(A) Major points.
(1) Local and minor. Local and minor streets shall be designed in such a manner that through traffic within the subdivision will be discouraged.
(2) Intersections. Streets shall be laid out so as to intersect as nearly as possible at right angles.
(3) Street names. Street names shall not duplicate an existing street name, unless the said street is an extension of the existing street. Street names shall be subject to the approval of the Town Council.
(4) Curves. The minimum curve radius shall be as designated in subsection (A)(9) of this section.
(5) Alleys. Alleys shall be provided if required by the Planning and Environmental Commission. The minimum width of the alley shall be 20 feet. Dead-end alleys shall not be permitted. All alleys shall be paved.
(6) Easements. Easements shall be provided for all utilities, drainageways, channels or streams which traverse across the subdivision.
(7) Block lengths. The length, width and shapes of blocks shall be determined by the type of use, zoning requirements, needs for convenient access, circulation and safety of street traffic, and limitations and opportunities of topography.
(8) Culs-de-sac. Cul-de-sac streets, however, shall be designed as minor or local with a right-of-way bulb of 50-foot radius and pavement radius of 40 feet.
(9) Street width. Street width shall conform to the following:
Class | ROW Width | Paved Width | Shoulder | Design Speed | Max. Grade % | Min. Curve Radius | Future ADT |
Arterial (Frontage) | 70 | 12 ft. per lane | 8 | 50 | 6 | 650 | 750 and over |
Collector | 50 | 24 | 4 | 40 | 7 | 250 | 300—750 |
Local | 50 | 22 | 3 | 30 | 8 | 60 | 150—300 |
Minor (Private) | 40 | 22 | 2 | 30 | 9 | 50 | 0—150 |
(B) Horizontal alignment.
(1) The major considerations in horizontal alignment design are: safety; grade profile; road type; design speed; sight distance; and topography. All these factors must be balanced to produce an alignment that is safest, most economical, and adequate for the type of road proposed.
(2) Horizontal alignment must provide at least the minimum stopping sight distance for the design speed at all points. This includes visibility at intersections, as well as around curves and roadside encroachments.
(C) Vertical alignment.
(1) Reference. The grade line is the reference line by which the elevation of the pavement and other features of the road are established. It is controlled mainly by topography, the factors of horizontal alignment, safety, sight distance, design speed, drainage and construction costs. The configuration of heavy duty vehicles must also be considered.
(2) Grade line. The grade line should be positioned with relation to the cross section as follows.
(a) It should coincide with the road centerline on two lane and multi-lane undivided roads.
(b) Separate grade lines may be required on divided multi-lane roads.
(D) Vertical curve.
(1) Properly designed vertical curves should provide adequate stopping and passing sight distance, headlight sight distance, driver comfort, good drainage and pleasing appearance.
(2) Long, flat vertical curves should be avoided as they may develop poor drainage at the level section and tend to create driver insecurity in passing maneuvers.
(3) Lengths of vertical curves shall be determined by the site distance requirements.
(E) Sight distance.
(1) All portions of the grade line must meet sight distance requirements for the design speed.
(2) The minimum stopping sight distance is the distance required by the driver of a vehicle, traveling at a given speed, to bring his or her vehicle to a stop after an object on the road becomes visible. Stopping sight distance is measured from the driver’s eyes, which are assumed to be three and three-fourths feet above the pavement surface to an object six inches high on the road. The required stopping sight distance for a given design speed shall be designed by the engineer.
(F) Crown slopes.
Type Of Surface | Min. Crown | Max. Slope |
Portland cement concrete | 1.5% | 4.0% |
Bituminous mix pavements | 2.0% | 4.0% |
(G) Clearances. The following are minimum clearances to structures or other roadside obstructions. Additional clearance must be provided for sight distance and other requirements. Where streets or highways under the jurisdiction of other agencies are involved the clearance, as required by said agency, if more restrictive than town standards, shall apply.
(1) Horizontal clearance. The minimum horizontal clearance from the edge of traveled way shall be ten feet to the right and four feet to the left when facing in the direction of travel.
(2) Vertical clearance. The minimum vertical clearance to major overhead structures shall be 15 feet above the traveled way and 14 feet above the shoulders. For minor overhead structures, such as signs, cables and the like, the minimum vertical clearance shall be 18 feet.
(H) Structural design.
(1) Design formula. The design of pavements is based upon the American Association of State Highway Officials group index rating, which is obtained by the use of a group index formula based upon the gradation, liquid limit and plasticity index of the soil. The group index formula is as follows:
Group Index = 0.2a + 0.005 ac + 0.01bd |
Where:
a = that portion of the gradation percentage passing the No. 200 sieve greater than 35% and not exceeding 75%, expressed as a positive whole number (1 to 40);
b = that portion of the gradation percentage passing the No. 200 sieve greater than 15% and not exceeding 55%, expressed as a positive whole number (1 to 40);
c = that portion of the numerical liquid limit greater than 40 and not exceeding 60, expressed as a positive whole number (1 to 20); and
d = that portion of the numerical plasticity index greater than ten and not exceeding 30, expressed as a positive whole number (1 to 20).
(2) Pavement thickness. The group index shall be computed for each soil sample taken and then applied to the proper design curve to obtain the minimum total pavement thickness required to the nearest one-half inch. That total thickness shall be used for pavements where the soil sample is representative. (See Figure 3.1.*)
(I) Sidewalks. Sidewalks are not generally required but may be required by the planning staff, Planning and Environmental Commission, or Town Engineer if deemed appropriate. (See Figure 3.2.*)
(J) Curbs and gutters. Approved types of concrete curb and gutter are not generally required but may be required by the planning staff, Planning and Environmental Commission or Town Engineer if deemed appropriate. (See Figures 3.2 and 3.3*, and § 13-10-11(E) of this chapter.)
