This section describes the geometric requirements for each road classification and the maximum and minimum standards applicable to the horizontal and vertical layout of the roads. The Section also covers subjects such as driveway entrances, pavement transitions and design, roadway structures, construction, etc. Almost all requirements are based on safety considerations; therefore, standards which provide a greater degree of safety may be used within reasonable economic limits, but standards which could provide conditions less safe than those described in this Section shall not be used.
Geometric designs shall be based on estimated traffic volumes projected 20 years into the future. Variations in this design period may only be authorized by the Director of Public Works.
All collector and arterial roads shall be designed to provide a level of service C or higher at the 20 year design point. Local roads may be designed to a level of service D.
The American Association of State Highway and Transportation Officials (AASHTO) has published policies on highway practice. These are approved references to be used in conjunction with this Section. AASHTO policies represent nationwide standards which do not always satisfy local conditions. When standards differ, the instruction in this Section shall govern.
All traffic control devices and road striping must be in accordance with the Manual on Uniform Traffic Control Devices (MUTCD) prepared by the U. S. Department of Transportation.
Unless otherwise approved by the Director of Public Works, all traffic generation estimates shall be in accordance with the publication Trip Generation by the Institute of Transportation Engineers.
Unless otherwise specified herein or specifically modified herein, the Colorado Department of Transportation’s M & S Standards and Standard Specifications for Road and Bridge Construction shall control the design and construction of roadway improvements or details not specifically covered by these Standards.
The right-of-way requirements shown in Figures 1 through 9 of Appendix 1 and Table 1 of Appendix 2 are based on the minimum space needed for each classification of road when it is constructed to meet ultimate development requirements. The right-of-way must also be adequate to accommodate those utility lines which should be laid outside the road pavement areas. Adequate right-of-way must be provided for cut or fill slopes, bike paths, sidewalks, traffic control, drainage structures, fire hydrants and other public facilities when required.
Reduced road right-of-way widths may be approved by the Board of County Commissioners when it can be demonstrated that for a specific location the standard width would be in excess of all known road improvement requirements or when existing or proposed development in the immediate area would make the standard width inappropriate. Normally, when reduced right-of- way widths are allowed a public utility and drainage easement is required between the reduced width and the standard width.
Right-of-way widths in excess of the standard width may be required in special circumstances such as when:
-- Cut or fill slopes cannot be confined within the standard width;
-- Minimum sight distance lines on horizontal curves are not within the standards;
--Minimum sight distances at intersections are not within the standards;
--Auxiliary lanes are to be provided;
--Additional right-of-way is required to accommodate storm water drainage facilities.
Vertical curbs are also called "barrier curbs". They deter vehicle operators from driving onto areas not intended for vehicular use, control parking and provide a channel for longitudinal road drainage. Vertical curbs are required on urban roads. Vertical curbs with gutters are to be constructed in accordance with CDOT Standard M-609-1, Type 2, Section IIB.
The "alternate ramp curb and gutter" depicted in Figure 27 of Appendix 1 may be approved by the Director of Public Works for certain urban roads subject to the following conditions:
• The road on which the curb is proposed is a low volume, low speed road classified as Local Access or Local Minor Residential.
• The proposed road improvements are fronting a predominately undeveloped area or new subdivision, making it difficult or impossible to ascertain eventual driveway locations.
• It can be demonstrated that all storm drainage entering onto the road will be handled in accordance with County standard criteria.
The minimum radii for curb returns measured to the back of the curbs shall be in accordance with the following:
Minimum 15 feet when both roads are classified local access.
Minimum 25 feet when one of the roads is classified minor collector or higher.
Minimum 25 feet when one or both are classified local commercial.
Minimum 30 feet when one or both are classified local industrial.
Minimum 30 feet when both are classified minor collector or above.
All curb returns shall be provided with sidewalk from PC to PT of the same width as that provided for the sidewalk behind the tangent curb section.
Horizontal alignment should provide for safe and continuous operation of motor vehicles at uniform design speed for substantial lengths of road. A horizontal curve is required when the angle of change in horizontal alignment is equal to or greater than one degree. The minimum radius of curvature will be determined by the design speed or by the stopping sight distance.
