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Chapter 12.52
DESIGN CRITERIA FOR ROADS AND STREETSSections:
12.52.010 Scope.
12.52.020 General requirements.
12.52.030 General design principles for new roads.
12.52.040 Design requirements—New and reconstructed roads.
12.52.050 Stormwater management.
12.52.060 New utilities.
12.52.070 Connections to existing utilities.
12.52.010 Scope.
The purpose of this chapter is to present criteria for the design of roads and streets. It is to be used by developers and their engineers in the design of both public, private roads, exempt private roads and streets for which approval by the county engineer is required. (Ord. TLS 07-04-30B Exh. B (part): Ord. TLS 04-02-30B Exh. A (part))
12.52.020 General requirements.
A. Minimum Standards. The provisions stipulated in this section are required minimum standards and shall be considered applicable to all parts of these specifications including any supplements and revisions. All construction within the public right-of-way shall be designed by or under the direct supervision of a registered professional engineer licensed to practice in the state of Washington. All drawings and support data submitted to the county for approval must bear his/her seal. The signature of the registered professional engineer will only be required on the approved final plans.
The design criteria, as presented, are intended to aid in preparation of plans and specifications and include minimum standards where applicable. These design criteria are considered minimum and a complete design will usually require more than is presented in this document. As with any design criteria, occasions may arise where the minimum standards are either inappropriate or cannot be justified economically and an equal but alternative solution may be proposed. A written request for each alternative shall be directed to the county engineer and shall conform to criteria in DCC Section 12.50.100, Alternatives.
B. Application to Private Roads. Although community road requirements are usually best served by public roads owned and maintained by the county, private roads may be appropriate for some local access roads for either residential or commercial/industrial property. These standards apply to private roads as part of the county’s responsibility to ensure adequate access for normal and emergency vehicle use to individual parcels throughout the county.
Private roads are permitted only when all of the following conditions are met:
1. Permanently established by tract or easement providing legal access to each affected lot, dwelling unit, or business and sufficient to accommodate required improvements, to include provision for future use by adjacent property owners when applicable.
2. Constructed to Douglas County Road Standards as set forth herein.
3. Accessible at all times for emergency and public service vehicle use.
4. Not obstructing, or part of, the present or future public neighborhood circulation or arterial plan developed in processes such as the Douglas County Comprehensive Plan, applicable community plan, or capital improvement plan.
5. Designed for an average daily traffic count (AADT) of one hundred sixty or less vehicles per day utilizing current ITE guidelines. New private roads directly accessing a county road that will generate more than one hundred sixty AADT will be permitted only by approval by the county hearing examiner and will be required to meet the appropriate county road rural or urban road standards contained herein (see Figures 3-2 through 3-8). There is no maximum AADT provision if a new development is served by an extension of, or spur off, an existing private road, however, should such existing private road be less than the standards considering the current and proposed AADT, the new development may be denied unless a lower level of improvement has been approved by the county hearing examiner upon recommendation by the county engineer or the applicant assumes responsibility for bringing the existing private road serving the new development up to these standards.
6. Maintained in accordance with these standards by a capable and legally responsible owner, homeowner’s association or other legal entity made up of all benefited property owners. A written road maintenance agreement addressing the rights and responsibilities of all benefited property owners shall be approved by the department prior to final approval of the land development. Said road maintenance agreement shall be recorded with the county and shall become a covenant with the affected properties. The term “benefited property owners” shall include the owners of record of all properties with frontage, including access rights, on the private road or otherwise have legal access, whether constructed or not, to the private road.
7. Clearly described as a private road not maintained by the county on the face of the plat, short plat or other development authorization.
8. Clearly signed at the road location as a private road.
C. Application to Exempt Private Roads. Although community road requirements are usually best served by public roads owned and maintained by the county, exempt private roads may be appropriate for some local access roads. Exempt private roads are exempt from county review for compliance with the comprehensive road standards.
Exempt private roads shall be permitted only when all of the following conditions are met:
1. All new or revised accesses onto a county road require an approved access permit as per the procedures in DCC Chapter 12.24, Approaches to County Roads. New exempt private roads directly accessing a county road that will generate more than one hundred sixty AADT will be permitted only by approval by the county hearing examiner upon recommendation by the county engineer.
2. Dimensions, slopes and details for all exempt private roads connecting to a county road shall at a minimum meet the standards included on Figure 4-1.
3. Clearly signed at the road location as a private road.
4. Clearly described as an exempt private road not maintained by the county. (Ord. TLS 07-04-30B Exh. B (part): Ord. TLS 04-02-30B Exh. A (part))
12.52.030 General design principles for new roads.
A. Road Network Circulation. The importance of good road network circulation for the health, welfare and safety of the public cannot be overemphasized. Poor circulation adds unnecessary miles to pedestrian and trail systems, school bus routes, mail delivery and other service deliveries, utility services and most importantly, emergency services such as police and fire. Through good road network circulation, the public will have better emergency access and police and fire safety will be enhanced.
