(A) Soil report required. All public and private roadway designs shall be based on the results of a soil investigation performed by a registered geotechnical engineer or a registered civil engineer with expertise in soil investigation. The report will address roadbed foundation conditions, grading considerations, slope stability (for slopes in excess of 15%) and special conditions expected, such as highly organic or soft soil or shallow bedrock, which may affect design or construction. The report shall specifically determine the design resistance (“R”) value of native materials at the proposed subgrade elevation to allow proper design of the roadbed structural section.
(B) Structural section.
(1) Thickness.
(a) All roadway designs shall be based on the recommendations of the required soil report. The thickness of the structural section elements shall be determined from the structural pavement design chart in the standard plans.
(b) A minimum asphalt concrete (A.C.) thickness of two inches shall be used on all streets except collectors, arterials and expressways where the minimum A.C. thickness shall be three inches.
(2) Traffic indexes (T.I.). Traffic indexes shall be determined in the traffic report or as required by the County Engineer. The following minimum shall apply:
T.I. |
T.I. | |
Common private driveway | 3.0 |
Cul-de-sac or loop | 4.0 |
Local residential | 4.5 |
Collector-non access | 5.0 |
Collector with access | 6.0 |
Arterial | 7.0 |
Expressway | 8.0 |
Freeway | Per CALTRANS |
(C) Design speed.
(1) Design of all roads shall be consistent with the speeds expected and shall follow the recommendation of the traffic report. The following speeds shall be the minimum allowed for road design:
Classification | Minimum Design Speed |
Classification | Minimum Design Speed |
Common-private drive | 10 mph |
Cul-de-sac | 20 mph |
Local/residential through street | 30 mph |
Collector | 40 mph |
Rural road | 50 mph |
Arterial | 60 mph |
Expressway | 70 mph |
Freeway | Per CALTRANS |
(2) The County Engineer may require higher design speeds at locations where higher speeds may occur.
(D) Horizontal layout.
(1) Intersection offsets. Streets located on opposite sides of an intersecting street shall have their centerlines directly opposite each other, otherwise the centerlines shall be separated by not less than 150 feet for local roads and 300 feet for collectors. Arterials and expressways shall be intersected only by collectors or higher classification roadways and only at intervals acceptable to the County Engineer.
(2) Continuation of existing streets. New subdivision streets that are continuations of existing streets shall be aligned so that their centerlines coincide. In cases where straight continuations are not physically possible, centerlines shall be continued by curves.
(3) Horizontal curves.
(a) Minimum curve radii. The minimum centerline curve radii, except where physical conditions make compliance impractical, shall be as follows:
Minimum Centerline Radius |
Minimum Centerline Radius | |
Expressways | 2,400 |
Arterials | 1,800 |
Collectors-access | 1,200 |
Collectors-non access | 1,200 |
Local/residential-through street | 300 |
Residential cul-de-sacs | 100 |
Common private driveway | 60 |
(b) Tangent length between curves. The minimum tangent length between reversing curves shall be 250 feet for expressways, 150 feet for arterials and collectors and 100 feet for all other classifications, except 50 feet for private drives or as approved by the County Engineer. Broken back curves are not allowed.
(c) Compound curves. The use of compound curves should be avoided where possible. Where special topography or other conditions exist, the use shall be as approved by the County Engineer.
(4) Corner radii. The corner property line, curb line and edge of pavement radii at street intersections shall be as follows:
Type of Intersection | Required Radius Ft. |
Cul-de-sac and local | 20 |
Collectors | 25 |
Arterial or expressway | 35 |
The highest classification of the intersecting roads shall govern. | |
(5) Intersection angle. Streets shall intersect at an angle as near to 90 degrees as practical and not less than 80 degrees. The central angle on the property line corner curve of adjacent corners shall be within ten degrees of each other. Intersections shall be designed on tangent sections at least 50 feet beyond the right-of-ways for the intersecting roads.
