Issues Concerning The Geometric Design of Roads And Highways
- Roadway Context
- Roadway Users
- Transportation Demand
- Measures of Effectiveness
- Sight Distance
Roadway ContextThe context of a roadway is a critical factor to consider in developing a project’s purpose and need, making fundamental design decisions such as cross-section determination, and selecting detailed design elements such as street light fixtures or other construction materials. Development of a roadway design that is sensitive to, and respectful of the surrounding context is important for project success. Historically, the highway design process has focused on a project’s transportation element, particularly those associated with motor vehicle travel. A context-sensitive design should begin with analysis of the contextual elements, such as environmental and community resources, of the area through which a roadway passes. The concept of area types has been developed to help the designer understand the users, constraints, and opportunities that may be encountered in different settings. Once the designer has an understanding of the area surrounding the road and the road’s users, the designer should consider the transportation elements of the roadway, its function within the regional transportation system, and the appropriate level of access control. Thus, three main elements of context considered in design are :
- Area Type – The surrounding built and natural environment
- Roadway Type – The role the roadway plays in terms of providing regional connectivity and local access
- Access Control – The degree of connection or separation between the roadway and the surrounding land use.
The context of a roadway begins with its environmental context, which includes nearby natural resources, terrain, and the manmade environment (development patterns, historic, cultural, and recreational assets). The environmental context can be a determinant of the desired type of accommodation for different users. This context often establishes the physical constraints of the roadway alignment and cross-section, and influences the selection of motor vehicle design speed. A roadway frequently traverses a variety of changing environs. Additionally, the volume and character of pedestrian, bicycle, public transit, and motor vehicle activity can change considerably along its route. Land use is the fundamental determinant in the function of a road; as land use changes along a road, the road's functions also change. Roadway must be designed in a manner that serves the existing land use while supporting the community's future land use goals.
The transportation network is composed of several types of roadways that provide different functions, traditionally referred to as an its functional class. The primary of some roads is to facilitate movement of vehicles (bicycles, cars, trucks, buses and light rail) between cities and towns. The primary purpose of other roads is to provide access to the adjoining land. Most roads provide a combination of these purposes. The roadway type should be selected to reflect the actual role that the roadway plays in the transportation system, as defined through the project development process. A typical trip will often entail travelling along a variety of roadway types, each of which provides a different degree of local access and a different degree of regional connectivity.
Access control is a term used to define how access to adjacent properties is regulated and designed along a roadway. Access control is among the most useful tools available to maintain safe and efficient roadway operations for all users. Judicious use of medium treatments, driveway permits, and safe driveway geometry can improve roadway safety and enhance the operation of the road without undue burden on accessing boarding property. The degree of access control is influenced by the roadway type and area type. For example, access controls are usually more stringent on arterials than on collectors and local roads, reflecting the mobility and land access functions of these roadways. Likewise, access controls are often given more consideration in developing areas where there is flexibility for future land use to conform to an access management plan than in developed areas where the pattern of land use has been established. However, the designer should consider existing access points along a roadway and the possibility for changes that are consistent with the project’s purpose and need. For example, it may be possible to relocate, redesign, or consolidate driveway along an existing roadway. A thorough understanding of access control will help the designer select an appropriate design speed, planning parameters, and desired level-of-service for the facility’s users. Access control is exercised by statute, zoning, right-of-way purchases, driveway controls, turning and parking regulations, geometric design (e.g., raised medians, grade separations, and frontage roads), and right-of-way, permitting frequently administered by PWD.
Roadway UsersA fundamental expectation in roadway design is that all users will be accommodated safely. Virtually all roadways serve a variety of users including pedestrians, cyclists, motor vehicle drivers and passengers. In a few cases, such as expressways, roadways serve almost extensively motor vehicle traffic. Early in the process, the designer needs to determine the composition of users anticipated for the facility. Appropriately accounting for all user characteristics is essential to obtain a safe and efficient roadway. Experience demonstrates that when human and vehicular factors are properly accommodated, the safety and effectiveness of the highway or road system is greatly enhanced. Consideration of roadway users’ characteristic and selection of appropriate accommodation can also influence the roadway’s effectiveness for businesses and residential users, the economic health of the region, the physical health of the population, and the quality of the built and natural environment. The characteristics of these varied roadway users are important controls that influences the physical design of a roadway, as described in the following sections.