1. Plans will be reviewed for the provision of the best possible road and pedestrian network circulation and for conformance with any adopted comprehensive plan. The road alignment may necessitate re-alignment in order to foster the long-range transportation objectives of the county. This includes greater scrutiny to provide continuity of pedestrian and other trail systems related to the proposed road network. The inclusion of permanent cul-de-sacs is strongly discouraged as they inhibit road and pedestrian network circulation (see DCC Section 12.52.040(L) for specific requirements) and are not consistent with the county comprehensive plan.
2. To facilitate the best possible road and pedestrian network circulation, if it is determined by the county engineer, after making an individualized determination, that the layout of roads are to provide for the continuation of existing roads in adjoining subdivisions, then the roads shall be constructed prior to final plat approval. When adjoining property is not subdivided, the county engineer shall determine whether roads in the proposed plat are to provide access to such unplatted property. The location for access to unplatted property shall be placed such that the objectives in these standards can be achieved. Reserve easement strips may be required to prevent unauthorized access until such time as the connecting roads are constructed.
3. If the roads are to remain private, the above still applies except a separate tract or easement will be shown on the final plat map and they will not be dedicated to the public. Specific information in the recorded covenants regarding the use of this easement will be required.
4. Unless otherwise approved, all lots within major subdivisions shall be accessed by means of an internal road network. This network may be public or private but not a mixture of both. Access easements or multi-dwelling driveways in lieu of public or private roads shall not be permitted.
B. On-Site Principles. An integral part of an overall traffic study relates to basic site planning principles. An integrated on-site roadway system should deliver vehicles from the external roadway system in a manner easily understood by typical drivers and that maximizes efficiency, accommodates anticipated traffic patterns and ensures public safety.
1. Alignment. Connecting street centerlines deflecting from each other more than ten degrees shall be connected by a curve the radius of which shall be approved by the county engineer. Street intersections shall be as nearly at right angles as is practicable, and street jogs having offsets of less than one hundred twenty-five feet shall be avoided.
2. Internal Vehicular Circulation. Internal circulation is the means by which vehicular traffic is delivered between entry points and parking areas, pick-up/drop-off points, and service areas, and should be planned to accommodate appropriate future traffic volumes.
3. Subdivision Boundary Streets. A street lying along the boundary of a subdivision may be dedicated with less than the width required by these standards if it is practicable to require the dedication of the remaining portion of such width when the adjoining property is subdivided. In such case there is required a reserve easement strip one foot wide along such street for the purpose of withholding access to the unsubdivided property from such street until a street is constructed to the full width required. The procedure shall also apply in the case of any street that dead-ends at the boundary of a subdivision.
4. Access Points. Roads and lots shall be laid out to provide individual lot access onto an internal roadway system, hence via the internal roadway system to the existing public road system. Direct access to a perimeter road shall normally be denied except for direct access onto a designated local access road.
5. Parking. Parking shall be provided to meet site-generated demands and be consistent with DCC Title 20 and other planning department policies.
6. Vehicular Queuing and Storage.
a. Access drives should provide adequate vehicular exit queuing.
b. Parking areas and access points of small developments should be designed so vehicles waiting to exit are aligned perpendicular to the off-site roadway system.
c. Queuing areas of large developments should be sufficient so vehicles queued at exits do not block internal circulation. Exits shall be signalized if warranted by the MUTCD at build out.
d. Documentation shall be provided to verify queue lengths for signalized intersections, on-site queuing reservoirs, and off-site left- and right-turn lanes.
7. Building Service Drives. Building service drives are roadways adjacent to a building and its entrances, and should be designed with sufficient width to serve as one or all of the following:
a. Fire and/or emergency vehicle access.
b. Pedestrian Pick-up/Drop-off Points. Pedestrian crossings and pick-up/drop-off points should be signed and striped to identify the vehicular/pedestrian conflict.
c. Internal circulation.
d. Recirculation in parking areas. Recirculation aisles shall have sufficient turning radii, clearances, sight distances and signing.
e. Transit passenger pick-up/drop-off areas.
8. Pedestrian, Bus, Bicycle, and Disabled Access Facilities. The overall site plans must consider pedestrians, bus, bicycle, and disabled access facilities.
a. Pedestrian Facilities. Pedestrian connections between public transportation facilities and site buildings shall be integrated into the overall project design. Pedestrian facilities shall be designed to reduce the motor vehicle use for trips within the development and between nearby developments.
b. Transit Facilities. Appropriate public transportation facilities, such as passenger shelters, ride sharing areas and bus staging areas shall be accommodated adjacent to service drive and entrance areas; at key locations along circulation drives; and at major pedestrian focal points along the external roadway system as determined the county and Link Transit.
c. Bicycle Facilities. Facilities for parking bicycles should be provided where bicycle use is expected. Refer to DCC Chapter 18.16.
d. Disabled Access Facilities. Access for disabled persons shall be provided in accordance with federal, state and county requirements.
