The following requirements shall be incorporated into the design of all roads.
(A) Horizontal alignment.
(1) Horizontal curves. When a roadway has a deflection angle of one degree or greater, the designer must design the roadway with a horizontal curve. The arterials and collectors should have a minimum centerline radius of 300 feet but should avoid minimum radii to minimize using maximum superelevation rates. All other types of streets shall have a minimum radius of 100 feet. The arterials and collectors should have a minimum length of curve of 500 feet if the deflection angle is five degrees or less. If the deflection angle is five degrees or greater, the minimum length of curve shall be consistent with the BLR Manual.
(2) Superelevation. The selection of a maximum allowable superelevation and superelevation rate shall conform to the appropriate articles of the latest revision of the BLR Manual.
(B) Vertical alignment.
(1) Grades. The maximum profile grade for all streets shall be 8% and the minimum grade on all streets shall be 0.50%. All grades shall be subject to the approval of the County Engineer. If an engineering design or construction situation arises not covered by the ordinance herein, the option of the County Engineer or county representative shall be final.
(2) Vertical curves. All grade changes shall be connected by vertical curves. The length of the vertical curve will depend on the algebraic difference between the two tangent grades and required site distance. The minimum crest and sag vertical curve length for all arterial and collector streets shall be consistent with the BLR Manual. The minimum crest and sag vertical curve length for all other roadways shall be 30 feet.
(C) Cross-section elements.
(1) Guidelines. All streets to be constructed by the subdivider shall be in accordance with the following guidelines and the BLR Manual, which are subject in every case to the discretion of the County Engineer. See Appendix A and Appendix B for additional details.
(2) Widths.
(a) Generally.
Type of Street | Pavement Width (Edge of Pavement to Edge of Pavement) |
Pavement Width for Rural Functional Classification Categories
| |
Type of Street | Pavement Width (Edge of Pavement to Edge of Pavement) |
Pavement Width for Rural Functional Classification Categories
| |
1. Minor arterials | 24 feet minimum |
2. Major collectors | 24 feet minimum |
3. Minor collectors | 24 feet minimum |
4. Local roads | 20 feet minimum |
5. Streets with a cul-de-sac | 24 feet minimum |
6. Alleys | 10 feet minimum |
Urban Functional Classification Categories
| |
7. Minor arterials | 24 feet minimum |
8. Collector streets | 20 feet minimum |
9. Local streets | 20 feet minimum |
10. Streets with a cul-de-sac | 24 feet minimum |
11. Alleys | 10 feet minimum |
(b) Increased width. Increased widths may be used depending in the anticipated traffic volumes and parking demands. The pavement widths shall be determined by the County Engineer after consultation with the subdivider’s engineer.
(3) Parking. The subdivider shall evaluate the demand for parking as required in the BLR Manual.
(4) Medians and islands. The subdivider shall evaluate the need for medians and islands as required in the BLR Manual. No sign may be installed in the median which blocks the sight distance at the intersection. If a sign is to be installed, detailed plans for the sign shall be submitted with the construction plans for approval. In no case shall an island or median contain any other sign or structure except as may be placed for traffic control under the direction of the County Engineer. The subdivider may landscape medians or islands. Ground cover may not exceed a maximum height of 12 inches. Any additional living plant material must be installed to allow visibility across, over, or through medians and islands at a height of three to ten feet above the adjacent roadway pavements.
(5) Curb and gutter. The subdivider shall design the barrier curb and gutter in accordance with the BLR Manual. Mountable curb and “V” gutters shall not be allowed adjacent to roadway pavement or shoulders.
(6) Cross slopes. All streets with the exception of an alley shall have a minimum transverse cross slope of 1.5% sloping away from the centerline of the roadway. Alley cross-sections may be V-shaped with transverse slopes up to 2.5% toward or away from the centerline of the roadway. If the alley has a transverse slope toward the center of the roadway, the vertical alignment shall be designed to ensure positive drainage to a storm sewer system.
