4 t MOBILE COUNTY COMMISSION DESIGN GUIDELINES for Improving Existing County Maintained Unpaved Local Roads 2012
FORWARD The Mobile County Commission is charged, by law, with authority in relation to the establishment, change or discontinuance of public roads in Mobile County, Code of Alabama, Section 11-3-10. The Commission has general superintendence of public roads so as to render travel over such roads as safe and convenient as practicable, Code of Alabama, Section 23-1 -80. The Commission is vested with the authority to establish, promulgate and enforce rules and maintenance of its public roads, Code of Alabama, Section 23-1 -80. The Commission has found that existing design policies do not recognize or adequately address the uniqueness of the unpaved road system maintained by the Mobile County Commission. Existing policies often require design elements not suited for conditions found in Mobile County and often advocate expenditure of funds for design elements that neither improve the efficiency nor enhance the safety of the types of unpaved roads found in Mobile County. Therefore, the Mobile County Commission has found it to be in the public s best interest to promulgate a guide on roadway design for paving existing unpaved local roads within Mobile County. Pursuant to the direction of the Mobile County Commission, the Engineering Department has prepared a guide on roadway design for the purpose of paving existing unpaved roads. The intent of this guide is to set the general roadway design elements for roadway construction directed by the Mobile County Commission. The design elements contained within this guide come from various engineering sources, technical publications, research studies, applied engineering judgment and current public acceptance within Mobile County. The intent of this guide is to create a safe roadway system at a reasonable cost, to give the citizens of Mobile County the best benefit/cost ratio possible, and to serve all the areas within the county with the ultimate goal of paving all existing roadways in Mobile County. Aesthetic, historic, and cultural resource impacts of the roadway design are to be principal considerations within this guide. This guide does not apply to the general maintenance of roads, but does apply to the paving of existing county maintained unpaved roads. This guide does not constitute a fixed standard, rule or regulation. As with any guide, it is meant as a guide and is not intended to be a substitute for sound engineering judgment. This guide recognizes that deviations from the terms of this guide may be necessary and warranted in the exercise of sound engineering judgment. I
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This guide will go into effect upon commission approval and will be applied to any project that is initiated after this date for the paving of an existing county maintained unpaved road in the unincorporated section of Mobile County. This guide will also be utilized for design of unpaved roadways accepted in Mobile County s Dirt Road Program (DRP). This guide will remain in effect pursuant to the direction of the Mobile County Commission and can only be altered, changed, or amended by a majority vote of the commission. Approved by the Mobile County Commission: On this 22. day of 2012 Commisserudgood Commiss oner Connie Hudson Commissioner Mike Dean 3
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Mobile County Commission Design Guidelines for Paving Existing County Maintained Unpaved Local Roads Table of Contents Forward Chapter 1 11 General Terms! Elements for Design 1.1 ClearZone 1.2 Context Sensitive Solutions 1.3 Drainage 1 1.4 Erosion Control Requirements.5 Functional Classification 1.6 LateralOffset 1.7 Roadway 1.8 Temporary Traffic Control 1.9 Traffic Calming 1.10 Traveled Way 1.11 Typical Traveled Way Cross Section 1.12 Vertical Clearance Chapter 2 17 2. Elements of Design for Major Local Roads 2.1 Roadway Design 2.2 Stopping Sight Distance 2.3 Intersection Sight Distance 2.4 Horizontal Alignment 2.5 Vertical Alignment 2.6 Cross Slope 2.7 Drainage 5
Chapter 3 25 3. Elements of Design for Minor Local Roads and Residential Roads 3.1 Roadway Design 3.2 Stopping Sight Distance 3.3 Intersection Sight Distance 3.4 Horizontal Alignment 3.5 Vertical Alignment 3.6 Cross Slope 3.7 Drainage Chapter 4 33 4. Elements of Design for Access Roads 4.1 Roadway Design 4.2 Stopping Sight Distance 4.3 Intersection Sight Distance 4.4 Horizontal Alignment 4.5 Vertical Alignment 4.6 Cross Slope 4.7 Drainage Chapter 5 41 5. Elements of Design for Access Ways 5.1 Roadway Design 5.2 Stopping Sight Distance 5.3 Intersection Sight Distance 5.4 Horizontal Alignment, Vertical Alignment and Cross Slope 5.5 Drainage 6
Chapter 6. 47 6. Functional Classification 6.1 Arterial and Collector Road General Characteristics 6.2 Local Road General Characteristics 6.21 Major Local Road 6.22 Minor Local Road 6.23 Residential Road 6.24 Access Road 6.25 Access Way 6.3 Reevaluation of the System Chapter 7 59 7. Design Waiver References 61 7
