis Tools
Intersection Type
Level of Detail
Sketch Level or Macroscopic
HCM Based (Mesoscopic)
Microsimulation (Microscopic)
Two-Way Stop Control (TWSC)
Not applicable
Synchro, HCS
Vissim, CORSIM
All-Way Stop Control (AWSC)
Not applicable
Synchro, HCS
Vissim, CORSIM
Traffic Signal
CAP-X
Synchro, HCS
Vissim, CORSIM
Roundabout
See
Chapter 12, Appendix M
See
Chapter 12, Appendix M
See
Chapter 12, Appendix M
Alternative Intersection (DLT, RCUT, MUT, CGT, gradeseparated)
CAP-X
1
 (Synchro for CGT or grade separated)
Synchro, HCS2
Vissim, CORSIM
1
Synchro can be used for CGT and grade-separated intersections
2
Vissim can be used for HCM-based (mesoscopic) analysis when the facility type is unavailable in HCS
Table 10-3: Operational Analysis Tool Inputs (Stage 2 ICE)
Tool
Necessary Data Inputs
Optional Data Inputs
Synchro
  • geometric conditions
  • turning movement volumes (veh/hr)
  • speeds
  • peak hour factor
  • control type
  • heavy vehicle percentages
  • number of bus blockages
  • number of parking maneuvers
  • percent of traffic from midblock sources
HCS
  • geometric conditions
  • turning movement volumes (veh/hr)
  • speeds
  • peak hour factor
  • growth factor
  • control type
  • heavy vehicle percentages
  • buses/hour/approach
  • parking per hour
  • arrival type
  • detector length
  • initial queue
Vissim
  • geometric conditions
  • turning movement speeds
  • static vehicle routes (turning movement volumes veh/hr)
  • conflict rules
  • vehicle inputs
  • control type and signal timing
  • driver behavior parameters
  • heavy vehicle percentages
CORSIM
  • geometric conditions
  • turning movement volumes (veh/hr)
  • speeds
  • control type
  • heavy vehicle percentages
Table 10-4: Signalized Intersection Capacities for Exclusive Left Turn Treatments
Case I: No Exclusive Left-Turn Lanes
Assumed critical signal phases*
2
Left-turn volumes
Critical major approach**: ≤ 125 veh/hr
Critical minor approach: ≤ 100 veh/hr
Planning-level capacity (veh/hr), sum of critical approach volumes***
Number of basic lanes,**** major approach
2
3
4
Number of basic lanes, minor approach
1
1,700
2,300
-
2
2,400
3,000
-
3
-
-
-
Case II: Exclusive Left-Turn Lane on Major Approaches Only
Assumed critical signal phases
3
Left-turn volumes
Critical major approach: 150-350 veh/hr
Critical minor approach: ≤ 125 veh/hr
Planning-level capacity (veh/hr), sum of critical approach volumes
Number of basic lanes, major approach
2
3
4
Number of basic lanes, minor approach
1
1,600
2,100
2,300
2
2,100
2,600
2,800
3
2,700
3,000
3,200
Case III: Exclusive Left-Turn Lane on Both Major and Minor Approaches
Assumed critical signal phases
3
Left-turn volumes
Critical major approach: 150-350 veh/hr
Critical minor approach: ≤ 125 veh/hr
Planning-level capacity (veh/hr), sum of critical approach volumes
Number of basic lanes, major approach
2
3
4
Number of basic lanes, minor approach
1
1,500
1,800
2,000
2
1,900
2,100
2,400
3
2,200
2,300
2,800
Notes:
* Critical signal phases are nonconcurrent phases
**A critical approach is the higher of two opposing approaches (assumes same number of lanes)
***Use fraction of capacity for design purposes (e.g., 85 or 90 percent)
****Basic lanes are through lanes, exclusive of turning lanes
Adapted from NCHRP 279, figure 4-11