Roadway Standards Section 250 - Geometric Design

The design of public and private roadways shall be largely based upon these Standards, the manuals and standards of Section 115 and the Standard Drawings.

The County reserves the right to restrict specific combinations of horizontal and\or vertical alignments which contain steep grades, minimum K values, minimum centerline radii, and broken back curves.

250.1 General

250.1.1 Roadway Cross Section

Standard Drawings C110 to C140 are to be used for the design of roadways under the jurisdiction of the County. These figures illustrate the required right-of-way width, paved widths, shoulder widths, lane configurations, easement widths, pedestrian facility widths, planter strips widths, curbs, bike facility widths, and design speeds for each functional classification.
Roadway cross sections shall consider the Regional Street Design Type Guidelines as described per Comprehensive Plan Chapter 5 as illustrated Map V-5.
Community and Design Plans are provided in the Comprehensive Plan that have exceptions to these Standards.
The Active Transportation Plan, projects adopted in the Comprehensive Plan and CIP should be referenced in determining the appropriate bicycle facility.
An urban street section shall be used on all roadways within the Urban Growth Boundary (UGB) and within unincorporated communities except as allowed by Engineering.
A rural road section shall be used on all roadways outside the UGB, unless located within an unincorporated community.

250.1.2 Design Speed

The design speed for all roads shall be determined by Engineering.
The minimum design speed for all public roads shall be 25 MPH.
Design speed shall be determined or assumed as one of the following:
Intended posted regulatory speed.
Existing posted regulatory speed.
85th percentile speed.
In the vicinity of a horizontal curve, the advisory speed posting plus 10 MPH.

250.1.3 Design & Control Vehicle

Engineering shall determine the appropriate design and control vehicle for a facility.
A "design vehicle" frequently uses a facility and must be accommodated without encroaching into opposing traffic lanes.
A "control vehicle" infrequently uses a facility but encroachment into opposing traffic lanes, multiple-point turns or minor encroachment into the roadside is acceptable.
The Engineer may be required to provide evidence that the design vehicle and control vehicle are accommodated in their designs.
Functional classification, safety and roadway use all play a role in determining the acceptability of lane encroachment by control vehicles. For example, on a local road, full lane encroachment by a control vehicle may be acceptable if sight distance was adequate while on a major arterial, such encroachment may not be permitted.

250.2 Curbs

When needed, curbs shall conform to Standard Drawings S100 to S180, as specified by Engineering.
Curbs may be omitted when it is demonstrated that surface water quality and storm water discharge considerations can be better achieved by collection in shallow, grass-lined swales paralleling the roadway and provisions for maintenance can be arranged. Additional pavement width along shoulders or on separated facilities shall be shown to demonstrate sufficient accommodations for pedestrian and bicycle traffic.

250.3 Pedestrian Improvements - General

All pedestrian improvements shall comply with the PROWAG standards for a pedestrian access route. The specific provisions of the PROWAG standards for pedestrian access routes can be found in the Sidewalk/Multiuse Path – ADA Design Review Checklist.
Pedestrian facilities are required within the UGB and unincorporated communities. Pedestrian facilities are not required outside the UGB except within unincorporated communities.
If right-of-way or public easement is adequate and a development is required to construct a pedestrian facility, the development is required to connect to an existing pedestrian facility that is terminated mid-block and within 15 feet of the proposed pedestrian facility, the development shall connect to that pedestrian facility. The development shall be required to construct the connecting pedestrian facility and associated improvements including curb, drainage and landscaping and comply with Section 250.3.9.
On roadways with an anticipated ADT of less than 400 where pedestrian facilities would otherwise be required, pedestrian facilities may be provided on only one side.
Minimum pedestrian facility widths are found in ZDO Section 1007. However, compliance with Section 250.1.1 is required.
Pedestrian facilities within the public right-of-way may have a running slope in the direction of pedestrian travel equal to the grade of the adjacent roadway. Pedestrian facilities outside the public right-of-way may not have a running slope in the direction of pedestrian travel in excess of 5%.