9. Service and Delivery Vehicles. Service and delivery vehicles require separate criteria for movement to and from the site:
a. Vehicle turning paths shall be sufficient to accommodate the largest vehicles anticipated, a minimum single unit truck (SU).
b. Service vehicle access points shall have turning paths sufficient to allow service vehicles to enter and exit the site without encroaching upon opposing lanes or curbed areas.
c. External and internal roads shall have sufficient separation for large vehicles to be queued on entry or exit without blocking access to parking spaces or internal roadways.
C. Fitting the Road into the Environment. When land development requires the construction of new roads, there are opportunities to “fit” the road into the existing landscape and environment in ways that are more pleasing to the eye than just simple straight lines.
The principles set forth in this section stress the importance of protecting, conserving, and enhancing the scenic qualities of a county road. They primarily address rural situations but can apply to urban streets in many cases, especially in major subdivisions with an extensive new road system. They are also consistent with good engineering and the necessity to provide a roadway which is safe to travel and economical to construct and maintain. Unless conditioned by “shall,” these principles are not regulatory but are advisory only to provide guidance to developers.
1. Relating Alignment to the Landscape. A unique visual quality of most county roads is the harmonious relationship their alignments have with the landscape. Increased volumes of traffic, poor sight distance, or other operational conditions may often necessitate modification of an existing alignment. If such a change is necessary, the roadway geometry usually must become more precise and directional. However, a new alignment should not be considered a straight line connecting two points. Rather, it should seek the same qualities of existing alignments by reinforcing and revealing the features of the landscape. The following guidelines will be useful for relating new alignments with the landscape:
a. Choose an alignment that blends with the terrain and adjusts to important scenic features.
b. In most instances, the appropriate alignment will be characterized by curves that continually adjust to the rolling topography of natural landform. A curvilinear alignment is visually and functionally preferable to long tangents that cut through hillsides, leaving steep unsightly and unstable embankments.
c. Where the land is level, or a strong lineal direction is created by landscape elements, such as a long row of trees or the patterns of fields, the use of a long tangent may be justified. When using a long tangent, try to direct it toward a natural or manmade focal point.
d. When climbing a hillside, the roadway should bend to the crest, traversing the contours, rather than climbing it straight on. However, care must be taken to avoid hiding a curve or driveway just beyond the brow of a hill.
e. When crossing a ridge, pick a saddle or low area in the top to locate the roadway.
f. Natural and manmade features provide variety and contrast which maintain the traveler’s interest. Whenever possible, alignments should be located to bring the more interesting features into view.
g. Near the edges of surface water, woods, or a break in topography, use alignments that echo or emphasize the shape of the edges. However, avoid moving roadways close to the water’s edge as it destroys habitat.
h. When approaching important features, it is preferable to allow a distant view of the object, curve the alignment away, and then bring it close for a contrasting view.
A road which blends with the form and pattern of the landscape is also desirable from the standpoint of construction and maintenance. Some of the advantages to be gained are reduction of cut and fill quantities, more efficient utilization of natural drainage, and better control of roadside erosion because natural vegetation is preserved.
2. Combining Horizontal and Vertical Alignment. The combination of horizontal and vertical alignments closely influences the appearance and safety of a roadway. When alignments are properly coordinated, a roadway will be visually pleasing and safer to travel. Alignment coordination primarily applies to major roadways, but the basic principles should also be recognized as important considerations when altering minor roadways. Set forth below is a partial list of suggestions to guide the combination of horizontal and vertical alignments:
a. Consistency in the scale of horizontal and vertical elements should be maintained whenever possible. Small dips and humps should be avoided in what is actually a uniform grade, and “kinks” should be avoided in what is actually a long curve.
b. The beginning and ending of horizontal and vertical alignments should not occur in the same location. The beginning of a horizontal curve should generally occur before beginning a vertical curve and be somewhat longer in length. This provides a gradual transition between the alignments and prevents one from accentuating the other.
c. The beginning of a horizontal curve should not coincide with the top of a hill. This situation is visually deceptive and hazardous, as the quick change in horizontal alignment cannot be seen by the driver.
d. Avoid dips in vertical alignment before beginning a horizontal curve. This will prevent the roadway from appearing disjointed.
e. Avoid “broken back” curves (two horizontal curves in the same direction with a short tangent in-between), compound curves or reverse curves except for local access roads with a design speed of twenty-five mph.
f. When an extremely long grade is necessary, it may be better to adjust the vertical alignment so the grade is steeper near the bottom of the hill and gradually lessens as it approaches the crest of the hill. Another alternative is to create an alignment with intervals of lesser grades
g. Sight distance requirements vary with the anticipated speed of vehicles. Adequate sight distance must be provided. This should be checked at all horizontal curves and crest vertical curves.