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CHAPTER 1 1. General Terms I Elements for Design The following information will be included in the design for paving existing county maintained unpaved local roads. 1.1 ClearZone The clear zone is the traversable area provided beyond the edge of the traveled way for the recovery of errant vehicles. Any vertical obstructions in this area shall be breakaway devices or shall be shielded. For traveled ways with design speed greater than 30 mph the clear zone shall be the same as the shoulder widths as shown in this guide. For traveled ways with design speed 30 mph, clear zone criteria shall not apply. 1.2 Context Sensitive Solutions Context sensitive solutions (CSS) are theoretical and practical approaches to transportation decision-making and design that takes into consideration the communities and lands which streets and roads pass through ( the context ). These streets and roads should fit the physical setting and preserve scenic, aesthetic, historic, natural, and environmental resources, while maintaining safety and mobility (being sensitive to location). Projects involve efficient and effective use of resources (time, budget, community) with minimal disruption to the community. All stakeholders involved in a project should communicate and collaborate to achieve a design that meets most expectations. 1.3 Drainage All drainage design shall conform to the Mobile County Flood Damage Prevention Ordinance. The purpose of this ordinance is to promote public health, safety, and general welfare, and to minimize public and private losses due to flood conditions. 1.4 Erosion Control Requirements All projects shall include the current requirements of NPDES. Reference the current edition of the Alabama Handbook for Erosion Control, Sediment Control, and Stormwater Management on Construction Sites and Urban Areas. All devices shall be clearly defined and quantified, including all applicable directions, notes and associated references to perform the construction. 11
1.5 Functional Classification Functional classification is the grouping of streets, roads, and ways by the character of service they provide. The functional classification of each road shall be noted on the plans. 1.6 LateralOffset Lateral offset is the area adjacent to the traveled way clear of all vertical obstructions such as signs, utility poles, luminaire supports, fire hydrants, etc., including breakaway devices. For this guide the lateral offset shall be 1.5 feet from the traveled way. 1.7 Roadway The roadway is the total composition of all design elements including the traveled way, shoulder, and drainage system. I 8 Temporary Traffic Control All projects shall include a temporary traffic control plan and will conform to the current edition of the MUTCD, Manual on Uniform Traffic Control Devices, Part VI. All devices shall be clearly defined and quantified, including all applicable directions, notes and associated references to perform the construction. 1.9 Traffic Calming Traffic calming involves design elements that incorporate changes in street alignment, installation of barriers and other physical measures to reduce traffic speeds and/or cut-through volumes, in the interest of street safety, livability, and other public purposes. 1.10 Traveled Way Traveled way is the portion of the roadway, exclusive of the shoulders and drainage system, which is used for the movement of vehicles. 12
1.11 Typical Traveled Way Cross Section Roadways for this guide may include the following cross section types: Open Ditch Curb and Gutter Roll Curb Asphalt Wing Inverted Crown No-Crown I Single Slope 1.12 Vertical Clearance Shoulders and traveled way should be clear of obstructions 16 feet above their surface. Any locations that cannot achieve the required vertical clearance shall be marked with appropriate traffic control devices. For traveled ways with design speed 30 mph the vertical clearance criteria shall not apply. 13
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Chapter 2 Elements of Design for Major Local Roads
CHAPTER 2 2. Elements of Design for Major Local Roads The following roadway design criteria will be used for existing county maintained unpaved roads functionally classified as Major Local Roads. The design speed for this classification should be in the 25 45 mph range. Under warranted situations lower design speeds may be used only with approval by Mobile County Engineering Department. Right-of-way width should be a minimum of 60 feet, unless the environment which it serves restricts the acquisition to a lesser width. 2.1 Roadway Design On some open-ditch type roadways I - 2 feet of paved shoulder may be approved by Mobile County Engineering Department. If approved, this paved area shall have the same cross slope as the traveled way, but would be considered as part of the shoulder width. I -750 ADT Design Speed Traveled Way Width Shoulder Width Clear Zone (mph) (feet) Minimum (feet) Minimum (feet) 15to30 18 4 n/a 35to45 18 4 4 For roadways with design speeds 30 mph clear zone shall not apply (n/a). Curb and gutter section roadways shall have a 1.5 feet clear zone measured from the face of curb. 751-2000 ADT Design Speed Traveled Way Width Shoulder Width Clear Zone (mph) (feet) Minimum (feet) Minimum (feet) 15to30 18 4 n/a 35to40 20 4 4 45 22 4 4 For roadways with design speeds 30 mph clear zone shall not apply (n/a). Curb and gutter section roadways shall have a 1.5 feet clear zone measured from the face of curb. 17
Design Speed Traveled Way Width Shoulder Width Clear Zone (mph) (feet) Minimum (feet) Minimum (feet) 15to30 20 4 n/a 35 22 4 4 For roadways with design speeds 30 mph clear zone shall not apply (n/a). Curb and gutter section roadways shall have a 1.5 feet clear zone measured from the face of curb. Design Stopping Sight Stopping Sight Stopping Sight Speed Distance for Distance for Distance for Level Roadways Downgrades Upgrades (feet) (feet) (feet) (mph) <3% 3% 6% 9% 3% 6% 9% 15 80 80 82 85 75 74 73 20 115 116 120 126 109 107 104 25 155 158 165 173 147 143 140 30 200 205 215 227 200 184 179 35 250 257 271 287 237 229 222 40 305 315 333 354 289 278 269 45 360 378 400 427 344 331 320 Formula source: AASHTO, A Guide on Geometric Design of Highways and Streets (3-3) Stopping sight distance shall be calculated on a driver eye height of 35 feet to an object height of 2 feet. The shoulder area shall not be utilized to determine the sight distance length. Where a site specific location has a stopping sight distance less than the calculated distance condition. 18 based on the design speed, traffic control devices shall be installed to alert the driver of the 30((3:2)±G) d= Distances should be adjusted for grades steeper than those shown. 2.2 Stopping Sight Distance 40 22 6 6 >2000 ADT
