An assembly baseline is typically located in one of two places, as shown in the following figures.
Central Baseline
Figure 2: Assembly for divided roadway
The assembly in figure 2 has a baseline in the center of a ditch between two roadways. This assembly can be used for a corridor designed symmetrically along a single central baseline. However, if you are designing a corridor section from the gutter flow line or another outside line, it usually makes more sense to place your baseline at that location. The best practice is to set your baseline along the primary alignment that controls the corridor’s location.
Outside Baseline
Figure 3: Assembly for curb return
Figure 3 shows a typical assembly for the design of a curb return in an intersection. The curb return is the curved region that joins the outside edges of two intersecting alignments. The assembly shows a single road lane with a gutter, curb, sidewalk, and standard daylight. The baseline is located where the edge of pavement meets the curb, as this is the alignment that controls the design. Another common location for a curb return baseline is the back edge of the curb.