3. Cross-Section. The small scale of cross-section elements is an important characteristic adding to the scenic quality of a county road. This aspect is most apparent in the width of the traveling surface, its adjacent shoulders, and the close proximity of the roadside. Where traffic volumes are low, speeds are slow, and meeting and passing of vehicles is infrequent, a narrow cross-section may be appropriate.
Modifications which require widening of the roadway will alter the existing scale of the county road and consequently its visual impact on the motorist and adjacent properties. The following considerations should guide the determination of an appropriate cross-section:
a. Appropriate widths should be determined by the function the road serves as part of the county road system, operational requirements for safe vehicular movement, and the characteristics of topography and other physical features (check results of planning considerations). In most cases, the minimum cross-section, based on rural/urban location, functional class and traffic volume, is shown in Figures 3-1 through 3-8 at the end of this chapter.
b. Consider all elements of the cross-section (traveling surface, shoulders, ditches, proper grading to stabilize cut and fill slopes, slope rounding, etc.).
4. Roadside Slopes. Proper molding of roadside slopes is essential during the grading operation. Slopes which do provide a smooth visual transition from the roadway to existing land forms have a pleasing appearance. Slopes shaped in this manner are also required for effective erosion control, adequate drainage, and reduced maintenance. Some general guidelines to follow when grading the roadside are set forth below:
a. Where the topography is flat to rolling and the landscape is open, slopes which are flattened and well-rounded are appropriate. Flattening of slopes to four to one (four horizontal to one vertical) should be carried out.
b. Where the topography is steep, uneven, and wooded, roadside slopes with grades of two to one or three to one should be favored to save roadside vegetation. However, check to make sure the slope is flat enough to be stable.
c. Vary the steepness of roadside slopes to save vegetation and other landscape features.
d. On areas of extreme cut, which may require easements or more right-of-way, the use of small benches, stepped down a steep slope, will slow water runoff and provide excellent locations where vegetation can quickly take hold. It is important to maintain a slight downhill pitch on these benches to provide adequate drainage.
e. On fill slopes of extreme length, larger benches can be formed to fulfill the same functions as above.
f. All slopes should be well-rounded to form a smooth transition from the shoulder edge to the existing grades. Deep ditches with well-defined bottoms are required where drainage or soils are poor. Rounded or shallow ditches are acceptable when there is little drainage and the soil is free draining.
g. All slopes should be warped by flattening the ends of cut and fill areas. This will avoid sharp breaks between new and existing grades and result in natural looking slopes that will more effectively support vegetation.
h. When grading the roadway, avoid disturbing important roadside vegetation and the creation of deep cuts that expose tree roots and leave steep banks that are susceptible to erosion and difficult to maintain.
5. Ditches. Ditches provide an important function in sustaining quality roads by providing adequate storm and subgrade groundwater drainage. However, excessively deep or wide ditches can severely impact vegetation, the rural feel of a road or safety. Several issues to consider when selecting a ditch section follows:
a. Slopes from the roadway to the ditch bottom shall be at least four to one or greater. The shallower pitches will allow for some vehicle recovery and less potential of a vehicle overturning on higher speed roadways.
b. Ditches must be deeper than the subgrade to allow drainage of roadway base and surfacing courses.
c. Where ditch construction may impact significant roadside features, short sections of culvert, curtain drains or shallow or no ditches at all should be considered, subject to approval by the county engineer.
d. Ditches must be constructed to adequately carry the anticipated water flow.
e. Biofiltration (or water quality) swales and grass buffer strips are encouraged at all locations where the road gradient makes them a practical alternative to regular roadway ditches. (Ord. TLS 04-02-30B Exh. A (part))
12.52.040 Design requirements—New and reconstructed roads.
The following minimum design standards shall apply to all new and reconstructed roadways. Exhibits showing the basic cross-section requirements for the various road classifications are shown in Figures 3-1 through 3-8 at the end of this chapter.
A. Road Base and Surfacing Requirements. The following road surfacing requirements shall apply to all new and reconstructed roads.