2.3 Intersection Sight Distance (mph) (feet) 15 170 20 225 25 280 30 335 35 390 40 445 45 500 50 555 55 610 Distances shown are for a passenger (P) design vehicle making a left turn maneuver onto a main road and may require additional distance for a larger design vehicle. For reference use: Guide on Geometric Design of Highways and Streets, Chapter Table 9-6. AASHTO, A 9, Intersections, Speed shown (mph) is the speed the vehicle is traveling along the main road and the distance shown (feet) is from the side road to the vehicle on the main road. Intersections that do not meet minimum distance may require an advisory intersection and speed sign. Distances shall be adjusted for grades more than 3 %. For reference use: AASHTO, on Geometric Design of Highways and Streets, Table 9-4, Adjustment Factors for Sight Distance Based on Approach Grade. A Guide 2.4 Horizontal Alignment Rmin = V2 15 ( O.Olemax+fmax) Formula source: AASHTO, A Guide on Geometric Design of Highways and Streets (3-8) Centerline Radius (feet) Minimum Design Speed (mph) 15 20 25 30 35 40 45 Cross Slope (%) fmax.31.26.22.20.18.16.15-2.0-2.2-2.5 55 115 210 340 515 765 1,040 55 115 215 345 520 775 1,060 55 120 220 350 530 795 1,085 19
20 Maximum grade for traveled way shall be 12%. difference in intersecting grades (A). K = LIA Rate of vertical curvature, K, is the length (L) of curve divided by the percent algebraic Design Speed K Value for Crest K Value for Sag 2.5 Vertical Alignment Cross Slope (%) fmax.31.26.22.20.18.16.15 8.0 40 80 140 220 320 450 590 7.0 45 85 145 225 330 465 615 6.0 45 85 150 235 345 490 645 5.0 45 90 160 245 360 510 680 4.0 45 90 165 255 375 535 715 3.0 45 95 170 265 390 565 755 2.5 50 95 175 270 400 580 775 2.2 50 100 175 275 410 590 790 2.0 50 100 180 280 415 595 800 (mph) Vertical Curves Vertical Curves 15 3 10 20 7 17 25 12 26 30 19 37 35 29 49 40 44 64 45 61 79 Design Speed (mph) 15 20 25 30 35 40 45 Centerline Radius (feet) Minimum emax =.06 (Urban Area) emax.08 (Rural Area) Superelevation
2.6 Cross Slope Pavement Cross Slope Open Ditch 2.0% - 2.5% Curb and Gutter 2.0% - 2.5% Roll Curb 2.0% - 2.5% Asphalt Wing 3.33% (typical) Grass Shoulder Cross Slope 4% - 6% 2% if adjacent to sidewalk Front Slope Desirable Maximum 3: 2: 1 1 Back Slope Desirable 1 Maximum I : 1 3: 2.7 Drainage Side Drain or Lateral Storm Sewer Minimum 10 yr. storm Crossing (Closed) Storm Sewer Minimum 10 yr. storm Sump Crossing (Closed) Storm Sewer Minimum 25 yr. storm Culvert (Open) Cross Drainage Minimum 25 yr. storm Bridge or Bridge Culvert * * All Bridge or Bridge Culvert requires Minimum 50 yr. storm HS-20 Structural Design. Drainage to 1 foot. designed than structures pass located within a federally 100 a minimum of a 100 storm without raising year designated year base the base elevation should be flood elevation more 21
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CHAPTER 3 3. Elements of Design for Minor Local Roads and Residential Roads The following roadway design criteria will be used for existing county maintained unpaved roads functionally classified as Minor Local Roads and Residential Roads. The design speed for these classifications should be in the 10 30 mph range. Traffic calming elements should be incorporated to keep roadway users from traveling at speeds higher than the design speed. Under warranted situations for Minor Local Roads only, higher design speeds >30 mph may be used, only with approval by Mobile County Engineering Department. Design speed for Minor Locals Roads >30 mph shall follow the guidelines of Chapter 2, Elements of Design for Major Local Roads. Context sensitive solutions should be incorporated in areas that are highly developed or that have an isolated location that should be preserved. Right-of-way width should be a minimum of 30 feet, unless the environment which it serves restricts the acquisition to a lesser width. The following criteria are intended to be designed within the existing right-of-way, but may require additional right-of-way at site-specific locations for roadway elements and/or for utility relocations adjacent to the roadway. 3.1 Roadway Design * Design Speed for Minor Locals Roads >30 mph shall follow the guidelines of Chapter 2, Elements of Design for Major Local Roads Design elements such as horizontal and vertical curves, traffic calming devices, short tangent lengths, etc., should be incorporated to reduce high running speeds. Some locations may require additional pavement width such as on the inside of horizontal curves with small radii or at site-specific locations that will reduce the need for future maintenance (drainage outfalls, driveways, intersections, etc.). 25
Maximum grade for traveled way shall be 16%. 28 difference in intersecting grades (A). K = L/A Rate of vertical curvature, K, is the length (L) of curve divided by the percent of algebraic 30 19 37 25 12 26 20 7 17 10 3 10 15 3 10 K Value for Crest Vertical Curves K Value for Sag Vertical Curves (mph) Design Speed 3.5 Vertical Alignment -3.0 35 60 120 225 355-3.3 35 60 120 225 365-3.5 35 60 120 230 370-4.0 35 60 125 235 375 35 55 120 220 350 35 55 115 210 340 35 55 115 215 345-20 -22-25.38.31.26.22.20 f max Cross Slope (%) Design Speed (mph) 10 15 20 25 30 Centerline Radius (feet) Minimum Formula source: AASHTO, A Guide on Geometric Design of Highways and Streets (3-8) 15 ( O.Olemax+ fmax) Rmin = V2 3.4 Horizontal Alignment