250.3.1 Sidewalks

Sidewalks should be designed to comply with Standard Drawing S960.

250.3.2 Shared-Use (Multi-Use) Paths

As an alternative or in addition to sidewalk and separated bike facilities, shared use paths may be allowed according to the criteria of the ZDO and as part of conditions of approval of development or allowed as part of a County initiated project.
The location of planned shared use paths is provided by Map 5-3 of the Comprehensive Plan.
The required shared use path width varies from an unobstructed minimum width of ten feet depending upon anticipated use. These circumstances will consider relative anticipated use of the facility, topography, preservation of significant trees, safety, and right-of-way.
Shared use paths under County jurisdiction shall be constructed of concrete.

250.3.3 Cycle Tracks

Separated pedestrian facilities constructed at the same grade as a cycle track shall be physically or visually separated from the cycle track. The design of these facilities should be based upon Standard Drawing S960.

250.3.4 Landscape Strips

All pedestrian facilities should be located adjacent to a landscape strip or other physical buffer from vehicular traffic unless otherwise approved.
Landscape strip width shall be determined per Standard Drawings C110-140 or by the Comprehensive Plan.
Landscape strips shall include landscaping elements of Section 255.

250.3.5 Right-of-way and Easements

All roadway improvements including sidewalks, cycle tracks, and shared use paths should be constructed within the public right-of-way. These improvements may be located within a public easement if approved.

250.3.6 Horizontal and Vertical Clearance

A minimum horizontal clearance around any obstacles shall be compliant with PROWAG and be provided on all pedestrian facilities.

The minimum vertical clearance above a sidewalk or shared use path is eight feet.

250.3.7 Pedestrian Facilities Cross Slope

The cross slopes of pedestrian facilities perpendicular to the direction of travel shall be designed to a maximum 1.5%, and shall not exceed 2% per ADA requirements as identified in the Sidewalk/Multiuse Path – ADA Design Review Checklist.

250.3.8 Curb Ramps

At any location where the route of pedestrian travel requires crossing a curb, a curb ramp shall be provided. Two curb ramps are typically required at each intersection corner, with one curb ramp to serve each direction of pedestrian travel. Curb ramp considerations include:

Curb ramps are to be placed as near as possible to continue the natural path of pedestrians using the adjacent sidewalk.
Design of curb ramps shall comply with ADA requirements as identified in the Curb Ramp – ADA Design Review Checklist. Typical curb ramp designs are shown on Oregon Standard Drawings RD755, RD756 and RD757. Typical designs for a ramp to allow pedestrians to transition from the end of a sidewalk, separated path or shared use path to the road surface are shown on Oregon Standard Drawing RD754.
For pedestrian facilities proposed to end mid-block that do not connect to an existing pedestrian facility, curb ramps shall be provided as such:
1. At both ends of the new pedestrian facility when the proposed construction length exceeds 100 feet.
2. At one end of the new pedestrian facility when the proposed construction length is between 50 feet and 100 feet.
3. No curb ramps if not required by 1) or 2).
For pedestrian facilities proposed to end mid-block that connect to an existing pedestrian facility, curb ramps shall be provided as such:
At the end of the new pedestrian facility when a curb ramp already exists. The existing curb ramp, even if located off-site from a development, shall be removed and replaced with appropriate curb and landscaping.
At the end of a new pedestrian facility when the proposed construction length exceeds 50 feet.
No curb ramps if not required by 1) or 2).
Exceptions to the ADA requirements for curb ramps may be requested using the process identified in Section 170 in cases of physical barriers that make full compliance infeasible.