1. Surfacing. The minimum road surfacing requirements for various traffic volumes and locations are shown in Table 3-1:
Table 3-1 Road Surfacing Requirements
Location
Figure
Traffic Volume (AADT)
Road Surface Type
Rural—Private
3-1, 3-2, 3-3
Up to 160
Over 160Crushed Surf. Top Course
BST Class ARural—Public
3-2, 3-3, 3-4, 3-5, 3-6
Up to 400
Over 400ACP1
ACPUrban—Private
3-7, 3-8
All
ACP or PCC
Urban—Public
3-7, 3-8
All
ACP or PCC
1 BST Class A will be allowed for local access roads up to 400 AADT. All other classes require ACP.
2. Structural Sections—Public Roads.
a. Design Procedure. A roadway section structural design procedure shall be performed for all new and reconstructed public roads. The design life for all roads shall be twenty years with a growth factor as determined by the county engineer. The design procedure shall be approved by the county engineer and shall consider the following design elements:
i. Design Load—HS 20-44.
ii. Total Design Life Traffic Loading. An estimate of the number and types of loadings the roadway will carry for the design life. This estimate of loading shall be determined using a procedure accepted by the county engineer and be expressed in eighteen KIP equivalent single axle loads (ESALs).
iii. Subgrade Support. One or more representative values for the stiffness of the native material on which the road will be built. These values shall be established by a procedure accepted by the county engineer and be expressed as resilient modulus.
iv. Analysis. A procedure for establishing the roadway structural section for a given traffic loading and resilient modulus. This procedure shall be approved by the county engineer.
b. Construction Requirements. All structural sections including surfacing shall use materials meeting the specifications of and be constructed in accordance with the WSDOT Standard Specifications. Minimum compaction requirements shall be ninety-two percent for asphalt concrete pavement and ninety-five percent for crushed surfacing and base courses. See DCC Chapter 12.56 for details and further guidance.
3. Structural Section—Private Roads. Private roads with a projected AADT under one hundred sixty do not require roadway section structural design; however the requirement for minimum thickness will apply. Private roads with a projected AADT under one hundred sixty that are constructed without a roadway structural design will not be eligible for future inclusion into the county road system without further structural analysis as may be directed by the county engineer.
4. Minimum Thickness.
a. In all cases, the minimum roadway structural section shall be as shown on Figures 3-1 through 3-8 at the end of this chapter.
b. Many areas of Douglas County have soils excessively susceptible to frost heave. The applicant or applicant’s engineer shall consult with the county engineer’s office as to those locations with known frost heave problems or with soils likely to generate excessive frost heave. These locations may require additional base thickness.
B. Design Speed. Design speeds for urban and rural roads and streets shall be as shown in Table 3-2:
Table 3-2 Design Speeds
Terrain—Design Speed in MPH
Location/AADT
Figure
Flat
Rolling
Mount.
Urban—Private 1
3-7
25
25
20
Urban—Local access
3-7
25
25
25
Urban—Collector
3-8
35
30
25
Urban—Arterial
3-8
45
45
45
Rural—Private (400 or less) 2
3-1
40
30
20
Rural—Local access 400 and less
3-2
50
40
25
Rural—Local access over 400
3-3
50
40
30
Rural—Collector 400 and less
3-4
50
40
30
Rural—Collector 401 to 2,000
3-5
60
50
40
Rural—Collector and arterial over 2,000
3-6
60
50
40
1 Private urban shall meet the urban local access standard.
2 Private rural over four hundred AADT shall meet the rural local access standard.
See DCC Section 12.51.020(E) for terrain definitions.
C. Right-of-Way. The basic minimum rights-of-way for all roads are shown in Figures 3-1 through 3-8 at the end of this chapter. Additional right-of-way shall be required to accommodate other road features such as additional lanes, planter strips and transit stops. Construction and maintenance easements may be allowed for cut and fill slopes.
D. Lane, Shoulder and Roadway Width. Basic lane, shoulder and total roadway and surfacing minimum width requirements are shown in Figures 3-1 through 3-8 at the end of this chapter. Minimum widths are based on a combination of roadway classification and traffic volume (AADT).
Urban roadway sections (Figures 3-7 and 3-8) assume parking on both sides. Rural and private roadway sections do not include additional width for parking; when parking space is required, additional width will be required. Additional width may also be required to accommodate removal and storage of snow.
Where truck traffic exceeds fifteen percent of the projected AADT, twelve-foot lanes will be required for all rural roadway classifications.
E. Ditch Slopes. The slope from edge of shoulder to bottom of ditch shall be four to one for all ditch roadway sections; except that three to one will be allowed for rural private roads under one hundred sixty AADT. See DCC Section 12.57.020 for cut and fill slope requirements.
F. Sight Distance. All new roads and streets shall be designed to achieve the following sight distances:
1. Stopping Sight Distance: 2001 AASHTO Green Book, Exhibit 3-1, page 112.
2. Passing Sight Distance: 2001 AASHTO Green Book, Exhibit 3-7, page 124.
3. Intersection Sight Distance: WSDOT “Design Manual M22-10,” Chapter 910.10, “Intersections at Grade: Sight Distance at Intersections.”