Safe, convenient and well-designed facilities are essential to encourage use of bicycle. Roads designed to accommodate cyclists with moderate skills will meet the needs of most riders. Young children are primarily the cyclists who may require special consideration, particularly on neighborhood streets, in recreational areas, and close to schools. When bicycles are used on public streets and roads, cyclists are subject to the same traffic rules as motor vehicle operation.
Transportation demands – volume, composition, and patterns – are important design controls. The greater the demand for a facility, the more important are its operation and safety characteristics. The designer must have a good understanding of existing and anticipated demands by pedestrians, cyclists, and drivers. Community planning goals, the selected design year, and performance measures for a project are key determinants of how the design achieves the project’s purpose and need.
Projects are designed to accommodate travel demands likely to occur within the life of the facility under reasonable maintenance. This involves projecting future conditions for a selected planning horizon year. Projections of future demand for major transportation investments are usually made for the 20 to 30 year range. For large projects, the designer should usually select 20 years from the expected facility completion date as the design year. This is a reasonable compromise between a facility's useful life, the uncertainties of long-range projections, and the consequences of inaccurate projections. For smaller, less capital intensive projects, a 5 to 10 year planning horizon is generally used. Forecasts of future activity levels should reflect community and regional plans, community setting, and the project’s purpose and need. Based on these considerations, a future conditions forecast represents a technical analysis and policy consensus on the type and developed intensity of land use, future regional economic activity, presence of transit service, the needs of pedestrian and cyclists, and many other factor. Forecasts of future activity levels should include estimates of pedestrian and bicycles activities. Particular care must be takien when forecasting pedestrian and bicycles volumes. Most of the times, there is latent demand above observed pedestrian and bicycle volumes because pedestrian and bicycle facilities do not yet exist in the project area, are substandard, or do not provide complete connectivity to attractions. It is important to evaluate future land development, including any potential attractors such as transit stops, schools, parks and retail uses that may be located near moderate and high-density residential development. Planners and designers need to determine the appropriate estimates of activity level design. For the typical project undertaken within a community, such as an intersection improvement or a corridor access management project, the forecast is based on existing conditions. First, traffic counts (including pedestrian and bicycle trips) are conducted to determine when the peak hour(s) of traffic occurs. Second, seasonal adjustment is made, if necessary, to ensure the count data are representative of at least average annual conditions. Lastly, future conditions are estimated by adding or subtracting from the existing traffic volumes to account for known development and transportation projects, and an annualized factor is generally applied to account for potential area wide growth or decline. Regional travel demand models are often used in planning larger transportation projects. Although the typical process for forecasting traffic volumes assumes that traffic will increase over time, there are situations where traffic volumes may decline or remain relatively constant over time. It is important that traffic forecasts for a roadway design project reflect likely conditions over the project's life and are not selected arbitrarily.
The composition of transportation demand is an important element in the design of roadways. The designer should develop a realistic design scenario including the volume and mix of activity for all modes as described below.
Pedestrian counts should be completed to determine pedestrian flows and patterns. The pedestrian counts should include sidewalk demands, crossing demands, and storage demands at corners, traffic islands, and median (total number of pedestrians waiting to cross the street). In addition to relying on counts of pedestrians, the designer should also evaluate the project area to determine if there is latent demand for pedestrian accommodation due to an uncomfortable existing walking environment, missing links in the pedestrian network, or expected changes in development patterns. The likelihood of latent demand can be assessed by looking at surrounding land uses and their propensity to generate pedestrian activity. One can also look for conditions like pathways worn along the roadside to determine if pedestrian connectivity is underserved. It may be important to complete pedestrian counts for other times of the day (beyond the typical morning and evening peak hours) and/or on weekends, depending on the project area. For example, if a project area is heavily influenced by a school, it is important to observe pedestrian flows during morning and mid-afternoon periods. Public assembly facilities and transit stops or stations also merit special consideration because they can produce high volumes of pedestrians over short durations. To determine the appropriate locations for pedestrian counts (including project area intersections), it is important to review current pedestrian routes between activity centers. Informal paths or crossing locations may warrant supplemental pedestrian observations during project planning.