The table shown below provides a minimum radius of curvature for each of several selected design speeds with and without superelevation of 0.02 ft/ft. Wherever possible, the radii used in design should be larger. If stopping sight distance conditions require a larger radius than one shown in the following table, then the larger radius shall be used. All listed radii are to centerline.
|Design Speed (MPH)||Minimum Radii (ft)|
|Normal Crown||0.02 ft/ft Superelevation|
When items such as walls, buildings, bridge piers, cut slopes, or vegetation growth are near the roadway on the inside of a curve, they can block a driver’s view of the road ahead. If they are too close, the driver will not have sufficient distance along the curved roadway to stop when a hazardous condition comes into view. It is assumed that the driver’s eye is 3.5 feet above the center of inside lane (the driving lane closest to the inside of curve) and that the hazardous condition is an object 0.5 feet high in the center of the inside lane. The line of sight is assumed to intercept the view obstruction at the mid-point of the line of site 2.0 feet above the center of the inside lane. The clear distance, M, is measured from the center of the inside lane to the view obstruction. The following is a table of minimum stopping sight distances for various design speeds:
|Design Speed (MPH)||Minimum SSD (FT)|
The following equations are to be used when the length of the curve on the center of the inside lane must be equal to or greater than the stopping sight distance for the roadway:
• Assuming that the roadway geometry and design speed are fixed, the stopping sight distance, 5, and the radius to the center of the inside lane, R, will be known. The distance, M, found by the following equation will be the closest that an obstruction can be placed to the center of the inside lane:
M = R[l-COS(28.65 S/R)]
• If the radius R (for example the minimum radius based on design speed) and the distance, M, are tentatively selected, then the length, L, of the arc in the middle of the inside lane may be found by the following equation:
L = (R/28.65)COS-1[(R-M)/R]
The reduction of a road design speed on a curve should be avoided; however, where physical restrictions prohibit increasing the radius of the curve or the clear distance, M, the design speed for the curved section may be reduced with the approval of the Director of Public Works. In such circumstances, signing in accordance with the MUTCD is required. The difference between the design speed for the roadway approaching the curve and the design speed for the curve must not be greater than 10 MPH. The design speed for a curved roadway section must not be reduced if the reduction would occur at the end of a long tangent or at any location where high approach speeds may be expected.
For central angles smaller than 1 degree no curve is required. In no event shall sight distance nor other safety considerations be sacrificed when a curve is not provided.
A compound curve should be avoided; however, if site conditions make the use of a compound curve unavoidable, the shorter radius shall be at least 2/3 the longer radius when the shorter radius is 1,000 feet or less. Compound curves are not permitted when design speeds require the shorter radius to be greater than 1,000 feet.
On two-lane roads, tangent sections are needed between two curves in the same direction. If the pavement cross sections through the curves do not have superelevation, the minimum lengths for tangent sections are listed in the following table:
|Design Speed (MPF)||Tangent Length (FT)|
If superelevation is provided in the curved portions of the roadway, then the tangent lengths will be determined by the superelevation transition lengths, which shall be in accordance with the AASHTO publication, A Policy on Geometric Design of Highways and Roads.
A tangent section must be provided between two curves that curve in the opposite direction. A tangent section must also be provided between an intersection and a curve. If the pavement cross sections through the curves do not have superelevation, the minimum lengths for such tangent sections are listed in the following table:
|Design Speed (MPF)||Tangent Length (FT)|
If the curve radii are at least 50% greater than the radii required by the design speed, the tangent sections may not be required, depending on grades, topography and vegetation. If the curves are superelevated the superelevation transition lengths indicated in Table 1 will determine the minimum length of tangent sections between reverse curves.
Spiral curves are not permitted.
The use of changes in the vertical alignment or grade of a road is necessary for many reasons including changes in topography, drainage requirements and aesthetic factors. A vertical curve is required when a grade change equal to or greater than 2.0% occurs. All sections of a road’s vertical alignment must meet stopping and passing sight distance requirements for the design speed established for the road. When considering alternative grade profiles, economic and aesthetic comparisons should be made. For further details, see the AASHTO publication, A Policy on Geometric Design of Highways and Roads.
