11.3.4.3 LOS Guidelines

Roundabout interchanges and signalized interchanges have separate LOS thresholds. LOS is typically found for each OD movement through the interchange rather than individual turning movements occurring at the different intersections. shows the delay-based LOS criteria for signalized interchanges for each OD pair. LOS is defined as F for any OD pair or lane group that exceeds a volume-to-capacity (v/c) ratio or queue-to-storage ratio of 1.0
Table 11-3: HCM LOS Criteria for Each OD Pair for Signalized Interchanges
ETT (s/veh)
LOS
≤15
A
>15-30
B
>30-55
C
>55-85
D
>85-120
E
>120
F (v/c > 1)
shows the delay-based LOS criteria for roundabout interchanges for each OD pair. The LOS criterion is the same regardless of whether the interchange is a single or double roundabout
Table 11-4: HCM LOS Criteria for Each OD Pair for Roundabout Interchanges
ETT (s/veh)
LOS
≤15
A
>15-25
B
>25-35
C
>35-50
D
>50-75
E
>75
F (v/c > 1)
When evaluating interchanges using deterministic traffic analysis software, the software does not provide ETT as an output. In those instances, manual (spreadsheet) calculation of ETT can be performed based on the outputs provided by the software. If manual calculation is not possible, it is acceptable to report intersection LOS for each intersection of the interchange. However, ETT results are recommended to be reported when analyzing interchanges in Microsimulation (microscopic) analysis software or HCS. Microsimulation (microscopic) analysis is often used for complex geometry, atypical conditions, and oversaturated conditions.
Additional information on LOS methodology for diamond, cloverleaf, diverging diamond, single-point urban, and roundabout interchanges is covered in Chapter 23 (methodology) of the HCM 7th Edition. Chapter 34 (applications) of HCM 7th Edition provides guidance on selecting an interchange type based on operational performance. The LOS methodology does not cover interchanges with two-way stopcontrolled intersections, interchanges consisting of both roundabout and signalized intersections, or freeway-to-freeway interchanges. Chapter 38 of the HCM presents methodologies for corridors involving freeways and surface streets, including queue spillback. Microsimulation (microscopic) analysis is typically used to analyze the impact of queuing.
When calculating LOS for signalized diamond interchanges, it is recommended that coordination with adjacent arterial signalized intersections be considered, along with optimized timing and the appropriate diamond timing sequence for traffic patterns present or projected.