250.3.9 Curb Ramp Closures

A crosswalk may be considered for closure and a curb ramp not required under any of the following criteria:
There is no sidewalk or shoulder at least five feet wide on the opposite side of the street on the natural path of pedestrian travel and the construction of the curb ramp would result in a safety issue;
Per the provisions of ORS 801.220 there is a marked crosswalk at the intersection serving the same direction of pedestrian travel and the construction of the curb ramp may result in a safety issue.
There are closely spaced crossings of offset T-intersections;
The crosswalk is within a maneuvering area or storage length of an intersection and crossing at that location would result in a safety issue;
An ADA compliant curb ramp cannot be designed, and a non-compliant curb ramp approved through an exception would be a safety hazard for users due to excessive slope, cross slope or other existing physical constraints;
The crosswalk would not have adequate stopping sight distance based on the design speed, or
A physical barrier exists that prevents roadway crossing.
In the event that a crosswalk is closed, appropriate signs/barriers should be provided.
A curb ramp should be constructed but signed as "crosswalk closed" if an accessible pedestrian path is not available on the opposite side of the street in the natural path of pedestrian travel but is reasonably expected to be built within five years.

250.3.10 Bulb Outs (Curb Extensions)

Bulb outs are typically used to span parking areas on arterials and collectors to make pedestrians more visible, reduce pedestrian crossing length and should be provided in the following instances:

At midblock crossing locations and intersections within the UGB with adjacent established on-street parking along arterial and collector roadways.
As required by the Sunnyside Village Community Plan or similar community or design plan area standards in the Comprehensive Plan.
At other locations determined by Engineering.

250.3.11 Midblock Crosswalks

Marked midblock crosswalks may be considered in the UGB in the following cases and may be provided if warranted per the MUTCD:
On arterial or collector roadways with a posted speed of 35 MPH or less where existing intersections, proposed intersections, or existing crossing opportunities are more than 330 feet apart.
In locations of existing or anticipated moderate to high pedestrian volumes.
Midblock crosswalks should be designed and constructed with the following features:
Generally pursuant to the recommendations of Table 9.5 of ITE's Context Sensitive Solutions in Designing Major Urban Thoroughfares for Walkable Communities: An ITE Proposed Recommended Practice.
With a raised concrete median per Section 250.10 when crossing three or more lanes of traffic.
Curb ramps meeting ADA requirements as identified in the ADA Assessment Checklist – Curb Ramps should be provided on both sides of the street per these Standards.

250.3.12 Pedestrian Facility Condition & Repair

For pedestrian facilities under the jurisdiction of Clackamas County, comply with the requirements of the County Code Section 7.03 with regard to vertical displacements, cracks and disrepair.
Concrete pedestrian facilities may be ground up to ½" in depth from the original pedestrian facility depth. When more than ½" depth is proposed or required for removal, the full panel of the pedestrian facility shall be replaced.

250.4 Bicycle Improvements

Bicycle facilities should be designed and constructed per the Oregon Bicycle and Pedestrian Design Guide, the AASHTO Guide for the Development of Bicycle Facilities and with consideration given to NACTO's Urban Bikeway Design Guide.
Separated bicycle facilities shall be provided on all collector and arterial roadways.
The location of planned bicycle facilities is established by Comprehensive Plan Map V-7a in the urban area and V-7b in the rural area.
The Comprehensive Plan and Active Transportation Plan provide guidance on bicycle facility selection.
Bicycle facilities shall be provide travel in both directions along a roadway, where planned.

250.4.1 Shared Use Paths

See Section 250.3.3.

250.4.2 Other Bicycle Facilities

See the Oregon Bicycle and Pedestrian Design Guide, the AASHTO Guide for the Development of Bicycle Facilities, and NACTO Urban Bikeway Design Guide for design guidance.

250.5 Transit Improvements

The applicant shall evaluate existing transit amenities and work with transit providers to determine if transit feature improvements are necessary based upon established transit agency guidelines. Each project shall provide reasonable accommodations for the incorporation of public transit per transit design guidelines.