Bicycle demands should be counted during peak hour's concurrent with vehicle turning movement counts. As the pedestrian activity, the designer should also evaluate the project area to determine if there is potential latent demand for bicycles accommodation. Additional consideration of bicycle demands during other periods of the day and/or on weekdays may warrant supplemental counts.
Daily, peak hour, and patterns of motor vehicle traffic are needed as input to the planning and design of roadway facilities. Some key definitions of traffic volume measures are listed here:
- Average Annual Daily Traffic (AADT): The total yearly volume of automobiles and trucks divided by the number of days in a year.
- Average Daily Traffic (ADT): The calculation of average traffic volumes in a time period greater than one day and less than one year. (ADT is often incorrectly used interchangeably with AADT.)
- Peak-Hour Traffic (PH): The highest number of vehicles passing over a section of highway during 60 consecutive minutes. T (PH) is the PH for truck traffic only.
- Peak-Hour Factor (PHF): A ratio of the total volume occurring during the peak hour to the maximum rate of flow during a given time period within the peak hour (typically is 15 minutes).
- Design Hourly Volume (DHV): the one-hour volume in the design year selected for determining the highway design. (In Many cases, designers look at the typical worst case weekday morning or evening peak or the 30th highest hour of the year to assess the geometric requirements of their design.)
The design hourly (DHV), or daily peak hours, will affect many design elements including the desired number of travel lanes, lane and shoulder width, and intersection layout. The design volume may also influence the level of service provided and the accommodation appropriate for pedestrians and cyclists. Daily traffic estimates are also useful in making design decisions related to the total user benefit of a proposed improvement. For example, the benefit of highway safety roadside improvements is directly related to the crash exposure (expressed in ADT) on the road. Sometimes selection of the design hour entails judgement regarding the conversion of daily traffic to peak hour traffic volumes. Other times, when data from continuous traffic count stations are used, the design hourly volume is based on the peaking characteristics of the facility over an entire year. For rural areas, the DHV is typically based on the 30th or 50th highest hour. In urban areas, the DHV typically represents the 100th highest hour. In some circumstances, a lesser design hour is appropriate. These design hour volumes are usually selected since they capture operating conditions expected to occur on a regular basis and have been shown to have dependable statistical relationship to measured ADT on a roadway. The choice of the design hour volume has a significant impact on the characteristics of a project. Designers should ensure that the design volume is selected such as the facility is well-matched to the traffic volumes it will carry on a regular basis and is not “over-designed”. For example, accommodating a high volume expected to occur infrequently will result in a project that is costly and has significant adverse impacts. Likewise, accommodating a lower design volume that is frequently exceeded may result in significant congestion and not meet the level-of-service expectations for various users. Large or heavy vehicles, such as trucks and buses, have different operating characteristics from passenger cars and bicycles and can affect traffic operations. Therefore, the number of trucks and buses expected to use a facility needs to be estimated for both the daily and peak hour conditions, in planning and design. For highway capacity purposes, “heavy vehicle’ are typically defined as all buses, single-unit trucks, and truck combinations other than light delivery trucks. (Light delivery trucks two axles with four tires). In addition, the impact of transit operations (such as buses making stops along a roadway) must be considered in operational analysis of the roadway.
Measures of EffectivenessThrough the project development process and with public input, the designer should evaluate the project (and its alternatives, if applicable) using several measures of effectiveness. Suggested measures of effectiveness and analysis techniques for consideration during project planning and design are described below. Many of these measures of effectiveness are included in the transportation evaluation criteria used by transportation agencies for project evaluation and prioritization. The following sections discuss transportation or contextual of effectiveness.