3.6 Cross Slope Pavement Cross Slope Open Ditch 2.0% - 2.5% Curb and Gutter 2.0% - 2.5% Roll Curb 2.0% - 2.5% Asphalt Wing 3.33% (typical) Inverted Crown 2.0% - 4.0% No-Crown I Single Slope 4.0% (max) Grass Shoulder Cross Slope 4% - 6% 2% if to sidewalk Should match No-Crown I Single Slope adjacent pavement cross slope for Front Slope: Desirable 3 : 1 Maximum 1 2: Back Slope: Desirable 3 : 1 Maximum 1 : 1 3.7 Drainage Side Drain or Lateral Storm Sewer Minimum 10 yr. storm Crossing ( Closed ) Storm Sewer Minimum 10 yr. storm Sump Crossing (Closed) Storm Sewer Minimum 25 yr. storm Culvert ( Open ) Cross Drainage Minimum 25 yr. storm Bridge or Bridge Culvert * Minimum 50 yr. storm * All Bridge or Bridge Culvert requires HS-20 Structural Design. Drainage to 1 foot. designed than structures pass located within a federally 100 a minimum of a 100 storm without raising the year designated year base base elevation should be flood elevation more 29
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CHAPTER 4 4. Elements of Design for Access Roads The following roadway design criteria will be used for existing county maintained unpaved roads functionally classified as Access Roads. Roads for this classification are typically twodirectional one-lane roads and should have an ADT of 400. The design speed for this classification should be in the 10 25 mph range. Traffic calming elements should be incorporated to keep roadway users from traveling higher than the design speed. Context sensitive solutions should be incorporated in areas that are highly developed or that have an isolated location that should be preserved. In context sensitive areas, especially where an established path has been used by the community that it serves, right-of-way width should conform to the surroundings. Right-of-way width in some areas may be the same as the roadway width, and may only include the prescriptive area of the roadway. The following criteria are intended to be designed within the existing right-of-way currently available, but may require additional right-of way at site-specific locations for roadway elements and/or for utility relocations adjacent to the roadway. 33
4.1 Roadway Design Design Speed Traveled Way Width Shoulder Width (mph) (feet) Minimum (feet) Minimum 10-15 10 2 20-25 12 4 The shoulder area may have paved turnouts that parallel the traveled way at sharp curves, at spacing intervals of approximately 500-1000 feet., or at other appropriate areas like driveways, for the purpose of allowing vehicles to pass with minimal backing. Wider shoulders should be used wherever possible unless right-of-way constraints, or context sensitive solutions restrict their use. Design elements along the roadway such as horizontal and vertical curves, traffic calming devices, short-tangent lengths, etc., should be considered to reduce high running speeds. Some locations may require additional pavement width such as on the inside of small-radii horizontal curves or at site-specific locations that will reduce the need for future maintenance (drainage outfalls, driveways, intersections), etc. Consideration should be given at intersections to widen to two-lane widths with appropriate radii to accommodate the vehicle design-type. 4.2 Stopping Sight Distance L Stopping Sight Sight Distance Sight Distance Distance for Adjustment for Adjustment for Level Roadway Downgrades Upgrades Design Speed (feet) (feet) (feet) (mph) Level 3% 6% 9% 3% 6% 9% 10 66 75 75 76 59 59 59 15 115 134 136 138 107 107 107 20 143 178 181 184 130 129 129 25 194 248 253 258 173 172 171 Stopping Sight Distances are calculated based on the driver s eye height of 3.5 feet. to an object height of 2 feet. The shoulder shall not be used as part of the sight distance length. Under extreme circumstances, where a site-specific location has a stopping sight distance less than the design speed for the entire project, traffic control devices may be used to alert the driver of this condition. For grades steeper than those shown adjustment should be made. c 34
d 30(( a V \\ 32.2 / Formula source: AASHTO, A Guide on Geometric Design of Highways and Streets (3-3) 4.3 Intersection Sight Distance Posted Speed (mph) Limit Intersection Sight Distance (feet) 10 110 15 170 20 225 25 280 30 335 35 390 40 445 45 500 50 555 55 610 Distances shown are for a passenger (P) design vehicle making a left turn maneuver onto a main road and may require additional distance for larger design vehicles. For reference use: AASHTO, Table 9-6. A Guide on Geometric Design of Highways and Streets, Chapter 9, Intersections, Speed shown (mph) is the speed the vehicle is traveling along the main road and the distance shown (feet) is from the side road to the vehicle on the main road. Intersections that do not meet minimum distance may require an advisory intersection and speed sign. Distances should be adjusted for grades more than 3 %. Intersection of the side road to the main road should be widened to allow two-lanes of operation for ingress and egress (to the side road). If both roadways are two-directional one lane roadways all directions should be considered to widen to two-lane widths, allowing a place to pull over and pass each other before continuing. 35
15 20 25 4.4 Horizontal Alignment Centerline Radius Design Speed (mph) Minimum Radius 100 300 450 750 (feet) Minimum Additional Pavement Width Required for Curves If the road will have a school bus, garbage truck, other large single unit vehicles, or a vehicle pulling a trailer (boat, utility, etc) on a regular basis, additional width of pavement will be necessary to keep off-tracking onto the shoulder to a minimum. The additional pavement width should be considered to be placed on the inside area of the radius. Radius (feet) 100 101-500 500-750 751-950 >950 loft.lane 4 3 2 1 nla 12 ft. Lane 2 1 nla nla nla 4.5 Vertical Alignment Design Speed K Value for Crest K Value for Sag (mph) Vertical Curves Vertical Curves 10 2 n/a 15 4 10 20 10 17 25 16 26 Rate of vertical curvature, K, is the length of curve divided by the percent algebraic difference in intersecting grades (A). K = LIA Maximum grade for traveled way shall be 16%. 36
