The transverse grades and the structural cross-section of the median are included in the following discussion. Other components of the median cross-section are covered elsewhere in this Part of the Manual.
E 667.1 Transverse Grade
In some cases, median strips are used primarily to divide the Highways into separate independent roadways. This is generally done because there is a large difference in elevation between the two roadways. When the median is used for this purpose, the cross-sectional grades are not critical. However, the maximum slopes of the median should not exceed 2 to 1 and the minimum slopes should not be less than 1 percent. If the curbs are level, the median cross-section should be designed in the shape of an inverted V, using 2.5 percent slopes, to permit transverse drainage. See Subsection E 667.3.
Where medians are subject to emergency vehicular or occasional pedestrian use, the maximum transverse slope should be 6 to 1, although flatter slopes not less than 1 percent are preferred.
E 667.2 Structural Cross-Section
When construction is not over existing pavement, the entire raised surface of median strips should be paved with a minimum thickness pavement of 6 inches of portland cement concrete and 4 inches of asphalt concrete or landscaping. A base is required under the roadway; it should also be placed under the raised median strip surfacing.
Median strips that are constructed by dowelling or extruding concrete curb over the existing pavement may have the raised surface paved with a minimum of 3 inches of portland cement concrete pavement. However, where the curbs are constructed directly on the pavement, and where the median strip area is less than 1000 square feet, it may be more economical to fill the space between the existing pavement surface and the raised median surface with solid concrete. Where the median area is more than 1000 square feet it is usually more economical to use a portland cement concrete pavement 3 inches thick and the same subbase material as is specified for use under the roadway pavement.
Asphalt concrete pavement is generally used only on a temporary basis, to construct asphalt concrete berm and pave the raised median surfaces. If the existing raised median surface adjoining a proposed improvement has an asphalt concrete pavement, it may be desirable to pave the new construction also with asphalt concrete for aesthetic reasons. A minimum thickness of 4 inches is to be used for asphalt concrete pavement.
E 667.3 Landscaping
The following criteria should be used in determining whether areas within the improvement limits are to be landscaped:
- Council Resolution requirement
- Petition request
- Size of area
- Agricultural suitability of soil
- Availability of water
- Availability of electricity
- Projected maintenance costs
- Grading Ordinance No. 123,970
If landscaping is proposed, prints of preliminary street plans should be submitted to the appropriate Landscape Architectural Section. The prints are for use in the preparation of the landscaping plans.
Median strips or traffic islands that are constructed with either non-traversable barrier curbs or mountable Type "D” curbs may receive landscape treatment. Some damage to landscaping is expected to occur infrequently, although more often on mountable than on barrier type curbs. The landscape treatment may consist of ground cover, shrubs, trees, or combinations thereof.
Shrubs and trees should not be installed where sight distance may be impaired. Since a water source is required, drainage disposal must be considered. See Section E 666 Drainage Considerations.
In general, median strips less than 10 feet in width or traffic islands with an area of less than 500 square feet may be provided with artificial grass or approved surface treatment. In median strips wider than 10 feet or other areas larger than 500 square feet, either live approved plantings or artificial grass may be installed. Where adequate soil depth cannot be provided economically, artificial grass or an approved surface treatment may be used.
Center medians in underpasses or under bridges should not be landscaped with live plantings. All slopes other than those under bridges or underpasses should be considered for planting with natural or artificial ground cover, shrubs, trees, or combinations thereof.
For drainage purposes, the minimum transverse slope for median strips and traffic islands having artificial grass is 1 percent. Where the slopes must be pitched upward from the curbs toward the middle of the island or median strip to form a crowned surface, a grade break is created at the junction of the intersecting grades. The maximum grade break over which the fibers of artificial grass should be installed is 6 percent.
The most practical base for artificial grass is portland cement concrete with a minimum thickness of 2 inches (on a Select Material Base if this is required on the rest of the project). The surface of the portland cement concrete should be a wood float finish. The surface should be 0.25-inch below the top of the abutting curb. This permits the fiber base of the artificial grass to fit flush or slightly below the top of curb.
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