National or state transportation policy places an emphasis on improving the condition of existing facilities. Projects on existing facilities should return a facility to a state of good repair by addressing existing structural, pavement surface, or other deficiencies. Techniques such as pavement testing and bridge inspections can be used to identify existing deficiencies.
The safety of transportation facilities is a primary concern in planning and design. Some projects are specifically proposed to address known safety problems; however, all projects should result in a facility that safely accommodates its users. Corridor safety audits and analysis of crash records can be useful for identifying existing safety hazards. Project design elements should be selected based on their historic safety performance and expected operating characteristics.
Many projects result in improved accommodation for particular modes. The effectiveness of these projects can be measured by the degree to which they allow users to choose the mode best-suited to their trip purpose and personal values within the broader framework of the community, the region, and the environment.
SpeedSpeed is an important factor considered by travellers in selecting a transportation mode or route. Speed can also influence the physical characteristics of the transportation infrastructure. Many design elements such as horizontal and vertical curvature and super elevation are directly related to speed. Other features, such as lane and shoulder width, and the width of the roadside recovery clear zones for errant vehicles, can vary with, but are not a direct function of the design speed. The objective in the planning and design of a roadway is to determine a speed that is appropriate for the context results in a safe facility for all users, is consistent with the community’s goals and objectives for the facility, and meets user’s expectations. Once an appropriate speed is selected, the designer needs to tailor design elements to that speed. Speed is defined as the distance travelled by an object in a certain period of time. Speed is commonly expressed in km/h in the context of transportation planning and design. Several measures and characteristics of speed are important to understand when designing a roadway, as described in the following sections. These measures are most often used to describe motor vehicle operations, although they are also applicable to pedestrian and bicycle movement.
Operating speed is the measured speed at which drivers are observed operating their vehicles in fair weather during off-peak hours. Operating speed is measured at discrete points along a roadway. Operating speeds are usually reported using percentile speeds with the 50th percentile (average) and 85th percentile (the speed at which 85 percent of vehicles are travelling at or below) speeds are often used to characterize the operating speed on a roadway. The roadway’s features such as curves and topography, width, access to adjacent properties, presence of pedestrians and cyclists, parking, traffic control devices, lighting, etc., affect the operating speed. During peak periods, when traffic congestion or intersection operations are controlling movement along a corridor, observed operating speeds may be substantially lower than the operating speed measured during off-peak conditions when the roadway’s design and context are controlling speed. Numerous studies have indicated that drivers will not significantly alter what they consider to be a safe operating speed, regardless of the posted speed limit unless there is constant heavy enforcement.
The target speed is the desired operating speed along a roadway. The appropriate target speed is determined early in the project development process, and should consider:
- The context of the roadway including area type, roadway type, and access control;
- The volume, mix, and safety of facility users; and
- The anticipated driver characteristics and familiarity with the route.
Design speed is the selected speed used to determine various geometric features of the roadway. The design speed should be a logical one with respect to the target speed and existing operating speed. When selecting a design speed, understanding the existing operating speed and target speed addresses: (1) the need to meet the expectations of drivers based on the roadway environment, and (2) the ways in which the setting influences the desired speed. It is important to understand the inter-relationship between speed and roadway geometry. Selection of a design speed influences the physical geometrics of the roadway. Similarly, the physical geometrics of the roadway are important determinates of the operating speed that will result on the facility. The relatively wide range of design speed recognizes the range of roadway types, context, and topography. The provision of a range in design speeds combined with general guidance on selection of a design speed represents perhaps the greatest flexibility afforded by the designer. Designers should exercise judgement in the selection of an appropriate design speed for particular circumstances and conditions. In general, an appropriate design speed should be within approximately 5km/h of travel speed. When determining the appropriate design speed the designer should also consider the volumes and composition of the expected non-vehicular and vehicular traffic, the anticipated driver characteristics, and driver familiarity with the route. The designer should also consider expected operations throughout the day, including both peak and non-peak hours. Indeed, no-peak traffic flow will generally control the selection of a reasonable