4.6 Cross Slope Pavement Cross Slope Match existing topography (4.0% max) Grass Shoulder Cross Slope Match pavement cross slope Front Slope Desirable 3 Maximum 2: : 1 1 Back Slope Desirable 3: Maximum I : 1 I 4.7 Drainage determined that sensitive solution for the area. Some site-specific drainage improvements may be determined and incorporated into the design of the roadway by the engineer without a Drainage may match existing topography comprehensive drainage system. if it would be a context If drainage is used, the following should apply: Side Drain or Lateral Storm Sewer Minimum 2 yr. storm Crossing ( Closed ) Storm Sewer Minimum 2 yr. storm Sump Crossing (Closed) Storm Sewer Minimum 10 yr. storm Culvert ( Open ) Cross Drainage Minimum 10 yr. storm Bridge or Bridge Culvert * Minimum 50 yr. storm * All Bridge or Bridge Culvert requires HS-20 Structural Design. Drainage structures located within a federally designated 100 year base elevation should be designed to pass a minimum of a 100 year storm without raising the base flood elevation more than 1 foot. 37
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Chapter 5 Elements of Design for Access Ways
CHAPTER 5 5. Elements of Design for Access Ways The following roadway functionally classified roads design criteria will be as Access Ways. used for existing county maintained unpaved An Access Way shall be paved only if it directly to an existing paved road. It may be paved if jointly with a roadway paving project will a continuous paved route to an existing paved road. associated connects that create Roads for this classification typically two-directional and should have an ADT of 200. ADT for this classification of roadway is an of trips by the local property and their If the of any property to this type of roadway is going to unfamiliar with this to travel with frequency, different criteria should be design users are guests. encourage users considered. purpose one-lane roads aggregate area repeated adjacent A moderate to high housing should be with this type of roadway and should 1 for every 125 linear feet of roadway, or (maximum of 18 Road length should be no than 1/3 mile (1700 feet). An Access Way may divide to different properties, or loop back onto itself. of the roadway may different road given for and may constitute a housing density. of roadway provide additional property and should be part of the Access Way. average have access house about names These segments considered density greater the about associated addressing purposes, less These segments adjacent lesser access houses). The design speed for this classification should be 10mph. Context sensitive solutions should be incorporated for this type of roadway. The and to property this type of road should be In context with a minimal history, especially where an Access Way by the community it right-of-way width should conform to the surroundings. nature access sensitive areas, established that crash that serves, serves preserved. secluded has been Right-of-way width in may be the roadway width, and may only include the prescriptive of retain the width that Mobile County intended to be within existing right-of-way, but may require additional right-of-way at site-specific locations for roadway for utility relocations to the way. designed the some areas area elements and/or same as the the Access Way. Prescriptive right-of-way roadways should are has maintained. The following criteria adjacent 41
5.1 Roadway Design Design Speed Traveled Way Shoulder Width Maximum Width 10 mph Existing Travel Width Existing Width (Maximum 14 feet) Areas along the Access Way may include additional site-specific paved areas wider than the maximum shown to aid possible conflicting traffic or roadway maintenance locations. Some locations may be at sharp curves, drainage structures, intersections, driveways, or at other appropriate areas. Some areas may be for the purpose of allowing vehicles to pass with minimal backing. 5.2 Stopping Sight Distance Stopping Sight Distances are calculated based on the driver s eye height of 3.5 feet to an object height of 2 feet. The shoulder area may be utilized to determine the sight distance length. The minimum stopping sight distance shall be 40 feet. Site specific locations may have traffic control devices installed to alert the driver. 5.3 Intersection Sight Distance Intersections shall be evaluated based on their functional classification and posted speed limit. An Access Way that intersects another Access Way should have the same intersection sight distance as stopping sight distance, 40 feet. On Access Ways that intersect roads which are functionally classified other than an Access Way, the following intersection sight distance table should be used. Posted Speed Limit Main Road Intersection Sight Distance Along Main Road (mph) (feet) 10 110 15 170 20 225 25 280 30 335 35 390 40 445 45 500 50 555 55 610 Distances shown in the table are for a passenger (P) design vehicle making a left turn maneuver onto a main road and may require additional distance for larger design vehicles. Speed shown (mph) is the speed the vehicle is traveling along the main road and the distance 42
shown (feet) is from the centerline of side road to the vehicle on the main road. Intersections that do not meet minimum distance may require an advisory intersection and speed sign to be posted on the main road. 5.4 Horizontal Alignment, Vertical Alignment and Cross Slope Access Ways should match existing topography. 5.5 Drainage Drainage should match existing topography. Typically no drainage improvements are required. If determined that context sensitive drainage solutions for the area should be included, sitespecific drainage improvements may be incorporated. If the drainage system is improved, the design should accommodate the following drainage criteria: Side Drain or Lateral Storm Sewer Minimum 2 yr. storm Crossing ( Closed ) Storm Sewer Minimum 2 yr. storm Sump Crossing (Closed) Storm Sewer Minimum 10 yr. storm Culvert ( Open ) Cross Drainage Minimum 10 yr. storm Bridge or Bridge Culvert * Minimum 50 yr. storm * All Bridge or Bridge Culvert requires HS-20 Structural Design. Drainage structures located within a federally designated 100 year base elevation should be designed to pass a minimum of a 100 year storm without raising the base flood elevation more than 1 foot. 43
