LOS for interchanges is based on a combination of guidelines provided in the HCM 7th Edition for isolated intersections and urban segments. Because most interchanges are made up of clustered intersections, each user experiences both control delay at each signalized or unsignalized intersection and extra travel time due to the longer distances between intersections. ETT is a way to measure delay at interchanges, introduced in the HCM. ETT measures both control delay and travel time delay.

Appendix L, Section 4

provides ETT computational aids for several alternative intersections and interchanges through the FHWA Techbrief: Publication No.: FHWA-HRT16-069.The LOS for interchanges is measured in terms of ETT. Computing ETTs makes it possible to compare operations between standard and alternative interchanges that depend on diverted movements and multiple decision points. ETT is the sum of control delay at every junction and the extra distance travel time (EDTT) through the interchange, as shown below:

ETT = Experienced Travel Time (sec)

d

i

= control delay (sec) at every junction i encountered by the driver from origin to destination.

EDTT (sec) due to the extra distance necessary to travel between the individual intersections of the interchange.

An ETT calculation example of capturing ETT for one movement going from point A to point D at a conventional diamond interchange is shown in

Figure 11-14

. The ETT is equal to the control delay of the eastbound left movement at intersection 1 and the control delay of the northbound through movement at intersection 2, plus any EDTT. The EDTT for this movement would be equal to the difference in travel time between the indirect left-turn maneuver and a direct 90-degree left-turn movement from the eastbound approach to the northbound approach.

The following equation is applied to calculate the ETT for the movement between points A and D:

Where:

ETT

AD

= Experienced Travel Time from A to D

d

ELB1

= Eastbound left control delay at first point

d

NBT2

= Northbound through control delay at second point

EDTT

AD

= Extra distance travel time between A and D

TT

ABC

= Travel time along path A-B-C

TT

AC

= Travel time along (hypothetical) path A-C