An on-grade inlet may be much more efficient if it intercepts only a portion of the total flow in the gutter instead of all of the flow in the gutter. The gutter flow not intercepted is called bypass flow or carryover. This design approach can only be used for on-grade configurations and is recommended where interception of the total flow is not necessary.

Figure 10-12 illustrates (in profile) approximately what happens when the inlet is designed to intercept all of the approaching flow. Note the large area of inlet opening that is not utilized efficiently.

Figure 10-13 illustrates (in profile) approximately what happens when the inlet is designed for bypass flow. Note that the inlet opening is used much more efficiently than the inlet illustrated in Figure 10-12.

Bypass flow is normally captured at some other location. The gutter between the two points must accommodate the additional flow. Bypass flow is not recommended upstream of intersections and driveways, at superelevation transitions where the cross slope begins to reverse, or below entrance/exit ramps. Bypass flow at these locations would be crossed by vehicular traffic and may pose a traffic hazard. Bypass flow is also not recommended to be allowed to flow where there is no outfall or designated capture point.