Highway Engineering Handbook - Chapter 2: Highway Location, Design, and Traffic PDF

# HIGHWAY ENGINEERING HANDBOOK ## Chapter 2: Highway Location, Design, and Traffic ### Author: Larry J. Shannon, P.E. **Highway Department Manager** **DLZ Ohio, Inc.** **Columbus, Ohio** This chapter describes the overall transportation development process, highway location and design, and intel...

# HIGHWAY ENGINEERING HANDBOOK ## Chapter 2: Highway Location, Design, and Traffic ### Author: Larry J. Shannon, P.E. **Highway Department Manager** **DLZ Ohio, Inc.** **Columbus, Ohio** This chapter describes the overall transportation development process, highway location and design, and intelligent vehicle highway systems. It covers horizontal and vertical alignment, sight distance, superelevation, roadway cross sections, intersections, ramps, service roads, highway construction plans, CADD drawings, and roadside safety. ## 2.1 Transportation Development Process ### 2.1.1 Statewide Systems Planning - The development of a statewide transportation planning program includes goals and objectives which take into account social, economic, environmental, and developmental goals of other state, federal, and local agencies. - The state Department of Transportation identifies transportation improvement needs throughout the state. - All modes of transportation are considered, including public transportation, railroads, water, aviation, bikeways, and pedestrian ways. ### 2.1.2 Transportation Programming Phase - Transportation inventories, traffic analyses, modal forecasts, future system requirements, levels of service, population data and forecasts, land use inventories, public facilities plans, and basic social, economic, and environmental data are collected. - Information is collected from both public and private sources and is updated regularly. - Statewide fiscal program is reviewed. - Public input is sought from regional to local levels, and is included in the development of the state's recommended transportation improvement plan. ### 2.1.3 Project Evaluation - Projects are evaluated to determine which projects can advance to detailed design and which require further evaluation in preliminary design development. - Criteria for projects that can advance directly to the design phase include: no additional required right-of-way, no major changes in access points, traffic volumes, traffic flows, vehicle mix, or traffic patterns, no involvement with a live or intermittent stream, and no involvement with a historic site. - Examples of these types of projects include: - Restoration and/or reconstruction of existing pavement surfaces - Modernization of an existing facility by adding or widening shoulders - Modernization of existing facilities by adding auxiliary lanes or pavement widening to accomplish a localized purpose (weaving, climbing, speed change, protected turn, etc.) - Intersection improvements - Reconstruction or rehabilitation of existing grade separation structures - Reconstruction or rehabilitation of existing stream crossings which do not involve any modification of a live stream or otherwise affect the water quality - Landscaping or rest area upgrading projects - Lighting, signing, pavement marking, signalization, freeway surveillance and control systems, railroad protective, etc. ### 2.1.4 Preliminary Development Phase - Studies are conducted outside of the existing corridor or when a facility for more than one alternative mode of transportation may be involved. - Feasible alternatives are limited to the existing corridor in projects where feasible alternatives are limited to the existing corridor but did not qualify to pass directly to the design phase. - A project inventory is developed for each project. - Preliminary alternatives are developed together with documentation of the anticipated effects on community, preliminary cost estimates, and other considerations. - Public hearings are held to gain input from the local public in the affected areas. - Environmental concerns are considered. ### 2.1.5 Detail Design Phase - Detail design elements are finalized and construction plans are developed. - Various reviews prior to final plan submission may include some or all of the following, depending on the complexity of the plan: - Traffic request/validation - Traffic signal warrant analysis - Airway-highway clearance study - Alignment, grade, and typical section review - Conceptual maintenance of traffic review - Structure type study - Retaining wall justification - Service road justification - Preliminary drainage review - Preliminary right-of-way review - Bridge type, size, and location study - Drive review - Slope review - Traffic control - Lighting - Waterline - Sanitary sewer - Final roadway, field and office check ## 2.2 Geometric Design ### 2.2.1 Design Controls - Design controls are established for each project. - Design controls are grouped into five categories: - Functional classification - Traffic data - Terrain - Locale - Design speed **Functional Classification:** - The initial division is between urban and rural roadways. - The urban classification may be defined differently in various parts of the country. - The rural classification is defined as areas outside of urban areas. **Roadways are further subdivided into other classifications defined as follows:** - Interstate: Roadways on the Federal system with the highest design speeds and the highest design standards. - Freeway: An expressway with full access control and no at-grade intersections. - Expressway: A divided arterial highway with full or partial control of access and generally having grade separations at major intersections. - Arterial: A facility