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CHAPTER 6 Functional Classification The classification of local roadways into different functional classes is necessary for communication among engineers, local agencies, and the general public. Different classifications apply for different purposes in both the rural and urban environment. Functional classification, the grouping of streets / roads by the character of service they provide, is the main purpose in planning, designing, meeting social needs, and establishing funding. The functional classification of roads and streets is an important management tool in the establishment of realistic improvements both for individual roadways and for the county road system as a whole. The idea is to combine adequacy with economy and to attain a rural road or urban street system giving the highest overall level of service per dollar of investment. A complete functional roadway system provides the motoring public with a series of distinct travel movements. From main movement at the national level, to the termination movement at an isolated location at the rural level, the function of each type of roadway should be designed and planned in a distinctly different manner. According to the guidelines as established in AASHTO, A Guide on Geometric Design of Highways and Streets, Current Edition, the current concept of a complete highway system consists of six stages or movements. In most cases these stages are main, transition, distribution, collection, access, and termination. In some cases, the termination of a trip may exclude some of these movements. (See fig. 6.1) Main movement involves the transportation of the highest volumes of vehicular traffic on freeway and primary arterial systems at national and state levels. Examples of this include roads like the Interstate System (1-10 and 1-65), U.S. Highway System ( U.S. Hwy 45, U.S. Hwy 98, State Hwy 188). This level requires the highest design standards for mobility and safety, while limiting access points throughout its entire system. Transition movement is the first step in directing traffic from a national, state, or inter-county level roadway system to the local environment level. The transition movement includes minor arterial and collector systems. Examples of this include roads like Lott Rd., Airport Blvd., Schillinger Rd., and Cottage Hill Rd. Transition movement design requires the designer or planner to start allocating for the need of public access to property. However mobility at this level is still the primary focus. Distribution movement consists of a broad spectrum of vehicular traffic trying to access a wide variety of major and minor collector and various local roads in a county-wide system. Examples of this include roads like Prine Rd., Spice Pond Rd., Tanner Williams Rd., and Bellingrath Rd. The demand for public access to property becomes varied and is the main focus in design. Mobility is restricted at different levels and for different purposes. The collection movement, access movement and termination movement constitute the rest of the various local road systems. Transportation at this level involves the movement of a wide variety of vehicular types to and from a wide variety of destinations. Access is the main focus for the design, blending with the movement with the various types of design vehicles. Such movement can range from high-density work or school traffic, to a single vehicle destined for a boat ramp on an isolated rural road (way). The design of these roads may and will vary considerably. 47
MAIN MOVEMENT FREEWAYS - ART ERIALS High MobUity - to Restricted Access MINOR LOCAL ROADS RESIDEN11AL ROADS ACCESS ROADS ACCESS WAYS COLLECTION MOVEMENT VARIOUS MAJOR, MINOR LOCAL ROADS & RESIDENTIAL ROADS Moderate to Minimal Mobility - to Modemte Access MINIMAL ACCESS TERMINATION POINT TRANSITION MOVEMENT ARTERIALS - COLLECTORS MODERATE ACCESS TERMINATION POINT High to Moderate Mobility - to Limited Access DISTRIBUTION MOVEMENT COLLECTORS - MAJOR LOCAL ROADS HIGH ACCESS TERMINATION POINT Moderate Mobility - to Minimal Access ACCESS MOVEMENT MINOR LOCAL ROADS RESIDENTIAL ROADS ACCESS ROADS ACCESS WAYS Minimal to Restricted Mobility - Access Relationship of Functionally Classified Systems afficmobiiity MAIN ACCESS TERMINATION POINT HIGHWAY SYSTEM OF MOVEMENT (Hg. 61) 48 High Limited High Moderate Minimal
The first step roadway boundary areas. AASHTO, in developing road classifications is to determine limits of urban and rural A Guide on Geometric Design of Highways and Streets, Current Edition, defines urban areas as all areas which have a population greater than 5,000 or more. Urban areas are further subdivided into urbanized areas (population of 50,000 and over) and small urban areas (population between 5,000 and 50,000). All other areas which have a lesser population are rural. The Alabama Department of Transportation has established by code (Section 101 of Title 23, U.S. Code) the boundary of Mobile County that has an urbanized area. In both the rural or urban areas all the vehicular movement levels occur, but the vehicular densities and the access to properties vary considerably. The quantity of vehicular traffic does not determine an urban area iust as the use of land does not determine a rural area. The next step in developing