Each turning movement (left, through, right, Uturn) of each approach of an interchange has an origin point at the approach leg and a destination point at the departure leg. Each OD pair experiences delay as it travels through the interchange. The sum of these delays makes up the ETT. The LOS for each OD pair is based on the average ETT of the users completing that movement.

Table 11-2

from HCM 7th Edition, Chapter 34, presents a worksheet for converting turning movement volumes to/from OD movement volumes at a diamond (including DDI) interchange. HCM also includes similar worksheets for a partial cloverleaf interchange, a double roundabout interchange, and a SPUI (one input intersection). Once OD volumes are obtained, the performance of various interchange types can be compared. The worksheets accommodate up to the following number of OD movements:

16 OD movements for roundabout interchanges (turning left, through, right, or U-turn per approach on four approaches);

14 OD movements for diamond (including DDI) interchanges (turning left, though, or right per approach on four approaches and making a U-turn on the major road approaches);

12 OD movements for SPUIs (turning left, though, or right per approach on four approaches); and

12 OD movements for partial cloverleaf interchanges (turning left, right, or U-turn on major road approaches and turning left, though, or right on minor road approaches)