G. Superelevation. Superelevation shall normally be applied to all new or reconstructed roads and streets. The maximum superelevation for roads with a design speed of thirty-five mph or greater shall not exceed six percent and for roads with a design speed under thirty-five mph shall not exceed four percent; provided, however, that the combination of superelevation and road gradient shall not exceed twelve percent at any point on the roadway surface.
H. Horizontal Alignment (Curvature). The minimum curve radius for all new or reconstructed rural highways and higher speed urban roads shall not be less than the “rounded radius” values in the AASHTO Green Book, Exhibit 3-14, page 145, using a maximum superelevation rate of six percent for design speeds of thirty-five mph or greater and a maximum superelevation rate of four percent for design speeds under thirty-five mph.
Low speed urban streets (design speed of thirty mph or less) may use the minimum curve radii as set forth in Exhibit 3-41, page 197 of the AASHTO Green Book based on a maximum superelevation rate of four percent. Lesser curve radii may be used only with the permission of the county engineer.
I. Vertical Alignment.
1. Maximum Grades. The maximum and minimum grades for each roadway classification are shown in Figures 3-1 through 3-8 at the end of this chapter.
2. Vertical Curves. Sag vertical curves shall be designed in accordance with the 2001 AASHTO Green Book, Exhibit 3-78, page 278, “Design Controls for Sag Vertical Curves—Open Road Conditions,” and Exhibit 3-79, page 280, “Design Controls for Sag Vertical Curves.”
Crest vertical curves shall be designed in accordance with the 2001 AASHTO Green Book, Exhibit 3-75, page 273, “Design Controls for Crest Vertical Curves—Open Road Conditions,” Exhibit 3-76, page 274, “Design Controls for Stopping Sight Distance and for Crest Vertical Curves,” and Exhibit 3-77, Page 276, “Design Controls for Crest Vertical Curves Based on Passing Sight Distance.”
J. Vertical Clearance. The minimum vertical clearance for all roadways under structures such as overpasses shall be sixteen and one-half feet.
K. Design Vehicle. The physical characteristics of vehicles and the proportions of various sized vehicles using the road system are positive controls in geometric design. For road design purposes, three general classes of vehicles have been selected: passenger cars, trucks and buses/recreational vehicles. The passenger car class includes compacts and subcompacts plus all light vehicles and light delivery trucks (vans and pickups). The truck class includes single-unit trucks, truck tractor-semitrailer combinations, and trucks or truck tractors with semitrailers in combination with full trailers. Buses/recreational vehicles include single-unit buses, articulated buses, school buses, motor homes, and passenger cars or motor homes pulling trailers or boats. In addition, where provision is made for bicycles on a road, the bicycle should also be considered a design vehicle.
The dimensions for the fifteen design vehicles representing vehicles within these general classes are provided in the 2001 AASHTO Green Book, Chapter 2. In the design of any road facility, the largest design vehicle likely to use that facility with considerable frequency or a design vehicle with special characteristics that must be taken into account in dimensioning the facility is used to determine the design of such critical features as radii at intersections and radii of turning roadways.
Unless unusual sized vehicles must be accommodated, the typical design vehicle used for design of roads shall be of the following classes:
Table 3-3 Design Vehicles
Roadway Class
Design Vehicles 1
Rural and urban arterials and rural collectors
Large school bus (S-BUS40)/Intermediate semitrailer (WB-50)
Urban collectors
Large school bus (S-BUS40)/Single unit truck (SU)
Rural and urban local access
Large school bus (S-BUS40)/Single unit truck (SU)
Private
Single unit truck (SU)/Passenger car (P)
1 Refer to AASHTO Green Book, Chapter 2 for specific design data.
L. Cul-de-Sacs and Dead-End Roads. Permanent cul-de-sacs and dead-end roads are not normally permitted within new developments. Permanent cul-de-sacs will only be permitted where the applicant can (1) demonstrate that his/her design does provide an acceptable level of network circulation considering the terrain and adjacent existing roadway network, and (2) demonstrate that the prohibition of cul-de-sacs will place an unreasonable economic impact on the applicant.
Where permitted, the following requirements shall apply:
1. Permanent road ends of one hundred fifty feet or less in length (measured from the edge of traveled way of the intersecting road to the end of the road) shall have a minimum right-of-way and roadway section as specified in Figures 3-1 through 3-8 at the end of this chapter and do require an access easement turnaround.
2. Permanent road ends between one hundred fifty and six hundred feet (measured from the edge of traveled way of the intersecting road to the beginning of the cul-de-sac) shall have a minimum right-of-way and roadway section as specified in Figures 3-1 through 3-8 and be provided with a cul-de-sac as shown in Figure 3-9 at the end of this chapter.