250.6 Horizontal Alignment

250.6.1 Horizontal Curves

The horizontal alignment of County, public and private roadways shall conform to the following requirements:
1. The centerline alignment of roadway improvements shall be common to the centerline of the right-of-way or access easement unless otherwise approved by Engineering.
2. The centerline of a proposed roadway extension shall be aligned with the existing centerline.
3. Horizontal curves shall meet the minimum radii requirements shown in Table 2-13. The minimum horizontal curve radii are determined by the following formula:
  R = V2/(15 (e+f))
  where R = minimum centerline radius (ft)
  V = design speed (MPH)
  e = rate of roadway superelevation (ft/ft)
  f = side friction factor
  Table 2-13
Very low volume local roads with ADT less than or equal to 400 may use a centerline radius of 178 ft per AASHTO's Guidelines for Geometric Design of Very Low-Volume Local Roads. Exceptions for very low-volume local roads can be found in Section 250.6.3.
Residential driveways that serve no more than three lots, and are less than 400 feet in length or have topographic constraints may use a 50 foot centerline radius for a 12 foot width or 40 foot centerline radius for a 20 foot width. Engineering and emergency service provider approval is required.

250.6.2 Design Intent for Horizontal Curves

Minimum radii may be used only as approved by Engineering. The following excerpts from AASHTO's A Policy on Geometric Design of Highways and Streets and ITE's Urban Street Geometric Design Handbook clarify the use of minimum radii and are adopted by Engineering as general design controls:

Per AASHTO Chapter 3 - Elements of Design, "General Controls for Horizontal Alignment":

"Winding alignment composed of short curves should be avoided because it usually leads to erratic operation."
"In an alignment developed for a given design speed, the minimum radius of curvature for that speed should be avoided wherever practical."
"Abrupt reversals in alignment should be avoided. Such changes in alignment make it difficult for drivers to keep within their own lane."
"The broken-back or flat-back arrangement of curves (with a short tangent between two curves in the same direction) should be avoided… "

Per AASHTO Chapter 3 - Elements of Design, "General Design Controls":

"Sharp horizontal curvature should not be introduced at or near the top of a pronounced crest vertical curve. This condition is undesirable because the driver may not perceive the horizontal change in alignment, especially at night."
"…sharp horizontal curvature should not be introduced near the bottom of a steep grade approaching or near the low point of a pronounced sag vertical curve."
AASHTO Chapter 5 - Local Roads and Streets, "Local Urban Streets", Page 5-12:
"[Local urban] street curves should be designed with as large a radius curve as practical, with a minimum radius of 100 feet."

Per ITE, Chapter One 1.6.3:

"Although local streets may be planned, constructed and operated with the primary purpose of providing access to adjacent property, some local streets also may serve a limited amount of through traffic due to street network deficiencies. In these situations, the designer should utilize geometric design and traffic control features more typical of collector streets to encourage the safe and efficient movement of all street users."

250.6.3 Exceptions for Very Low Volume (≤400 ADT) Local Streets with a Speed of 25 MPH or less

The following are allowed under the listed circumstances on a limited basis:

Horizontal curves on local roadways within residential areas may have a minimum centerline radius of one hundred (100) feet as limited in this section.
A single 15 MPH maneuver is allowed on a local road on a limited basis when physical constraints or property boundary limitations exist.
A 100 foot tangent length shall be provided between low speed maneuvers. The tangent length provides the driver adequate time to recognize the maneuver and slow down to accomplish the turn.

250.6.4 Roadway and Marking Transitions

Shifts in roadway alignment, widening, or narrowing within motor vehicle travel lanes shall be accomplished through roadway transitions as described below.

Roadway transitions within through lanes or left turn lanes:
L = S x W (S ≥ 45 MPH)
L = WS²/60 (S < 45 MPH)
where L = minimum taper length (ft)
S = design speed (MPH)
W = offset (shift) width (ft)
Exclusive right turn lanes shall have a minimum 8 (length) to 1 (offset) widening taper for design speeds 35 MPH and below and 15:1 for design speed of 40 and above.
On all bike facilities and all paved shoulders, roadway width transitions shall have a minimum 8 (length) to 1 (offset) pavement taper for design speeds 35 MPH and below and 10:1 for design speed of 40 and above.
Along local and connector roads within the UGB, roadway width transitions are not required if traffic is not expected to utilize the shifting roadway.