design speed. The design speed may vary from any given route as it traverses rural, suburban, and urban areas. Once these factors have been evaluated and an appropriate design speed is determined, the geometric elements should be designed consistently to the level. The designer should document the factors leading to the selection of an appropriate design speed. This documentation is important for selected design speed below the existing posted speed limit, below the “reasonable and proper” speed for the type of roadway and area or below the measured operating speed. Where it is not possible to meet the selected design speed for one location or design element along a corridor, a design exception and appropriate warning signage may be justified. Higher design speeds impose greater challenges and constraints on designers. Designers faced with difficult or constrained conditions may consider selecting a lower design speed for an element or portion of the highway. This practice can cause problems such as a large number of drivers may not “behave” as the designer desires or intends them to. Designs based on artificially low speed can result in inappropriate geometric features that violate driver expectations and degrade the safety of the highway. The emphasis should be on the consistency of design so as not to surprise the motorist with unexpected features. Therefore, the design speed should only be based on the speed limit if the speed limit is consistent with existing operating speed or physical constraints of the built environment. Designers should not propose an alternative design speed for a highway or segment of a project as design exception. A serious fundamental problem with accepting or allowing a design exception for design speed is based on its important relative to all features of the highway. A reduction in the design may be unlikely to affect overall operating speed. It will potentially result in the unnecessary reduction of all the speed-related design criteria rather than just the one or two features that led to the need for the exception. The acceptable alternative approach to a design speed exception is to evaluate each geometric feature individually, addressing exceptions for each feature within the context of the appropriate design speed.
Occasionally, projects retain geometric elements, such as tight curves, super elevation, or restricted sight distances that are designed for a speed lower than the design speed for the corridor. This may be due to adjacent land use, or to environmental or historic constraints. In these cases, the designer should recommend a posted speed consistent. In these cases, the designer should recommend a posted speed consistent with the geometric features. Where it is desirable to maintain a higher consistent speed throughout a corridor, the designer should install appropriate cautionary signing at locations with design elements that do not meet the criteria for the posted speed.
The term traffic-calming refers to a variety of physical measures to reduce vehicular speed primarily in residential neighborhoods. The lowering of operating speed is often the appropriate solution to addressing safety problems. Such problems typically involve vehicle conflicts with pedestrians, cyclists, and school children. Research has shown that measurable reductions in operating speed are possible through traffic-calming. A local road or street, and in some instances other roadways that function as a local road or street, may have an existing operating speed far in excess of the speed limit or the target speed. In these cases it may be acceptable, and consistent with good engineering practice, to develop a design that will lower the operating speed.
Sight DistanceSight distance is the length of roadway ahead that is visible to the roadway user. In most cases, specific sight distance measures apply to motor vehicles and cyclists. The following aspects are commonly discussed for motor vehicle sight distance:
- Stopping sight distance
- Passing sight distance and
- Decision sight distance
The provision of adequate stopping sight distance (SSD) is a critical sight distance consideration for design and is described in the more detail below.
Stopping sight distance is the distance necessary for a vehicle travelling at the design speed to stop before reaching a stationary object in its path. The sight distance at every point along a roadway should be at least the stopping sight distance. The motor vehicle stopping sight distance is given in Table 1.
For two-lane highways, passing manoeuvers in which faster vehicles move ahead of slower vehicle must be accomplished on lanes regularly used by opposing traffic. If passing is to be accomplished safely, passing sight distance is necessary to allow the passing driver to see a sufficient distance ahead, clear of traffic, to complete the passing manoeuvers without cutting off the passed vehicle and before meeting an opposing vehicle that appears during the manoeuver.
Decision sight distance adds a dimension of time to stopping sight distance to allow a driver to detect and react to an unexpected condition along a roadway. Decision sight distance is suggested when there is evidence that it would be prudent to provide longer sight distance, such as when complex decisions are needed or when information is difficult to perceive. It is the distance needed for a driver to detect an unexpected or otherwise difficult-to-perceive information source or condition in a roadway environment that may be visually cluttered, recognize the condition or its potential threat, select an appropriate speed and path, and initiate and complete and manoeuvre safely and efficiently.