road classifications is to determine the general characteristics of the road. Some of the general characteristics would include linkage with other roads, the service it provides to the area, design speeds, access to property, truck usage, and level of service. The current roadway system, as recognized by NACE and defined by AASHTO, designates four basic classifications: Principal Arterial, Minor Arterial, Collector (Major and Minor) and Local. (See fig.6.2) Functional Classification System Urban Area Urban Mileage (Percent) Rural Area Rural Mileage (Percent) Principal Arterial System 5-10 Principal Arterial System 2 4 Principal Arterial plus Minor Arterial System 15-25 Principal Arterial plus 6 12 Minor Arterial System Collector System 5-10 Collector System 20-25 Local System 65-80 Local System 65 75 (Source: AASHTO, A Guide on Geometric Design of Highways and Streets, Table 1-1 and Table 1-2) (Fig. 6.2) 6.1 Arterial and Collector Road General Characteristics N 6.11 Principal Arterial A. B. Major link from state to state Provide high mobility for longer trips C. Access is limited to other roads only and rarely to property D. Design speeds are high, 45-70 mph E. Truck usage is high and will accommodate the largest vehicle type F. Level of Service B desirable 49
6.12 Minor Arterial A. Majorlinkwithin state orcounty level B. Serves as high traffic corridor to primary arterials C. Provide high to moderate mobility for moderate trips D. Access to abutting property should be limited and sometimes restricted E. Design speeds are moderate to high, 35-70 mph F. Truck usage is high with possible restrictions in vehicle type G. Level of Service C desirable 6.13 Collector A. Major link within county B. Serves as major traffic generator not directly served by arterials C. Spaced at intervals that accumulate traffic for local road systems D. Provide moderate mobility within county E. Access to abutting property is minimal to limited F. Design speeds are moderate to low 20-60 mph Note: low speeds 20-30 mph should only be used in mountainous terrain G. Truck usage is moderate with some restrictions in vehicle type H. Level of Service C to D desirable 50
6.2 Local Road General Characteristics The Local Road functional classification is the next level and approximately 65-80% of total roadway mileage. on AASHTO this classification the criteria However, not all Local or should have the in design, function, and environment. of diversity in following local road classifications have County. fig. 6.3) by Mobile the design characteristics characteristics, the (See characteristics. Based Roads have Because constitutes the has been established Local Road Functional Classification same general same Local Roads Urban Rural Mileage Mileage (Percent) (Percent) Major Local Roads 10 30 20-30 MinorLocaiRoads 5 15 5-20 Residential Roads 25 50 15-35 Access Roads 5 20 5 20 Access Ways 1 5 1 5 621 Major Local Road (Fig.6.3) This classification of roadway is the local level links to equal or higher functional classification at end. It may also include roadways link isolated or like subdivisions, industrial or recreational in urban or commercial farms, timberlands, small communities, residential industrial or recreational in rural to higher functional classification roadways. (developed areas roadways planned) areas sites highest each areas that that sites areas, A. Serves as a link equal or higher functional classification roadways, or may loop into an isolated and to the type of roadway between area reconnect same B. Serves as a link to communities or significant typically only connecting roadway from a higher classification roadway serves as the areas within a jurisdiction, or C. Access to abutting property is moderate to high D. Design Under speeds are moderate, some 25-45 mph conditions a lower design speed may be considered E. Right-of-way width should be a minimum of 60 feet, environment which it restricts acquisition to a serves the unless the lesser width F. Truck usage is moderate to high G. The level of service should provide moderate traffic flow times of heavy and may have restrictions congestion 51 passing except during
6.22 Minor Local Road This classification of roadway links isolated areas to a higher functional classification roadway. Isolated areas may be areas where the road extends to; a business, recreational area, scenic place, agriculture / land management areas, industrial site, or to a minimal number of houses. A. Links isolated areas to higher functional classification roadways B. May serve as the only access point C. Access to abutting property is high (main function) In the urban geographical environment (city / urban community) it characteristically consists of small parcels of land serving various businesses In the rural geographical environment (country / rural community), it characteristically consists of possible large parcels of land serving a low population density and/or business D. Design speeds are low, 10-30 mph For some locations, where greater than 30-mph operational speeds may need to be impeded, traffic calming elements should be considered in the design Under some warranted situations, only when the distance to travel is lengthy, higher design speeds >30 mph should be considered E. Right-of-way should be a minimum of 30 feet In context sensitive areas, especially where an established road has been used by the community that it serves, right-of-way width should conform to the surroundings F. Truck usage may be high in business areas G. The level of service for short distance travel length should characteristically provide slow traffic-speed flows 52