Maximum and minimum grades for each functional classification are listed in Table 1. Longitudinal road grades which do not comply with Table 1 must be approved by the Director of Public Works. Approval will not be granted if safety is compromised, or in the absence of compelling physical constraints.
In mountainous areas, often it is not physically or economically feasible to design a grade profile that will allow uniform operating speeds for all vehicles. Sometimes a long, sustained gradient is unavoidable. A grade profile with sections of maximum gradient, broken by lengths of flatter grade, is preferable to a long, sustained grade only slightly below the maximum allowable.
Properly designed vertical curves should provide adequate sight distance, safety, comfortable driving, good drainage, and pleasing appearance.
Flat vertical curves may develop poor drainage at the level section. This difficulty may be overcome by a slight adjustment in the grade of gutter or other roadside drainage facility or by shortening the vertical curve. On 2-lane roads where extremely long vertical curves are necessary (over 1,320 feet), it is sometimes more economical to use 4-lane construction than to obtain passing sight distance by the use of a long vertical curve. Broken-back vertical curves (two vertical curves in the same direction separated by a short grade tangent) should be avoided.
1. Type of Curve
A parabolic vertical curve is to be used. Figure 10 in Appendix 1 gives all the necessary mathematical relations for computing a vertical curve, either crest or sag.
2. Sight Distance Requirements
Sight distance is the continuous length of road ahead which is visible to the driver. In design, two sight distances are considered: passing sight distance and stopping sight distance. Stopping sight distance is the minimum sight distance to be provided at all points on multi-lane roads and on two-lane local roads where passing sight distance is not required. Stopping sight distance shall also be provided for all elements of intersections at grade, including private road connections.
a. Stopping Sight Distance
The minimum stopping sight distance is the distance required by the driver of a vehicle, traveling at a given speed, to bring the vehicle to a stop after an object on the road becomes visible. Stopping sight distance is measured from the driver’s eyes, which are assumed to be 3.5 feet above the road surface, to an object 0.5 feet high on the road.
b. Passing Sight Distance
Passing sight distance is the minimum sight distance that must be available to enable the driver of one vehicle to pass another vehicle safely and comfortably, without interfering with the speed of an oncoming vehicle traveling at the design speed should it come into view after the passing maneuver is started. The sight distance available for passing at any location is the longest distance at which a driver whose eyes are 3.5 feet above the road surface can see the top of an object 4.25 feet high on the road.
c. Sight Distance Standards
The following table shows the minimum sight distances to be used for specific design speeds:
|Design Speed (MPF)||Minimum Sight Distance|
|Stopping (ft)||Passing (ft)|
Basic considerations regarding these sight distances are covered in the AASHTO publication A Policy on Geometric Design of Highways and Roads.
3. Minimum Vertical Curve Length
Minimum vertical curve lengths are determined by sight distance requirements for a given design speed and the algebraic difference in grade for which the curve is being designed.
a. Crest Vertical Curve Lengths
Minimum crest vertical curve lengths are determined by either the stopping sight distances or the passing sight distances. A minimum curve length based upon passing sight distance for a given road design speed and algebraic grade difference will be several times greater than the curve length based upon stopping sight distance using the same parameters.
1. Roads with Four or More Traffic Lanes
Because these roads have traffic lanes in which vehicles may pass without meeting traffic moving in the opposite direction, the minimum crest vertical curve lengths must only meet stopping sight distance requirement; however, it is desirable that they also meet passing sight distance requirements.
2. Roads with Two Traffic Lanes
The minimum crest vertical curve lengths for two-lane roads shall meet passing sight distance requirements. When crest curve construction in accordance with passing sight distance requirements would result in the creation of drainage problems or excessive cuts or fills, the curve length may be reduced with the approval of the Director of Public Works and the installation of no-passing signs and pavement markings.
