The criteria presented herein are major controlling factors in the design of roads or streets. It is expected that the designers will exercise careful attention to detail in the application of these criteria todesign circumstances. Suitable transitional elements must be provided between changes in geometric configuration, pavement and curb character, and drainage carrying aspects of the ultimate street design.
(A) General design criteria. Table 2 summarizes criteria considered minimum in the county for various classifications of roads or street. Design speeds given are intended to establish levels to which facilities are to be designed; posted legal speeds are established only after appropriate examination of the completed road or street by the county.
(B) Geometric criteria. In general, criteria for the horizontal and geometric of road or street design given in Table 2 will be the minimum acceptable values. Other factors must also be considered in a balanced design.
(1) Vertical alignment. Long, flat gradients are undesirable because of poor drainage characteristics. The minimum desirable gradient consistent with acceptable drainage is 0.5% with curbs and gutter and 0.25% with no curbs and gutters and, as such, should be observed as a general design principle. Grades in valley areas and other special circumstances may be flatter than 0.5% if approved by the Public Works Director. Long, step gradients are also undesirable since such are difficult for heavier vehicles to negotiate at desirable traffic speeds. Vertical curve criteria stated in Table 2 are intended to provide adequate safety consistent with applicable design speeds. In the application of these criteria, the designer will be expected to apply good judgment in combining vertical geometry with horizontal geometry.
(2) Extreme vertical undulation is not acceptable. Vertical changes in grade occurring simultaneously with horizontal alignment changes must be carefully considered to preserve the acceptable sight distance consistent with the designspeed of the street. Engineering determination of safestopping sight distance should be demonstrated in these cases. Adequate intersection sight distances must be maintained in all designs.
(3) Balance design. Abrupt, inconsistent changes in either horizontal or vertical alignment arenot acceptable. Minor roads or streets shall be designed, however, in order to discourage excessive speeds of traffic. Major roads or streets shall be designed to permit larger flows of traffic at design speeds.
(C) Intersection design.
(1) Angle of intersection. Roads and streets must be designed to intersect at right angles as nearly as practical consistent with topography and sound design. The acute angles at intersection for all roads or streets shall be 80 degrees minimum. Intersection designs must provide for adequate clear sight distances.
(2) Spacing of intersections. Intersections of roads or streets along arterial roads or streets are to be minimized. Following are limiting values to be observed:
(a) Roads or streets intersecting arterials must generally be spaced no closer than one-sixth mile on center.
(b) Intersection of roads or streets shall be continuous in alignment if feasible. Offset alignment of intersecting roads or streets is highly discouraged. Note: Driveways should be at most distance from intersection.
(c) Intersections of roads or streets which are not on continuous alignment through the road or street intersected are to be spaced as follows:
1. Intersections of non-continuous roads or streets must be spaced at least 150 feet between centerlines of roads or streets on local roads or streets.
2. At least 300 feet on collector roads or streets.
3. At least 400 feet on all arterial roads or streets.
(d) Variances from these criteria will require written approval of the Public Works Director.
(3) Curb return radii. Minimum acceptable return radius for residential is 25 feet. All radii are measured to the back of the curb section.
(4) Intersection grading. Street crown may be reduced through intersections of major roads or streets of approximately equal classification if desirable to promote comfort. Crown reduction should not generally exceed one-half of standard crown unless special circumstances govern and the joint concurrence of the County is obtained. Concrete valley gutters and fillets are necessary to provide rapid drainage and eliminate pavement rutting (see Standard Details — see Appendix A to Ord. No. 2006-02).
(5) Intersection sight distance. Intersection of local roads or streets with major roads or streets classified as collector or above shall not be located at or near horizontal curves without special evaluation of intersection sight distance. The location of an intersection on the “inside” of a horizontal curve is a situation that will typically result intersection visibility problems. The location of any property lines, fences or other obstructions will need to be evaluated to insure that the minimum sight distance is maintained.
(6) ADA compliant. Submittals shall be subject to regulations and requirements of the federal Americans with Disabilities Act or the State as it relates to providing for design and construction to provide for accessibility of the handicapped.
(D) Curb and gutter and drainage criteria.
(1) Standard 6-inch high barrier-type curb and gutter with 1-inch gutter depth as shown in Standard Details must be used as the exterior curb section for all classes of roads or streets required to have curb and gutter. Deviation from these standards will require written approval and concurrence by the Public Works Director.
(2) If both traffic requirements and drainage requirements can be met to satisfaction of thePublic Works Director, mountable curb types as shown in Standard Details maybe used on local roads or streets.
(3) Concrete curbs and gutter shall be constructed of Class “A” 4,000-psi Portland cement concrete. Expansion joints are to be installed every 24 feet and joint and construction joints installed every 8 feet at a minimum.
(4) For roads without curbs and gutter, all driveways shall be provided with a 12-inch diameter culvert or larger if required by drainage flow calculations for a 10-year, 24-hour event, at all driveways accessing a public or county road. The culvert shall be placed to have positive downhill drainage and shall be placed in the location graded to carry drainage, commonly known as the swale.
(E) Pavement design criteria.
(1) Design of pavement structures of arterial roads or streets must be based on acceptable design procedures. Current acceptable design procedures include latest edition of AASHTO Interim Guide for Design of Pavement Structures, NMSHD Bulletin 102 (See Standard Details), as amended.
(2) Design of pavement structures for collector, industrial, and commercial area roads or streets and residential (local) roads or streets will conform to Standard Details. Subgrade soils: Designing geotechnical engineer shall submit report to county stating existing subgrade bearing values and proposed structural pavement design.
(3) Structural design of private roads or streets shall conform to division (E)(2) above.
(F) Drivepad design standards.
(1) Construction of drivepads must conform to the current Standard Details.
(2) Drivepads shall cross the sidewalk on the sidewalk grade line without depression of the sidewalks. However, if a drivepad gradient in excess of 10% would be required to avoid depression of the sidewalk, the sidewalk shall be transitioned to match the drivepad within 6 feet of edge of drivepad and the drivepad gradient shall be maintained equal to or less than 10%.
(3) ADA compliant. Submittals shall be subject to regulations and requirements of the federal Americans with Disabilities Act or the State as it relates to providing for design and construction to provide for accessibility of the handicapped.
(Ord. 2006-02, passed 3-17-2006) Penalty, see § 94.999