(6) Cul-de-sac streets. Cul-de-sac streets may be allowed as follows:
(a) Maximum length. Cul-de-sac streets shall not exceed the lengths specified herein. In no case shall more than 50 dwelling units be served by a cul-de-sac street.
(b) Turnaround. Cul-de-sac streets shall be terminated by an improved turnaround having a minimum right-of-way radius of 50 feet.
(c) Alternate turnaround. Hammerheads, loops, offset bulbs and other geometric designs may only be used under special circumstances and only with the approval of the County Engineer.
(7) Sight distance at intersections. Streets shall not be designed with intersections on the inside of curves or at any location in general where sight distance will be inadequate for drivers to tell if they can safely enter the traffic flow or cross the street. The minimum distance from an intersection to a curve shall be determined according to the California Department of Transportation Highway Design Manual. Exceptions may be made by the County Engineer for especially difficult design circumstances only if visibility easements to provide adequate sight distances are established. In lieu of visibility easements, additional street right-of-way may be dedicated.
(8) Horizontal sight distance.
(a) Where objects that may obstruct a driver’s vision are adjacent to the roadway, the design of horizontal curves shall be such as to attain the required stopping site distance as set forth in the California Department of Transportation Highway Design Manual, latest edition.
(b) Where a driveway or roadway intersects a roadway with a curved centerline, a limited use area, clear of obstructions, including parking, landscaping walls and structures, shall be maintained for sight distance. The areas shall be delineated on the improvements plans and restricted by map provisions or by deeds.
(E) Profile standards.
(1) Minimum grades. The minimum grade in all streets shall be 0.25 %. This minimum may be reduced to 0.20% around the outside of horizontal curves. Flowline grades in cul-de-sac shall be maintained at a minimum of 0.25%. These restrictions do not refer to centerline grades of vertical curves. Curb and gutter elevations on vertical curves shall be adjusted to meet a 0.25% minimum grade. Where matching existing conditions, minimum grade may be reduced only with the approval of the County Engineer.
(2) Cross slopes. The standard cross slope shall be 2%. For street widening projects, the minimum cross slope shall be 1.5% and the maximum cross slope shall be 3%. Superelevated sections may deviate from this section. The cross slope of a street widening project shall match the cross slope of the existing pavement whenever possible. Road widening cross-slope must equal or exceed cross-slope of existing, or overlay to a constant cross-slope will be required.
(3) Vertical curves.
(a) Required. Vertical curves shall be required whenever the algebraic difference of grades is 1% or greater for local streets and 0.5% or greater for collectors, arterials, expressways and rural roads.
(b) Minimum length. The minimum length of vertical curves shall be determined by consideration of passing and stopping distance requirements, headlight sight distance, drainage control and aesthetic appearance. Where other considerations do not govern the minimum length of the vertical curve for collectors, arterials and expressways, shall be L=AV2/46.5 at sags and L=AV2/150 at crests, where A is the algebraic difference in grades in percent divided by 100 and V is the design speed in miles per hour. In no case shall less than 100 feet be used.
(4) Stopping sight distance. The minimum stopping sight distance shall be measured from a point three and one-half feet above the pavement surface (assumed height of driver’s eyes) to a point one-half feet above the pavement (assumed object height). Minimum distances are as follows:
Recommended Design Speed | Minimum Stopping Sight Distance |
Recommended Design Speed | Minimum Stopping Sight Distance |
10 mph | 50 feet |
20 mph | 125 feet |
30 mph | 200 feet |
40 mph | 300 feet |
50 mph | 430 feet |
60 mph | 580 feet |
70 mph | 750 feet |
(5) Intersections. Profile of intersecting streets shall have a maximum slope of 3% within 50 feet of the curb line of the intersection. Variations to this requirement may be approved by the County Engineer for unusually steep terrain or other extenuating circumstances.
(F) Curb, gutter and sidewalk.
(1) Curb, gutter and sidewalk is required in urban and suburban residential areas (minimum lot size less than five acres).