primarily used for through traffic, usually on a continuous route. - Collector: An intermediate roadway system which connects arterials with the local road or street systems. - Local road or street: A road whose primary function is to provide access to residences, businesses, or other abutting properties. **Traffic Data:** - Traffic data is used to forecast future traffic volumes. - Highway design is usually based on what traffic demands will be 20 years from the current year. **Time Periods** - **20 years:** used for the design of most highways. Shorter time periods such as 10 years, are used to resurface projects or other minor repair projects. - **10 years:** used for resurfacing projects or other minor repair projects. **Traffic Data Types:** - **ADT** (Average Daily Traffic): The average number of vehicles using a roadway in a 24-hour period. - **DHV** (Design Hourly Volume): The estimated number of vehicles using the roadway in the 30th highest hour of the year. - **DDHV** (Directional Design Hourly Volume): The estimated number of vehicles traveling in one direction of a two-way roadway in the 30th highest hour of the year. - **Truck percentage (T):** The portion of the ADT which consists of B and C trucks. **Terrain** - Terrain is a factor that can significantly influence design features. - Terrain is categorized as follows: - **Level terrain:** Grades are generally limited to 1 or 2 percent and heavy vehicles can maintain approximately the same speed as passenger cars. - **Rolling terrain:** Heavy vehicles will reduce their speeds substantially below those of passenger cars, but not to operate at crawl speeds. - **Hilly terrain:** Heavy vehicles operate at crawl speeds, which is defined as the maximum sustained speed vehicles can maintain on an extended upgarde. **Locale** - Locale describes the character and extent of development in the vicinity, and is categorized as commercial, industrial, or residential, or rural or urban. **Design Speed** - Design speed is defined as "a selected speed used to determine the various geometric design features of the roadway". - Design speed should always equal or exceed the proposed legal speed of the roadway. - Table 2.1 shows the relationship of the functional classification, traffic data, terrain, locale, and design speed to the various geometric design features. ## 2.2.2 Sight Distance - Sight distance is the distance ahead that a motorist should be able to see so that the vehicle can be brought safely to a stop short of an obstruction or foreign object in the road. - Sight distance includes the driver's reaction or perception distance and the distance traveled while the brakes are being applied. - Values in Table 2.2 were developed using a reaction time of 2.5 s and a braking deceleration rate of 11.2 ft/s². - The height of eye is assumed to be 3.50 ft and the height of the object 2.00 ft. - Stopping sight distance requirements are addressed in detail in the text and are summarized in Table 2.2. **Types of Sight Distance** - **Horizontal sight distance** is checked by objects such as bridge piers, buildings, concrete barriers, guiderail, cut slopes etc. - **Vertical sight distance** is restricted by vertical curves in the roadway profile. **Intersection Sight Distance:** - Intersection sight distance is the distance required for a motorist stopped at an intersection to make safe crossing or turning maneuvers. - The minimum acceptable distance should be 7.5 s for a passenger vehicle turning left and a gap of 6.5 s for a crossing or right-turning vehicle. **Passing Sight Distance:** - Passing sight distance is the distance required to pass an overtaken vehicle. - The minimum distance required for passing is calculated based on various design speeds and is applicable for two-lane roadways only. - Table 2.3B contains K values for designing crest vertical curves to provide passing sight distance. **Decision Sight Distance:** - Decision sight distance provides a greater length for drivers to reduce the likelihood of error in receiving information, making decisions, or controlling the vehicle. - Recommended decision sight distances are provided in Table 2.4.. ## 2.2.3 Horizontal Alignment and Superelevation - Horizontal alignment of a roadway should be designed to provide motorists with a facility for driving in a safe and comfortable manner and ensure adequate stopping sight distance. - Changes in direction should be accompanied by the use of curves and superelevation when appropriate, following established guidelines. - The maximum deflection angle which may be permitted without the use of a horizontal curve for each design speed is provided in Table 2.5. **Superelevation:** - Superelevation banks the roadway to help counteract the effect of centrifugal force on the vehicle as it moves through the curve. - The relationship of superelevation and side friction is expressed as: $e + f = \frac{V^2}{15R}$. - Factors that affect the selection of a maximum superelevation rate include climate, terrain, and urban vs. rural areas. - Different maximum superelevation rates are used for urban areas and rural areas, as well as different regions, based on the potential for snow or ice. - Side friction factors are also dependent on factors such as pavement texture, weather conditions, and tire condition. - Higher speeds tend to have lower side friction factors. **Minimum Radius** - For a given design speed and maximum superelevation rate, there exists a minimum radius of curvature. - The minimum radius can be calculated using the formula: $R_{min}=\frac{V^2}{15(e+f)}$ - The degree of curve can be calculated using the formula $D=\frac{5729.6}{R}$ **Superelevation