250.6.5 Lane Widths

The Comprehensive Plan and these Standards present a range of lane widths. Engineering will utilize will consider the following when making decisions about lane widths:

Lane widths should be kept as narrow as possible.
Wider lane widths should be considered when the mix of heavy vehicles is greater.
When no bicycle facility or shoulder is present, a shy distance from a vertical curb should be considered.
If the purpose of the design is a lower speed environment, narrower travel lanes should be selected.
The Active Transportation Plan for guidance on the type and width of bicycle facilities.

250.7 Vertical Alignment

The vertical alignment of the County's public and private roadways shall conform to the following requirements:

250.7.1 Minimum Roadway Gradient

The minimum tangent roadway gradient shall be 1% along the crown and vertical curb line.
A minimum of 0.5% may be designed with concrete curb and gutter with Engineering approval.
Through curb ramps, the minimum gradient shall be designed to 1.0% to 1.5%

250.7.2 Maximum Roadway Gradient

The maximum roadway gradient shall be based on Standard Drawings C110-C140. Grades in excess of these maximums may be approved by Engineering on a case-by-case basis per Section 170.

250.7.3 Intersection Landing

At intersections, a landing shall be provided on the secondary or subordinate approach or on a stop-controlled approach.
Landings should be 20 feet in length for private driveways, 50 feet in length for local roadways and one hundred 100 feet in length for all other roadways.
Landings should be measured from the edge of pavement of the intersected roadway at full development and shall have an average grade of not greater than 5%.

250.7.4 Vertical Curves

Vertical curves shall be used when design grade breaks of more than 1% are necessary.
Vertical curves shall conform to the values in Table 2-14 and calculated as below.
K = L/A
where A = algebraic difference in grades (percent)
L = length of vertical curve (feet)
Table 2-14
The minimum vertical curve length shall be fifty (50) feet.
K-Sag values may be reduced to K-Crest values if adequate street lighting is present along the entire sag vertical curve.

250.7.5 Roadway Widening

Required road widening for land use approvals generally require a half street improvement; however, when one or more of the following apply up to a full street widening may apply:
1. Setting new curbs;
2. Centering up road improvements in the right-of-way;
3. Existing cross slopes are below the minimum or above the maximum standards;
4. Cross slopes vary from one side to the other; or
5. Offset crowns exist.
Road widening shall not reduce existing road structural sections beyond the minimum standard structural section unless the road structural section is reconstructed to standards.
Road widening designs require designed cross sections at 25' on center that illustrate the elevations at:
1. Centerline of right-of-way,
2. Crown of road,
3. Saw cut line, and
4. Gutter line
  1. Show the existing and new cross slopes between elevation points;
  2. Provide the station for each cross slope;
  3. These cross slopes are usually for plan review to insure that the cross slopes and crown are designed within the acceptable standards and can be provided as a separate exhibit to the plans unless they are also needed to provide information to the contractor.
5. The maximum grade break between existing and proposed cross slopes shall be 2%.
6. Cross sections through existing driveways and intersections shall be provided.

250.7.6 Superelevation

Design elements for superelevation should be based on AASHTO guidelines.
Superelevation is not allowed on roadways with a design speed of 35 MPH or less.
The maximum rate of roadway superelevation for urban conditions should be 4% (emax = 0.04).
The use of superelevation in the urban area is discouraged and shall be approved by Engineering before used.