6.23 Residential Road This classification of roadway links developed or planned subdivisions, or isolated dwelling areas to a higher functional classification roadway. Isolated dwelling areas may be areas where the road extends to a number of houses that may be associated with a business or an agriculture / land management area. A. Links residential areas to higher functional classification roadways, or links isolated areas B. May be part of an internal grid of residential roadways, has a linkage system to parcels, or serves as the only access point C. Access to abutting property is high (main function) In the urban geographical environment (city / urban community) it consists of small parcels of land and a high population density In the rural geographical environment (country I rural community), it consists of possible large parcels of land and a low population density D. Design speeds are low, 10-30 mph. For some locations, where greater than 30-mph operational speeds may need to be impeded, traffic calming elements should be considered in the design E. Right-of-way should be a minimum of 30 feet In context sensitive areas, especially where an established road has been used by the community that the surroundings it serves, right-of-way width should conform to F Truck usage is low G. The level of service should provide slow traffic-speed flows 53
6.24 Access Road This classification of roadway links isolated access areas to a higher functional classification roadway. Isolated access areas may be areas where the path extends to a minimal number of houses. Roads for this classification are typically two-directional one-lane roads and should have an ADT of 400. This classification is not intended to accommodate future developments. A. Links isolated areas to any functional class of roadway B. Serves as the only access point C. Access to properties is the only function D. Design speeds are very low, 10-25 mph. E. Traffic calming elements should be considered in the design F. Right-of-way may be the same as the roadway width, and may only include the prescriptive area of the roadway In context sensitive areas, especially where an established path has been used by the community that it serves, right-of-way width should conform to the surroundings G. Truck usage is negligible and may require backing its entire length. H. The level of service should provide slow traffic-speed flows For some types of design the driver may be required to stop to allow opposing traffic to pass, or may require backing to allow opposing traffic to pass 54
6.25 Access Way This classification of roadway links limited housing to a higher functional classification roadway. Limited housing property I pathway a very low-volume of traffic, local traffic only, 200 ADT, with housing no than 18 units over 1/3 mile (1,700 feet)of travel. Travel on this type of roadway may divide, or loop unto itself to properties. This classification is not intended to future ingress user accommodate egress areas where the reach developments. has density access areas access areas are greater A. Links limited housing areas to any functional class of roadway B. Serves as the only access point C. Access to properties is the only function D. Design speeds should be 10 mph maximum E. Traffic users are the property users of the road and their guest F. Right-of-way may be include prescriptive the the same as area the roadway width, and may only of the roadway In context especially where an path by the community that it right-of-way width should conform to used the surroundings sensitive areas, serves, established has been G. Truck usage is negligible and may require backing its entire length H. The local property users should provide very slow traffic-speed flows I. For alignment turnouts, use some and/or (types of design) driver may be required to stop, back to allow opposing traffic to the pass 6.25 Reevaluation of the System The roadway functional classification should be at a local level by individuals who have the knowledge the distribution and location of the various environmental, industrial, residential, recreational, and commercial jurisdictional In most this would be having jurisdiction and control for design, construction and of the local roadway local should have good information to and by the traveling public. The reevaluation of the functional classification a management tool should be on an approximate ten to twenty-year cycle. to an existing functionally classified road can occur at any time if environment along the roadway boundaries. system about cases maintenance the as the frequency system as Change associated 55 developed those usage system. These areas within their dictates. those agencies updated
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CHAPTER 7 Design Waiver Design waivers shall be permitted in accordance with the following procedure. The engineer shall submit a written proposal that will detail the design change and outline the specific reasons and consequences of the change to the County Engineer. The County Engineer will submit the waiver with his recommendation to the County Commission for their consideration. For clear zone and lateral offset, the criteria in this design guide should be treated as guidance and not as a county standard requiring a design waiver if not numerically met. 59
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References 1. AASHTO. A Guide on Geometric Design of Highways and Streets. Washington, D. C.: AASHTO, 2011. 2. Cost Effective Local Road Safety PlannIng and Implementation, National Association of County Engineers (NACE) and American Traffic Safety Services Association (ATSSA), 2011 3. NCHRP Report 500: Guide Series, National Cooperative Highway Research Program, 2009 4. Transportation Research Board, National Research Council. Determination of Stopping Sight Distances, NCHRP Report 400. Washington, D.C.: 1997. 5. AASHTO, Guidelines for Geometric Design of Very Low-Volume Local Roads (ADT 400). AASHTO, 2001 61
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