Methods** - Superelevation and side friction are directly proportional to the degree of curve in Method 1. - The side friction is used to offset centrifugal force in direct proportion to the degree of curve, for curves up to the point where fx is required, and e is increased in direct proportion to the increasing degree of curvature until emax is reached in Method 2. - Superelevation is used to offset centrifugal force in direct proportion to the degree of curve for curves up to the point where emax is required, and fis increased in direct proportion to the increasing degree of curvature until fmax is reached in Method 3. - Method 4 is similar to Method 3, but is based on average running speed instead of design speed. - Superelevation and side friction are a curvilinear relationship with the degree of curve, with resulting values between those of Method 1 and Method 3 in Method 5. **Recommended Superelevation Rates** - Recommended superelevation rates for horizontal curves in relation to other design elements are given in Tables 2.7 through 2.11. - Each table represents a different **emax**. - Method 5 is used to assign the appropriate superelevation rate to each degree of curve. **Transition Runoff Lengths** - Transition runoff lengths are provided in the tables. **Superelevation Transition** - The length of highway needed to change from a normal crowned section to a fully superelevated section is referred to as the superelevation transition. - The tangent runout is the length required to remove the adverse pavement cross slope. - The superelevation runoff is the length required to raise the outside edge of pavement from a half-flat section to a fully superelevated section. - The length of transition required to remove the pavement crown is generally equal to twice the T distance. - The minimum superelevation transition length should be equal in feet to 3 times the design speed in miles per hour. - The transition length is determined by multiplying the edge of pavement correction by the equivalent slope rate (G) shown in Table 2.13. **Superelevation Transitions between Reverse Horizontal Curves** - The superelevation transitions calculated independently may overlap. - The designer should coordinate the transitions to provide a smooth and uniform change from the full superelevation of the first curve to the full superelevation of the second curve. - The L₁ and L₂ values for each curve are dependent on the degree of curvature, but the total superelevation transition length should be at least 50 percent nor greater than 70 percent of L₁ + L₂. **Spiral Transitions** - Spiral transitions smooth out changes in direction and provide a gradual change in the degree of curve. - They are used in combination with simple curves or spiral curves. **Horizontal Alignment Considerations** - When establishing a new horizontal alignment, consider the following: - The alignment should be directional as possible while still consistent with topography and the preservation of developed properties and community values. - Maximum allowable curvature should be avoided whenever possible. - Consistent alignment should be sought. - Curves should be long enough to avoid the appearance of a kink. - Tangents and or flat curves should be provided on high, long fills. - Compound curves should be used only with caution. - Abrupt alignment reversals should be avoided. - Two curves in the same direction, separated by a short tangent, should be avoided. ## 2.2.4 Vertical Alignment - Steep grades can slow down large, heavy vehicles in the traffic stream in the uphill direction and can adversely affect stopping ability in the downhill direction. - Flat grades over extended distances will slow down the rate at which the pavement surface drains. - Vertical curves should be designed to provide adequate stopping sight distance. **Tangent Grades** - Table 2.14 shows how the maximum percent grade can vary under different circumstances. **Critical Length of Grade:** - The critical length of grade is the length of a particular upgrade which reduces the operating speed of a truck with a weight-to-horsepower ratio of 200 to 10 mi/h below the operating speed of the remaining traffic. - Figure 2.11 provides the amount of speed reduction for these trucks given a range of percent upgrades and length of grades. **Vertical Curves:** - Vertical curves are used to transition between vertical tangents. - The recommended minimum length of a vertical curve is based on the K value, which represents the distance required for the vertical tangent to change by 1 percent. **Allowable Grade Breaks:** - Grade breaks can be permitted at certain design speeds, without using a vertical curve. - Table 2.15 lists the maximum grade break permitted. **Crest Vertical Curves:** - Crest vertical curves should be designed to provide adequate stopping sight distance. - Table 2.16 lists the calculated design stopping sight distance values and the corresponding K values. **Sag Vertical Curves:** - Sag vertical curves should be designed for headlight sight distance. - Table 2.17 lists the calculated design stopping sight distance values and the corresponding K values. - Sag vertical curves should also be designed for comfortable operation and a pleasing appearance. - The minimum length should be at least 3 times the design speed in miles per hour. **Vertical Alignment Considerations** - When establishing new vertical alignment, consider the following: - The profile should be smooth with gradual changes consistent with the type of facility and the character of the surrounding terrain. - Undulating grade lines involving substantial lengths of steeper grades should be appraised for their