250.8 Intersections

250.8.1 Minimum Curb Radii

Minimum curb radii at intersections within the UGB are shown in Table 2-15.
Minimum curb radii at intersections outside the UGB are determined by Engineering.
The minimum right-of-way radii shall be sufficient to maintain at least the same distance from right-of-way to edge of pavement or curb as the lower classified roadway.
Curb radii shall be designed to accommodate the design and control vehicle per Section 250.1.3.
Curb radii shall be approved by Engineering based upon an assessment of design and control vehicle considerations as well as pedestrian and design speed considerations.
Table 2-15

50.8.2 Intersection Angle

The intersection angle at intersecting roadways shall be kept as near to 90 degrees as possible. Intersection angles from 80 to 85 degrees and 95 to 100 may be considered per Section 170.

250.8.3 Roadway/Lane Offset

New lanes, roadways or driveways intersecting an existing intersection should not be significantly offset from the existing alignment. Minor offsets may be approved where low speed maneuvers are predominant or where otherwise acceptably safe operations would occur.

250.8.4 Tangent Section

In order to improve the safety at intersections, new intersections should conform to the following tangent requirements unless otherwise approved by Engineering. The following tangent sections should be provided per Table 2-16.

Table 2-16

250.8.5 Residential Intersection Design

Four-legged intersections should be considered for neighborhood traffic circles per Section 265.

250.8.6 Roundabouts

Roundabout intersections shall be designed in accordance with FHWA's Roundabouts: An Informational Guide.
Roundabouts shall be considered per Section 260.1.1 prior to the approval of a traffic signal.
Roundabouts with pedestrian facilities shall be designed to comply with ADA requirements as identified in PROWAG Section R306.3.

250.8.7 Intersection Sight Distance

Comply with intersection sight distance requirements of Section 240.

250.8.8 Turn Lane Design

The need for left or right turn lanes shall be based upon a traffic study per Section 295.18.1 and/or as dictated by the Comprehensive Plan or CIP.
Queue storage estimates shall be based upon a traffic study per Section 295.16.
Left turn lanes, when provided, shall have a storage queue of at least 50 feet.
Design of left or right turn lanes should be based upon ODOT's Highway Design Manual.

250.9 Roadway Grading

Roadway grading should conform to clear zone requirements of Section 245 and cross section requirements of Standard Drawings C110 to C140.
Slopes along and adjacent to the roadway should be as specified in Standard Drawings C110 to C140. The maximum slopes outside the clear zone as detailed in Section 245 are as follows:
Cut Slope - 2 to 1
Fill Slope - 2 to 1
Roadway embankment should be constructed with crushed aggregate no larger than 6"-0 and no larger than 1½"-0 to 3"-0 can be used within one-foot of the structural section of the roadway.
Flatter slopes are preferred and may be required by the County if soils are unstable as determined by a geotechnical analysis.
Side slopes exceeding four feet in height shall be constructed in conformance with design and specifications prepared by an Engineering Geologist or Geotechnical Engineer. All side slopes shall be stabilized by grass sod, seeding, riprap, or other acceptable ground stabilizing materials as recommended by a geotechnical engineer.
Side slopes necessary for roadway stability extending outside the public right-of-way will require slope easements.

250.10 Non-traversable Medians and Accessible Route Islands

Non-traversable medians should be required in the following cases:
On arterial roadways within the UGB with five or more travel lanes.
When described as an element of a project listed an adopted plan.
On roadways where right-in/right-out driveway access maneuvers are required.
Non-traversable medians should be considered in the following cases:
On arterial or collector roadways within the UGB with three or more travel lanes.
On roadways where improved access management is desirable.
Medians should be designed and constructed with the following features:
Landscaping per Section 255.
Landscaping with a mature height of 2.5 feet should not be planted within 50 feet of an intersection.
With a minimum width of eight feet when designed to serve as a pedestrian refuge.
With a minimum width of one foot when not designed to serve as a pedestrian refuge.
To contain a "shy" distance from adjacent traffic of varying width depending upon the design speed of the roadway.
Medians that are crossed by a pedestrian access route and accessible route islands shall comply with ADA requirements as identified in ADA Design Review Checklist – Medians/Traffic Islands and depicted in Oregon Standard Drawing RD710.

A part of the Roadway Standards. Contact engineering@clackamas.us for drawings or information.