3. Minimum Curve Length Determined by Stopping Sight Distance
The following equations are to be used to determine the minimum crest vertical curve lengths based upon stopping sight distance requirements:
When SS < L, L = ASS2/1329
When SS >L, L = 2SS - 1329/A
SS = Stopping sight distance in feet for a given design speed
L = Length of curve in feet
A = Algebraic grade difference in percent
4. Minimum Curve Length Determined by Passing Sight Distance
When Sp < L, L = ASp2/3093
When Sp > L, L = 2Sp - 3093/A
Sp = Passing sight distance in feet for a given design speed
L = Length of curve in feet
A = Algebraic grade difference in percent
b. Sag Vertical Curve Lengths
Minimum sag vertical curve lengths are determined by either the stopping sight distance or by comfort factors. The longer of the two possible minimum curve lengths will be used.
1. Minimum Curve Length Determined by Stopping Sight Distance
The following equations are to be used to determine the minimum sag vertical curve lengths based upon stopping sight distance requirements:
When SS < L, L = ASS2/(400 + 3.5SS)
When SS > L, L = 2SS - (400 + 3.5SS)/A
SS = Stopping sight distance in feet for a given design speed
L = Length of curve in feet
A = Algebraic grade difference in percent
2. Minimum Curve Length Determined by Comfort Factors
The following equation is to be used to determine the minimum sag vertical curve length based upon comfort factors:
L = AV2/46.5
L = Curve length in feet
A = Algebraic grade difference in percent
V = Design speed in miles per hour
A right-angle intersection provides the shortest crossing distance for intersecting traffic streams. It also provides the most favorable condition for drivers to judge the relative position and speed of intersecting vehicles. Intersection angles which vary from a right-angle by more than 4 degrees are not permitted except on local roads where a divergence up to 15 degrees is permitted when approved by the Director of Public Works.
Intersections occurring on horizontal or crest vertical curves are undesirable from the standpoint of operation and sight distance. When there is latitude in the selection of intersection locations, vertical or horizontal curvature should be avoided. A line or grade change is frequently warranted when major intersections are involved. If a curve is unavoidable, it should be as flat as site conditions permit. Where the grade of the through roadway is steep, flattening through the intersection area is desirable as a safety measure.
In order to provide the opportunity for vehicles on a stop-controlled intersection leg to safely cross or make left or right turns onto a non-controlled intersection leg, adequate sight distance must be provided. Two sight distance triangles may be drawn to represent the areas which must be free of all objects, vegetation and topography in excess of two feet above the road surface below the driver’s eye on the stop-controlled intersection leg. The AASHTO publication, A Policy on Geometric Design of Highways and Roads identifies the acceptable means for determining the size of the sight distance triangles based upon many variables, including speed, width of the non- controlled leg, etc. It is not practical to attempt to tabulate all possible combinations of the many variables. Each new road intersection or proposed modification of an existing road intersection shall be evaluated in accordance with the AASHTO procedure.
1. Spacing and Location
If a road has a raised median, it may not be possible to have an opening in the median for every road intersection. Generally, median openings in arterial roads should be provided only for major cross roads. Median openings should be spaced at intervals no closer than 660 feet. If a median opening falls within 50 feet of an access driveway, it should be placed to include the access driveway.
2. Configuration of Openings
The configuration of median openings is to be determined by the AASHTO publication A Policy on Geometric Design of Highways and Roads.
3. Cross Slope
The cross-slope in the median opening shall be limited to a maximum of 0.02 foot per foot. Median openings on curves with superelevation rates exceeding 0.02 foot per foot will not be permitted.
1. Locations Where Cross Pans are Prohibited
Cross pans may not be used across roads in the following types of intersections:
Arterial - Arterial
Arterial - Collector
Collector - Collector
Exceptions to this restriction must be approved by the Director of Public Works. The purpose of this restriction is to prevent the flow of nuisance water across arterial and collector roads on a frequent basis, and to maintain the traffic flow for which these classifications are required. The gutter profiles for arterial roads and collector roads should be designed with sag curves or sump conditions located as far away from the intersection as practical. This will allow the interception or removal of light storm water and nuisance water, with only the larger flows still reaching and passing through the intersection.
2. Cross Pan Widths
Where cross pans are normally allowed, they shall be designed and constructed in accordance with Colorado Department of Transportation M Standards. If the Director of Public Works approves the use of a cross pan across arterial or collector roads the width must be designed to meet road design speed requirements. In no case will cross pans be less than 8 feet in width at road intersections on the stop controlled road, nor less than 12 feet in width for all other locations.