effect on traffic operation, since they may encourage excessive truck speeds. **Superelevation** - Superelevation rates are provided in Tables 2.7 through 2.11. - Each table represents a different **emax**. - Method 5 is used to assign the appropriate superelevation rate to each degree of curve. # 2.3 Superelevation Rates and Runoff Lengths This section covers the superelevation rates for different design speeds and curves. The tables cover design speeds from 15 to 80mph and curves up to 75ft (Table 2.8-Table 2.9). <start_of_image> breakdowns of each table are below: **R:** Radius of curve **V:** Assumed design speed **E:** Rate of superelevation **L:** Minimum length of runoff (does not include tangent runout) **NC:** Normal crown section **RC:** Remove adverse crown, superelevate at normal crown slope # 2.4 Transition Runoff - This section covers the transition between tangent sections and horizontal curves as well as the superelevation of a curve. - It describes the calculation of transition runoff length. # 2.5 Pavement Superelevation and Transitions - Superelevation transitions allow the pavement to transition smoothly from a normal crowned section to a superelevated section.  - There are three basic methods for developing superelevation on a crowned pavement: revolving the pavement about the centerline, revolving about the inner or lower edge, and revolving about the outer or higher edge of the pavement. - Figure 2.9 illustrates all three methods. **Minimum Superelevation Transition Length** - The minimum superelevation transition length should be equal in feet to 3 times the design speed in miles per hour.  - This includes the tangent runout as described in the text. **Superelevation Transitions Between Reverse Horizontal Curves** - The superelevation transitions calculated independently for each curve may overlap. - Figure 2.10 illustrates the coordination of the superelevation transitions for simple curves and spiral curves. - The total superelevation transition length should be at least 50 percent nor greater than 70 percent of L₁ + L₂.  **Spiral Transitions** - Spiral transitions smooth out changes in direction for a circular horizontal curve. - The degree of curve varies gradually from zero at the tangent end to the degree of the circular arc at the curve end.   - They should be used on roadways where the design speed is 50 mi/h or greater and the degree of curvature is 1°30′ or greater.  # 2.6 Horizontal Alignment - Factors to consider when establishing a new horizontal alignment are listed. # 2.7 Vertical Alignment - Vertical alignment affects safety and comfort of the driver. - Steep grades can slow down heavy vehicles in the uphill direction and can adversely affect stopping ability in the downhill direction. - Flat grades will slow down the rate at which the pavement surface drains. - Vertical curves should be designed to provide adequate stopping sight distance. - The maximum percent grade permitted for a given roadway is determined by the surrounding terrain and design speeds. - Table 2.14 shows how the maximum percent grade can vary. - The maximum grade is higher for local roads and at lower design speeds than higher functional class roadways and at higher design speeds. - Flat and level grades may be used on uncurbed roadways. **Critical Length of Grade:** - The critical length of grade is the length of a particular upgrade which reduces the operating speed of a truck with a weight-to-horsepower ratio of 200 to 10 mi/h below the operating speed of the remaining traffic. - If the critical length of grade is exceeded, consider constructing added uphill lanes on critical lengths of grade. - Figure 2.11 illustrates the amount of speed reduction for trucks given a range of percent upgrades and length of grades. **Vertical Curves:** - Vertical curves provide a smooth transition between vertical tangents. - They are parabolic curves and are usually centered on the intersection point of the vertical tangents. - The K value is used to determine the minimum length of vertical curves necessary to provide minimum stopping sight distance.  - The K value represents the distance required for the vertical tangent to change by 1 percent. **Allowable Grade Breaks:** - In situations where the difference in grades is not large enough, a vertical alignment may be permitted without using a vertical curve. - Table 2.15 lists the maximum grade break permitted depending on the design speed. **Crest Vertical Curves:** - The principle design consideration for crest vertical curves is the provision of adequate stopping sight distance. - Table 2.16 provides the design stopping sight distance values and corresponding K values for design speeds from 20 to 70 mi/h in 1-mi/h increments. - The minimum length of a crest curve in feet should at least be 3 times the design speed. **Sag Vertical Curves:** - Sag vertical curves are designed for headlight sight distance. - Table 2.17 provides the design stopping sight distance values and corresponding K values. - The minimum length of a sag curve in feet should at least be 3 times the design speed. **Vertical Alignment Considerations** - The following should be considered when establishing new vertical alignment: - The profile should be smooth with gradual changes consistent with the type of facility and the character of the surrounding terrain. - A "roller-coaster" or "hidden dip" profile should be avoided. - Undulating grade lines involving substantial lengths of steeper grades should be appraised for their effect on traffic operation, since they may encourage excessive truck speeds. The last page of the book shows the index of chapters and their corresponding pages.