3. Permanent road ends in excess of six hundred feet are discouraged but may be allowed in cases where lots are large and/or difficult terrain exists, provided, the number of single-family lots served by the road does not exceed twenty or the projected AADT generated from the properties served by the road does not exceed two hundred. The roadway shall have a minimum right-of-way and roadway section as specified in Figures 3-1 through 3-8 and be provided with a cul-de-sac as shown in Figure 3-9 at the end of this chapter.
4. The maximum gradient in any direction and at any point within a cul-de-sac shall not exceed four percent.
M. Intersections. All intersections shall be designed in accordance with Chapter 9, “Intersections” in the 2001 AASHTO Green Book or Chapter 910, “Intersections at Grade” in the current edition of the WSDOT “Design Manual M22-01.” All intersections with a state highway shall require approval from the WSDOT.
Corner lots, located at road intersections, shall be rounded with a minimum twenty-foot radius adjacent to roads with sixty-foot or more rights-of-way and twenty-five-foot radius adjacent to roads with less than sixty-foot rights-of-way.
N. Boundary (Half) Roads. Boundary or half roads may be permitted to be dedicated with less than the width required by these standards if the county determines that it is practicable to require the remaining portion of such width when the adjoining property is subdivided. Boundary or half roads with widths less than those required by these standards are not allowed adjacent to public parkland or properties owned by public school districts.
O. Transit Stops and Pull-Outs. Property owners and/or developers of proposed developments or other types of land uses located within the Link Transit service area and which generate two hundred average daily or twenty peak hour vehicle trips, as determined by the county engineer, shall negotiate with the public transit authority. Improvements that would enhance the area for public transit may include bus shelters. Improvements may include bus shelters, pullouts, transit stops, and/or other necessary facilities to offset transportation system impacts of the development and shall be analyzed as part of a traffic impact analysis prepared in accordance with DCC Chapter 20.30.
P. Railroad Grade Crossings. All proposed railroad crossings on public right-of-way must be submitted to the county engineer prior to being processed through the railroad and the utilities and transportation commission for approval. Additional railroad crossings, especially across main line track, will not be allowed if alternative access is available.
Where additional railroad crossings are allowed, they shall be designed in accordance with the 2001 AASHTO Green Book, “Railroad-Highway Grade Crossings,” pages 735 through 745.
Q. Curb and Gutter.
1. Cement concrete curbs and gutters shall be utilized for street edges under the following conditions:
a. In areas where urban road standards are to be used.
b. On frontages with commercial usage.
2. Rolled edge, thickened edge or mountable curbs are not permitted as a substitute for curbs and gutters except on private roads, and may only be used in rural areas when approved by the county engineer.
3. On all sections constructed with curb and gutter a closed drainage system consisting of catch basins, storm sewer pipes and manholes shall be required unless alternative and appropriately designed methods of collecting and dispersing stormwater such as bio-infiltration swales and drywells are provided.
4. Curb and gutter shall be constructed in accordance with WSDOT Standard Plan F-1 and F-1a.
R. Sidewalks.
1. Sidewalks shall be provided on both sides of all arterials, collectors, local access roads and commercial streets in urban areas. Sidewalks shall be required on only one side of the road on all perimeter arterial and major and minor collectors or half-roads of a development being constructed under urban standards.
2. Alternatives to requirements of subsection (R)(1) of this section may be approved under the provisions of DCC Section 12.50.100. Typical conditions that may warrant approval of an alternative or waiver of the requirements include existing streets where it would be unduly difficult or impractical to construct sidewalks due to grade or steep slope problems or in developments where the basic design allows for an off-road walkway system; provided, that said walkway is an improved surface and provision for maintenance is guaranteed.
3. Sidewalks shall be constructed with Portland cement concrete. Sidewalks shall be at least five feet in width and four inches in thickness for urban local access streets. Urban collector and urban arterial sidewalks shall be at least six feet in width. When adjacent to school property and in commercial areas, the sidewalks shall be at least eight feet in width. Sidewalk configurations shall be in accordance with the WSDOT Design Manual and the WSDOT Standard Plans F-3 except for sidewalk width. All sidewalk ramps and features shall be ADA compliant. (Ord. TLS 04-02-30B Exh. A (part))
12.52.050 Stormwater management.
A. All project submittals shall be in compliance with the provisions of DCC Chapter 20.34, “Stormwater Drainage.” In addition, all drainage facilities within current or future county right-of-way must be of the type and nature that can be easily maintained by the county. This typically includes as a minimum twelve-inch diameter storm sewer pipe and standard catch basins and manholes for curb and gutter roadway sections. All other facilities such as French drains, curtain drains, drywells and stormwater detention ponds shall be installed outside the county’s right-of-way and be maintained by the applicant or homeowners’ association. See also DCC Section 12.55.050, “Plan Elements,” Item J “Standard Plan Notes” for catch basin and grate requirements.
