If the cross-section design does not meet the above conditions (has sharper grade
breaks), the profile should be designed using a template of the design vehicle.
For tree lawns less than 6 ft (1.8 m) wide, Fig. 2.51 shows the profile treatment.
Clearance for the design vehicle is achieved by depressing the sidewalk 1 in (25.4 mm) at
the driveway. The sidewalk cross slope of 1⁄4 in/ft (2.1%) is retained. The design may be
used directly with curbed highways having cross-section criteria as listed above and the
profile conditions of Fig. 2.50. For other cross sections, a template of the design vehicle
may be used to design the profile.
Figure 2.51 shows an isometric view and profile for a driveway where only a 3-ft
(0.9-m) tree lawn is available. This design is shown not because it is desirable, but
because right-of-way width and property development may require this type of design.
Whenever feasible, the tree lawn should be 8 ft (2.4 m) or wider. Where the total width of
tree lawn and sidewalk is less than 6 ft (1.8 m), the minimum 3-ft (0.9-m) apron designs
are inappropriate and cannot be used, as they extend curb or sharp flares into the sidewalk
area. For this condition, the sidewalk and curb are transitioned to meet the drive profile as
shown on the lower portion of Fig. 2.51. The profile of the drive meets the 1-in (25.4-mm)
depressed grade of the sidewalk, as shown in the drive profile.
The tree lawn and walk design shown in Figs. 2.50 and 2.51 will keep storm water,
flowing at the curb design height or less, from flowing over the sidewalk. If it is necessary
to lower the curb and sidewalk more than 1 in (25 mm), the drainage condition
should be checked thoroughly.
Commercial drive profiles usually use a dropped curb across the approach.
However, some commercial drives serving large traffic generators may be designed as
at-grade intersections, without dropped curbs, because of their high traffic volumes.
Figure 2.58 shows the recommended grade controls for commercial driveways. The
grade should be as flat as possible and still meet drainage requirements. The 20-ft