(2) Curb, gutter and sidewalk in commercial or industrial areas shall conform to conditions of development approval.
(3) Curb, gutter and sidewalk on arterials and expressways may be required in urban and suburban areas.
(4) Cross gutters are not allowed on any road except local residential roads with the approval of the County Engineer. The developer shall submit evidence that the intersection cannot reasonable be drained to an underground system before cross gutter will be considered.
(5) Median curbs may be constructed using modified type B curb.
(6) Barrier curbs may be required by the County Engineer at such locations as deemed necessary to control drainage, delineate traveled ways, provide for safe pedestrian and vehicular passage and the like. Where required, barrier curbs shall conform to Type A1-6 of the California Department of Transportation Standard Plans.
(7) Handicapped ramps shall be constructed at all curb returns in residential and commercial areas and at such other locations required by the County Engineer. Ramps shall conform to standard drawing.
(8) Sidewalk widths shall be measured from face of curb to back of walk. Minimum widths shall be per applicable standard drawing.
(9) Sidewalk widening may be required at the discretion of the County Engineer in areas such as school zones, local commercial areas, bus stops, rural postal drop boxes, near bicycle ways and trails or other areas deemed appropriate. Where sidewalk widening is required, the County Engineer shall give width, length and transition requirements.
(10) In special situations with the approval of the County Engineer, sidewalks may be separated from the curb and gutter by a landscaping strip. The developer shall submit complete detail of proposed dimensions and landscaping.
(G) Driveways. Driveway design shall conform to approved standard drawings.
(H) Sound barrier design.
(1) Location requirements. Sound barriers may be required along the rear and side property lines of residential developments adjacent to limited access roads and other ground level noise elements to achieve the noise control objectives of the county’s general plan noise element or conditions of development approval.
(2) Sound study. When it appears to the County Engineer that a sound barrier may be necessary or when a sound barrier is a condition of development, a sound study prepared by an acoustical consultant shall be submitted to the Department of Public Works before the improvement plans will be approved by the County Engineer. The sound study shall include a recommended height and termination points for the sound barrier including all backup material leading to the recommendations.
(3) Design.
(a) The sound barrier shall be designed to conform to levels in the general plan at the affected building setback line or as required by the Department.
(b) The County Engineer may approve new sound barriers upon request. The request shall include plan details and calculations prepared and signed by an appropriate consulting engineer.
(c) Sound barriers shall be designed for a minimum service life of 30 years.
(d) Sound barriers normally will not be allowed within public rights-of-way.
(4) Plan requirements. All construction details for sound barriers, including the locations and limits, shall be shown on the site improvement plans.
(I) Survey monuments.
(1) Required locations. The developer shall place permanent survey monuments at the locations shown on standard drawings.
(2) Map Act compliance required. Qualifications of persons setting monuments, ties to nearby permanent objects and preparation of all required maps to be recorded shall rigidly adhere to the requirements of the State of California Subdivision Map Act, Land Surveyors Act and Professional Engineers Act.
(3) Protection of existing monuments. All improvement plans shall contain a note stating that the contractor is responsible to preserve and protect all existing survey monuments or other survey markers. Any monuments, damaged, displaced, obliterated or lost shall be re-established or replaced by persons authorized to practice land surveying at the contractor’s sole expense.
(J) Signage.
(1) Barricades. Barricades shall be required at the end of the paved section (or traveled way for non-paved street) of all terminating streets. Barricades shall comply with standard drawing.
(2) Street names.
(a) All street names for new streets shall be named by the developer subject to the approval of the Communications Department. Duplicate names, near duplicate and phonetically similar names to street names already in use shall not be allowed. Names with more than 13 letters are not allowed.
(b) To the extent possible, extensions of existing streets or streets that will in the future be part of a planned existing street extension, shall have the same name as the existing street.
(c) Street names and street name sign locations shall appear on plans submitted for approval. Sign details shall be as shown on standard drawing.
(3) Street sign locations.