(K) Drainage.
(1) The primary objective of drainage design shall be the protection of town streets and property while minimizing the possible flood damage to surrounding properties and structures. It should be emphasized that good drainage is one of the most important factors in road design. It preserves the good appearance as well as the level of service of the street while at the same time minimizing the cost of maintenance.
(2) Culverts under collector and arterial streets are to be designed to accommodate a 50-year frequency storm runoff, utilizing the maximum available head. The maximum available head shall be determined by the uppermost ponding elevation, so chosen as not to cause flood damage to upstream properties.
(3) On-site facilities and culverts under minor streets shall be designed to accommodate the 25-year frequency storm runoff.
(4) Inlets and other facilities draining the street surface shall be designed to accommodate the ten-year frequency storm runoff.
(5) All drainage installations shall also be designed to permit free unobstructed passage of debris and silt, or provide for their deflection and/or collection at a point upstream in such a manner as not to create an expensive maintenance problem.
(6) Storm runoff estimates: unless approved by the Town Engineer, the Soil Conservation Service method shall be utilized for drainage calculations. The appropriate design manual shall be Procedures for Determining Peak Flows in Colorado.
(a) Precipitation and snowmelt rates to be used are itemized below:
Frequency (years) | Precipitation (inches) | Snow Melt (cfs/acre) |
2 | 1.0 | 0.040 |
5 | 1.4 | 0.048 |
6 | 1.6 | 0.060 |
25 | 1.9 | 0.067 |
50 | 2.2 | 0.072 |
100 | 2.4 | 0.080 |
(b) 1. Unless an extensive drainage report is prepared by a registered professional engineer with supporting data on soil types, vegetation and historic flows, the following design procedures shall be used:
Calculation Of Runoff | Curve Numbers |
Impervious surface | CN = 98 |
Lawn areas | CN = 60 |
Wood or forest land | CN = 70 |
2. A weighted CN shall be calculated for the immediate area proposed for development. Open space areas will be lumped in separate tributary basins and a CN of 70 assigned to them independent of the developed area.
SAMPLE:
Total drainage shed = 5.0 acres
Immediate developed areas = 0.5 acres
Open space area = 4.5 acres
Impervious surface = 70% of developed area
Apply CN = 70 to 4.5 acres and calculate runoff
Weighted CN for developed area:
Weighted CN = .7 x 0.5 x 98+.3 x 0.5 x 60 = 86.6 = 87
.5
(c) Calculation of runoff depth.
1. Open space: From CN = 70 and 25-year precipitation of 1.9 inches; runoff depth = 0.20 inches.
2. Developed area: From CN = 87 and 25-year precipitation of 1.9 inches; runoff depth = 0.83 inches
(d) Calculation of flow (cubic feet/second).
Open space: Use Figure SI-A, flat slope (16%)
peak discharge (cfs/inch of runoff) = 8.3
Q = 8.3 x 0.2 = 1.7 cfs
Developed area: use Figure SI-A, flat slope (1%)
peak discharge = 1.6
Q:= 1.6 x.83 = 1.3 cfs
For piped systems, tributary areas for each inlet should be calculated and the flows calculated as above.
(7) Culverts. Culverts are to be located at each natural draw or watercourse as conditions warrant to prevent excessive accumulation of flow in street-side ditches or along the toe of slopes. Draws and watercourses are to be cleared of debris for a distance of 100 feet upstream from all culvert inlets.
(a) Inverts at the inlet should be slightly elevated above the normal flow line in steep natural draws to avoid plugging by debris. Excessive ponding shall be avoided.
(b) The culvert should slope downward in the direction of natural flow and designed to be self-cleaning wherever possible. The outlet should be designed so as not to discharge on unprotected fills or unstable material or at adverse angles to streams or open channels. Headwalls, riprap or other means of protection are required at inlets or outlets where erosion may occur.
(c) Minimum diameter for round pipe shall be 12 inches. The minimum rise of arch pipes and box culverts shall be 12 inches.
(d) Cleanout access shall be provided at least every 200 feet for pipes 24 inches in diameter or less, and at least every 400 feet for larger pipes. Cleanout accesses shall also be required at each angle point and at each change in grade.
(8) Subsurface drainage. Subgrades subject to poor drainage, underground seepage or a high water table must be adequately drained for roadbed stabilization. Drains must be installed to control or prevent the high ground water level from coming to within four feet of the roadway pavement. Surface drainage will be considered as an alternate.
(9) Inlet structures. Acceptable designs for inlet structures will be permitted on approval by the Town Engineer. Curb openings with protection bars are preferred. Special grates are permitted where pedestrians, bicycles, or debris laden flows are anticipated. (See Figures 3.4 and 3.5.*)
(L) Traffic control devices.
(1) All signs, striping, marking, delineators, signals and other traffic control devices are to conform to the requirements of the state as outlined in uniform traffic control devices.
(2) In new developments, all required street name signs are to be installed and paid for by the developer. Nonstandard signs or other traffic control devices are subject to rigid state control and approval by the Town Engineer must be obtained for their use. Requests for using nonstandard signs or other devices must be submitted to the Town Engineer along with all data required to support the request.
(Ord. 2(1983) § 1; Ord. 22(1999) § 2)
Editor’s note:
*Figures 3.1, 3.2, 3.3, 3.4 and 3.5 are available in the Town Engineer’s office.