B. All cross culverts and ditch channelizations shall be first evaluated for the presence of fish and, should it be determined by the county that the culvert or channelization be designed to accommodate fish passage including stream bed and/or stream bank enhancement, the culvert or channelization shall be constructed to meet current Washington State Department of Fish and Wildlife standards.
C. Stormwater facilities shall also be designed to accommodate the stormwater from the addition of frontage improvements including tributary area. In locations where future development is expected at a higher elevation and adjacent to the proposed development, the storm sewer pipe shall be extended and deadheaded at the development property line to ease future system connection.
D. Projects requesting stormwater management fee credit from the county for the stormwater facilities as provided for in DCC Chapter 19.40, “Surface and Storm Water Management Utility Code,” shall be required to provide the county with a project engineer’s certification of the facilities prior to release of the financial security. The financial security shall not be released until all facilities are completed and repaired as per the approved plans.
E. The maximum spacing on surface drainage courses between inlets or catch basins shall normally be one hundred fifty feet on road grades less than one percent and two hundred feet on grades from one percent to three percent. When the road grade is greater than three percent, the maximum spacing shall be three hundred feet. Additional catch basins may be required to confine drainage to the gutter and prevent road drainage from sheet flowing across roadways or intersections. The applicant shall locate any additional catch basins or make other drainage system improvements to ensure that any road drainage does not encroach more than one-half the traveled way lane width nor exceed one-half the curb height during a rainfall design event of two inches in ninety minutes. Maximum spacing on main storm sewers between access structures, whether catch basins or manholes, shall be three hundred feet.
F. All materials used shall conform to the requirements of the standard specifications. (Ord. TLS 04-02-30B Exh. A (part))
12.52.060 New utilities.
A. Location of Utilities—Underground.
1. Underground utilities to be installed within the right-of-way on new roads (or on roads where existing topography, utilities or storm drains are not in conflict), shall be located as shown in Figures 3-10 and 3-11 at the end of this chapter. Where existing utilities or storm drains are in place, new utilities shall conform to these standards as nearly as practicable and yet be compatible with the existing installations. Utilities to be installed outside the road right-of-way shall be installed within a designated utility easement and shall meet the installation requirements of the utility.
2. Gravity systems, whether sanitary or storm drainage, shall have precedence over other systems in planning and installation except where a nongravity system has already been installed under previous approved permit and subject to applicable provisions of such permits or franchises.
3. Individual water service lines shall:
a. Be placed with minimum thirty-six-inch cover from finished grade, ditch bottom or natural ground.
b. Use road right-of-way only as necessary to make side connections.
c. For any one connection, not extend more than sixty feet along or through the right-of-way, or the minimum width of the existing right-of-way.
d. Water meter boxes, when placed or re-placed, shall be located on the right-of-way line immediately adjacent to the property being served, unless otherwise approved by the county engineer. Meter box locations within the right-of-way may be approved by the county engineer based on site conditions that make routine service access difficult or impractical.
4. Sanitary Sewers.
a. In the case of individual sanitary sewer service lines which are force mains the pipe shall:
i. Be minimum two inches I.D., or as required by the utility to maintain internal scouring velocity.
ii. If nonmetallic, contain wire or other acceptable proximity detection features; or be placed in a cast iron or other acceptable metal casing.
iii. Be placed with minimum three-foot cover from finished grade, ditch bottom or natural ground, within ten degrees of perpendicular to road centerline, and extend to right-of-way line.
b. Sanitary and water lines shall be separated in accordance with good engineering practice such as the Criteria for Sewage Work Design, Washington Department of Ecology, latest edition.
5. Service Connections—All. Mains and service connections to all lots shall be completed prior to placing of surface materials.
6. Materials and Installation—All. All underground utilities shall utilize materials and be installed in conformance with the requirements of the particular utility standards.
B. Location of Utilities—Aboveground.
1. All poles, transformer cases, and other aboveground utility appurtenances shall be located to avoid becoming a roadside obstacle. See DCC Section 12.57.080 for further guidance.
2. Aboveground utilities located within intersections shall be placed so as to avoid conflict with placement of curb ramps. (Ord. TLS 04-02-30B Exh. A (part))
12.52.070 Connections to existing utilities.
Typically, new utility installations, both underground and overhead, constructed in conjunction with land development require a connection to existing utilities. Where such connections must utilize existing county right-of-way, the connection must be performed in accordance with DCC Chapter 12.20, “Accommodation of Utilities on Road Rights-of-Way.” (Ord. TLS 04-02-30B Exh. A (part))
Figure 3-1
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igure 3-2
Figure 3-3
Figure 3-4
Figure 3-5
Figure 3-6
Figure 3-7a
Figure 3-7b
Figure 3-8
Figure 3-9
Figure 3-10
Figure 3-11