A cul-de-sac is a road that serves more than one property owner and has only one intersection with the public road system. The following requirements apply the creation by plat or deed of new cul-de-sac roads, both public and private. The length of a cul-de-sac is measured between the centerline of the intersecting road and the radius point of the cul-de-sac. The minimum length of a cul-de-sac road is two times the radius. A cul-de-sac road shall not be longer than 660 feet and it shall not serve more than 20 single family dwelling units (and shall not serve uses projected to generate more than 200 vehicle trips per day in areas where the land use is other than single family residential).
Dead end roads will be allowed only where a future extension of the road would be necessary to serve adjacent properties when developed at a future date. When a dead end road is allowed, a temporary turn-around shall be provided. The maximum length of a dead end road shall be the same as the maximum length of a cul-de-sac road. Temporary turn-arounds shall match the physical requirements for cul-de-sac roads, and shall be provided with a temporary easement allowing for maintenance. Traffic control devices will be required that are designed to advise the motoring public of the existence of the dead end and to mark the end of the road.
The typical road cross sections are found between road intersections where there are no dip sections for drainage flow across the road. Undivided roads should have a normal crown which is a two-way cross slope with the cross section high point on the road centerline. Divided roads should have a cross slope on each pavement section with the high point of each section on the edge of the pavement nearest to the median. Unusual conditions may cause cross slope requirements to vary, but normally the cross slope should be in accordance with the following:
Standards for Pavement Cross Slopes
|Surface Type||Road Type||Slope (%)|
|Portland Cement Concrete||All||2|
|Bituminous Mix Pavements||All||2.0-2.5|
Where storm drainage runoff flows must cross the road, dip sections are required. The pavement through the dip section to carry the flow shall have a one-way slope (no crown) and curbing and medians must not be raised. Transitions back to normal road cross slopes will be required at both ends of the dip section.
The width of a median is measured from back of median curb to back of median curb. If the median has no curb, the width is measured between the centers of the continuous, painted median stripes.
Median widths are shown in Figures 1 and 2 in Appendix 1 for principal arterials and minor arterials. The widths shown shall be considered the minimum allowable widths. In special circumstances, the Director of Public Works may approve widths other than those listed, but in no case shall a median be constructed with a width less than 5 feet. If a median is to be landscaped, it shall not be less than 8 feet wide.
A median less than 8 feet wide shall be paved. The paved surface should be crowned and have the same cross slope as the road pavement.
Medians that are 8 feet or more wide are normally not paved. The grading of the unpaved areas shall be subject to the approval of the Public Works Department and shall assure positive drainage away from the traveled roadways via storm sewer, culverts or other means that do not result in runoff flowing on or across the roadway surface. Additionally, unpaved medians shall be vegetated pursuant to a vegetation plan approved by the Director of Public Works.
The design of geometric features such as horizontal and vertical curves will depend upon the design speed selected for the road. The choice of the design speed is primarily determined by the road classification. The design speed is the maximum speed for safe operation of a vehicle that can be maintained over a specific section of a road when conditions are so favorable that the design features of the road govern. Design speeds for the various classifications of roads may be found in Table 1. The use of design speeds other than those shown in Table 1 in Appendix 2 must be approved by the Director of Public Works.
Superelevation rates of 0.02 ft/ft may be used on all classes of roads. Superelevation rates greater than 0.02 ft/ft may not be used except when specifically approved by the Director of Public Works. In no case may it exceed 0.06 ft/ft.
The length of superelevation transition shall be based upon the superelevation rate and the width of rotation. The axis of rotation shall generally be about the pavement centerline. The transition lengths for a superelevation of 0.02 ft/ft are provided in Table 1 in Appendix 2.
With respect to the beginning or ending of a horizontal curve, one-third (1/3) of the transition will be on the curve and two-third (2/3) of the transition will be on the tangent section.
Whenever superelevation is allowed on a divided road, a storm drainage system to collect the runoff along the median curb shall be provided. In no case shall nuisance water from the higher traveled way be allowed to cross over the lower traveled way.