(7) Sidewalks. All sidewalks constructed by the subdivider shall comply with the State Accessibility Code, being 71 Ill. Admin. Code §§ 400.110 et seq. The minimum sidewalk width shall be five feet and minimum thickness shall be five inches. The minimum clear width at any isolated point along an accessible route shall be three feet. The sidewalk cross slope shall not exceed 2.0%. Sidewalks should be separated from roadways by curbs, planted parkways, or other barriers, which will be continuous except at driveways or alleys.
(D) Driveways. Residential driveway widths shall be 12 feet minimum and shall not exceed 24 feet, with a minimum thickness of six inches of PCC pavement. Commercial and industrial driveway widths vary from 24 feet to 35 feet depending on volume of traffic. For two-way driveways, the centerline should be at right angles to the roadway. Angles less than 60 degrees should only be used for one-way residential driveways.
(E) Cul-de-sacs. There shall be a minimum outside radius of 50 feet for residential, commercial, and industrial cul-de-sacs. Streets with cul-de-sacs should not be greater than 500 feet in length unless approved by the Committee and County Board.
(F) Intersections.
(1) Curb or radius pavement returns at street intersections shall be rounded by radii of at least 25 feet.
(2) Roads shall be laid out to intersect as nearly as possible at right angles, and the angle of intersection should be within 15 degrees of the perpendicular. Under restricted conditions, where obtaining the right-of-way to straighten the angle of intersection would be impractical, an intersection angle up to 30 degrees from the perpendicular may be used.
(3) The arrangement of streets in new subdivisions shall make provisions for the continuation of the principal existing streets in adjoining additions (or their proper protection where adjoining property is not subdivided) insofar as they may be necessary for public requirements. The width of the streets in new subdivisions shall not be less than the minimum street widths established herein. The street arrangement must also be so as to cause no hardship to owners of adjoining property when they plat their own land and seek to provide convenient access thereto.
(G) Electric distribution. The underground electrical distribution system shall be installed on utility easements along rear or side lot lines per § 154.086.
(H) Street lighting. The subdivider shall have the option to install street lights along public streets in the county in accordance of the American National Standard For Roadway Lighting prepared by the American National Standards Institute, published by the National Electrical Manufacturers Association (NEMA), and duly adopted by reference from time to time by the County Board. If the subdivider chooses to install street lighting, the layout of the streetlights are required on a photometric plan for approval by the Committee and the County Engineer. The lighting performance standards shall meet the City of Decatur Zoning Ordinance, § XXVIII.
(I) Pavement design.
(1) The subdivider shall design the pavement structure according to soil boring information taken from the proposed site with the methods detailed in the BLR Manual. The minimum pavement thicknesses shall not be less than the pavement thicknesses detailed in Appendix A and Appendix B. The pavement structure shall be constructed according to the requirements of the SSRB adopted by IDOT including supplement specifications of this state.
(2) These requirements are the minimum standards of construction only and do not preclude construction of roads and streets to higher standards, at the subdivider’s expense upon approval by the County Engineer.
(J) Drainage design.
(1) Methods of determining stormwater runoff rate and volume. Drainage and storage facilities shall be designed using the Rational Method of calculating runoff discharge rate and total volume for watersheds 50 acres or less. The Natural Resource Conservation Service (NRCS) Method (TR20 or TR55) can be used for any watershed but shall be required for watersheds over 50 acres.
(2) Detention. The detention reservoir shall be sized using one of the methods stated in division (J)(1) above releasing a ten-year storm event for the existing pre-developed condition while detaining the 50-year storm event for the proposed post-developed condition. If the imperviousness is reduced by the proposed development, no detention shall be required. The stormwater storage area shall be provided with a method of emergency overflow in the event that a storm in excess of the 50-year return frequency storm occurs. The emergency overflow shall be designed to safely convey a 100-year storm.
(3) Controlling entity. If the detention reservoir is being released into a watershed governed by a more controlling entity, the detention reservoir shall meet the controlling entities’ design criteria on detention reservoir design.