(a) Minor intersections. One street name sign installation [with two sign plates (four faces) on each post] is required at each intersection where each intersecting street has a right-of-way width of less than 84 feet. At a four-way intersection, the installation shall be located on one of the far right-hand corners of the intersection relative to the street having the greater right-of-way width or relative to the more important street if the right-of-way widths are equal.
(b) Major intersections. Two street name sign installations [with four sign plates (four faces) on each post] are required at each intersection where one or both of the intersecting streets has a right-of-way width of 84 feet or greater. At a four-way intersection, the installation shall be located on both far right hand corners of the intersection relative to the street having the greater right-of-way width or relative to the more important street if right-of-way widths are equal.
(c) Asymmetric intersections. For asymmetric intersections, or for expressways, arterials and freeways, street sign location shall be determined by the County Engineer.
(4) Traffic signs. Traffic signs, including warning signs, information signs, speed limit signs and the like shall be installed for all streets as required by the County Engineer. Signage shall conform to the requirements of standard details. Where required signs are not detailed in the standard drawings, California Department of Transportation Traffic Manual, Highway Design Manual and Manual of Uniform Traffic Control Devices (MUTCD) shall be used for guidance on sign size, shape, lettering and location, subject to the approval of the County Engineer.
(K) Striping and marking. Design of pavement striping and marker placement shall be in accordance with State of California Department of Transportation Traffic Manual and MUTCD subject to the approval of the County Engineer.
(L) Bridges and overpasses.
(1) Bridges and overpasses spanning 20 feet or more shall have their structural elements designed by an Engineer with appropriate registration who shall obtain the approval of the County Engineer for all design assumptions.
(2) The design width of overpasses and bridges shall be the full width of the right-of-way unless narrower widths are approved by the County Engineer.
(3) Bridges over water courses shall be designed such that a three-foot freeboard is provided between the 100-year design flow water surface elevation and the lowest portion of the bridge span. Detailed hydrologic and hydraulic calculations shall be submitted by a Registered Civil Engineer to document the 100-year water surface elevation.
(4) The geometric design on bridges and the approaches on either side shall consider the potential for bridge icing during extreme weather conditions, particularly in higher elevation areas of the county. Curved alignments should be avoided.
(M) Street lights.
(1) When required.
(a) Street lights shall be required whenever the County Engineer deems them necessary to provide vehicular safety during hours of darkness. In general, street lights are required in urban and suburban areas, at local street intersections, along collectors, local commercial and industrial roads and minor arterials. Expressways, major arterials and freeways shall have special lighting considerations at turning pockets, intersections and interchanges.
(b) Street lights are generally not required in rural areas along rural roads except at intersections or at the direction of the County Engineer.
(2) Relocation. Existing street lights that must be relocated, repositioned or reconfigured as a result of construction of new streets, widening of existing streets or construction of driveways into a development shall be the responsibility of the developer.
(3) Utility company authorization. A written notice from the serving utility company stating that line clearances and service requirements have been checked and are adequate shall be submitted to the County Engineer for all developments.
(4) Luminaries. The type of street light and the appropriate wattage shall be specified on the plans. Luminaries shall be high pressure sodium type with internal ballast unless otherwise specified by the County Engineer.
(5) Existing utility-owned poles.
(a) Where there are existing or planned utility owned poles adjacent to the roadway, street light installation must be coordinated with the utility company. Whenever possible street lights should be installed on existing utility poles, and planned street light locations may be adjusted up to 25 feet to use an existing utility pole.
(b) In the Pacific Gas and Electric Company (PG&E) service area, the developer shall arrange with PG&E to install PG&E owned and maintained street lights on PG&E poles in accordance with current utility rate schedules.
(6) Design standards. Street lighting shall be designed in accordance with these standards, the construction specifications and the American National Standards Institute, “American National Standard Practice for Roadway Lighting”.
(7) Calculations required. All data and calculations supporting the street light design and location shall be submitted to the County Engineer for review with the improvement plans.