For the purposes of this section the term bridge shall mean any structure for the purpose of allowing a public road or trail to cross over any stream, gulch, ditch, drainageway, etc. and having a span of 4 or more feet. This shall include box culverts and pipe culverts 48 inches or larger in diameter.
All bridge structures to be constructed within the public right-of-way shall be designed by a professional engineer licensed in the State of Colorado and qualified to perform such work. All bridge designs will be in accordance with the Standard Specifications for Highway Bridges adopted by AASHTO, latest edition, and the Colorado Department of Transportation’s design and detail memos. Design loading for all bridge structures within a public right-of-way shall be HS 20-44.
The required minimum hydraulic capacity for bridges shall be determined using a design storm frequency based on the following criteria (Q50 = peak flow from a 50 year storm event):
|Road type||Q50||storm frequency (yr)|
|Major Collector (urban)||---||100|
|Major Collector (rural)||> 4000 cfs||50|
|< 4000 cfs||25|
|Minor Collector (urban)||---||100|
|Minor Collector (rural)||> 4000 cfs||50|
|< 4000 cfs||25|
|Local||> 4000 cfs||50|
|< 4000 cfs||25|
For any structure to be located within a FEMA designated flood hazard area, documentation will be submitted by the design engineer demonstrating that no increase in the 100 year flood elevation will occur due to the structure. Documentation will include Hec 2 analysis and an approved flood hazard area development permit.
No bridge structure will be accepted for maintenance by Pueblo County until the Public Works Department has been provided with inspection reports prepared by a state approved bridge inspector demonstrating a minimum sufficiency rating of 95 pursuant to the CDOT Structure Inventory Coding Guide. At the discretion of the Director of Public Works the County may have the bridge inspected along with other County structures during a regular inspection year and bill the bridge owner for the cost of the inspection and resulting report.
Installation of guardrail may be required by the Public Works Department. The AASHTO publication Roadside Design Guide and CDOT M standards will be used as guidelines by the Public Works Department in evaluating the need for guardrail. When required, the installation of guardrail shall comply with CDOT specifications and M standards.
There are numerous road rights-of-way within Pueblo County which were created and dedicated to the public by subdivision plats that pre-date the present regulations (i.e. prior to August 1972). In many instances the roads were never actually constructed or were constructed too poorly to be accepted by the County for maintenance. Instances arise where a private property owner has no legal access to his/her property except over and across a platted, deeded or dedicated right-of-way in which no road exists. Pueblo County will not issue building permits nor certificates of occupancy for any property whose sole access is a public right-of-way not maintained by Pueblo County unless the person or persons desiring to use the right-of-way do one of the following:
A. Construct the road from its intersection with a maintained public road to a point along the frontage of the property to be accessed which would provide a road frontage equal to or greater than the minimum lot width required by the zone district the property is located in, or the entire frontage of the property, whichever is less. Construction shall be in compliance with all County standards applicable to the road’s functional classification. Upon completion of the construction in accordance with all applicable sections of these STANDARDS, application may be made to the Board of County Commissioners for acceptance of the road for County maintenance.
B. Apply to the Board of County Commissioners for a vacation of the public road right-of-way. Prior to issuance of the building permit, the person or persons wishing to use the resulting private road for the sole access to their property will be required to record at the Pueblo Clerk and Recorder’s office a waiver of maintenance, acknowledging that Pueblo County does not and will not maintain the road.
Mailboxes may be located within the public road right-of-way provided they do not create a roadside hazard, do not obstruct vehicular or pedestrian traffic, and do not unreasonably interfere with road maintenance activities such as snow plowing and weed mowing. On roads with traffic volumes in excess of 2000 vehicle trips per day, mailboxes shall be located at least 8 feet away from the edge of the traveled way and shoulder areas shall be provided which are adequate to support all weather vehicular traffic without damage to the public roadway, shoulder, or edge of roadway. Any variance to this requirement will be at the sole discretion of the Director of Public Works, and will be due to physical restraints beyond the control of the mailbox owner. In high density areas group or clustered mailboxes may be required. Mailboxes and their supporting structures shall not be allowed within sidewalks.
Installation of mailboxes shall comply with Figures 12 through 14 in Appendix 1.