(4) Retention. Retention reservoir stormwater system shall be designed with all the items required for detention storage and the following. There shall be a restrictor, weir, or other device to reduce flow out of the reservoir in order to store the required volume above normal pool elevation. Minimum normal water depth shall be six feet. If fish are to be used to keep the reservoir clean, a minimum normal depth of ten feet shall be required. Aeration hardware to prevent pond stagnation shall be provided, if necessary. Design calculations to prove the effectiveness of the aeration hardware shall be submitted with the construction plans.
(5) Freeboard. Detention/retention reservoirs shall have adequate capacity to contain the storage volume of tributary stormwater runoff with a minimum of one foot of freeboard above the water surface of flow in a 100-year emergency overflow storm event.
(6) Stormwater facilities. Where development of a property presents the threat of flooding or damage by flash flood runoff to downstream residents, the facilities for stormwater runoff control shall be constructed prior to any other construction on the project site.
(7) Ditch. When the rural estate section for streets is allowed, the pipe running longitudinally along the roadway shall conform to the requirements of division (J)(9) below, storm sewers, and the details shown in Appendices B and D to this chapter. If the ditch capacity calculations show the headwater for the maximum design year encroaching onto the roadway, the subdivider shall provide additional appropriately spaced inlets to convey the water away from the roadway. If it is not feasible to outlet the water from the proposed site to a natural drainage course using the rural estate section, with prior permission from the County Engineer, the subdivider may use the open ditch method for rural residential streets. If the open ditch method is utilized, the minimum ditch grade allowed shall be 0.3% to minimize ponding. All ditch grades greater than 4% shall be paved with concrete, as directed by the County Engineer. The open ditch shall be trapezoidal in shape with a one-foot minimum ditch bottom. The foreslopes and backslopes shall not be greater than 1V:2H. Design storm frequencies for rural and urban roadways shall be in accordance with the BLR Manual. All ditch capacities shall be determined with the methods described in the latest revision of the IDOT Drainage Manual.
(8) Typical residential lot grading. All lot drainage patterns for residential development shall convey positive drainage away from the proposed residence as shown in Appendix C and Appendix D.
(9) Storm sewers. Storm sewers shall be designed in accordance with the BLR Manual. Size the storm sewer system using a ten-year flood event. The minimum size for storm sewers shall be 12 inches for main lines and eight inches for lateral lines along all roadways. The minimum velocity allowed in storm sewer systems is three feet/second flowing full. Pipe and fittings shall be solid wall conforming to Class B material requirements in § 550 of the SSRB. Single-wall corrugated plastic pipe will not be allowed.
(10) Culverts. All culverts shall be designed in accordance with the IDOT Drainage Manual. The minimum permissible diameter culvert for entrances is 12 inches and 18 inches for all other roadway facilities.
(11) Surface and subsurface drains. All lots shall have access to a drain inlet placed on public right-of-way for surface and subsurface stormwater sources. All surface and subsurface drains such as sump pumps, roof drains, and the like shall be connected to these inlets underground. Maintenance of these drainage systems shall be the responsibility of the homeowner from the point of connection at the inlet back to the source of water collection. Water from these sources shall not be allowed to dump from a lot onto the surface of the roadway right-of-way. Directing this drainage towards an existing surface swale or drainage way such as a creek or stream will be allowed, provided all other applicable drainage design requirements of this chapter are satisfied.
(a) Curb and gutter sections. Inlets and manholes shall be placed on public right-of-way allowing access for each lot to connect to a drainage structure.
(b) Rural estate sections. The subdivider shall require the home builder to install an inlet or catch basin on the storm sewer line. This requirement shall be included on the subdivision plat. The drainage structure shall be installed upstream of the driveway entrance to the lot if possible. The subdivider shall install the storm sewer in such a manner that each lot will have access to it.
(K) Final plans. Final engineering plans shall show complete details for all items covered in this section. Plans, specifications, and all calculations for stormwater runoff control and storm sewers shall be submitted for review and approval as part of the final plat submittal.
(Ord. O-72-5-08, passed 5-8-2008)