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|A good intersection design: a blend of safety and aesthetics.||
Refer to Chapter XI of the AASHTO Green Book
At grade intersections are one of the most critical and most complicated elements in highway design. The efficiency, safety, speed, cost of operation, and capacity of the highway system depend on the design of its intersections. Design criteria that are used to create the most efficient roadways are easily thwarted when that roadway meets up with intersecting traffic vying for the same limited roadway space. In urban and suburban areas in particular, the capacity of signalized intersections can effectively define the capacity of the highway system. Add the need safely to accommodate bicyclists and pedestrians with varying degrees of mobility, and the need to handle left and right turns, and the challenge faced by designers becomes even more complicated. Page
The Basics of Intersection Design
As stated in the AASHTO Green Book, the main objective of intersection design is to:
Intersection design can vary widely in terms of size, shape, number of travel lanes, and number of turn lanes. Basically, there are three types of atgrade intersections, determined by the number of intersecting legs, topography, traffic patterns, and the desired type of operation. Each roadway radiating from an intersection is called a "leg." Most intersections have four legs, which is generally accepted as the maximum recommended number for safety and capacity reasons. The three basic intersection types are:
INTERSECTION DESIGN ELEMENTS
As is the case with other aspects of the highway design process, designers can use a wide range of intersection design elements in combination to provide both operational quality and safety. These include:
The following paragraphs summarize of primary intersection design guidelines.
Angle of Intersection
Crossing roadways should intersect at 90 degrees, if possible, and at no less than 75 degrees. Skew angles of 60 degrees or less may need geometric countermeasures, such as reconstruction, or traffic control, such as signalization.
Horizontal and Vertical alignment
The alignment before and through an intersection must promote driver awareness, operate well under frequent braking, and be easy to drive, so that the navigational task is not too difficult. The Green Book has recommended values for the minimum stopping sight distance needed based on the design speed of the approach roads. The design of intersections should also incorporate provisions for intersection sight distance.
Medians, either raised or painted, provide a physical separation between opposing traffic flows. They also provide a refuge area for pedestrians to wait at crossing locations. Medians are a standard form of channelization at rural roadways and urban street intersections carrying four or more lanes. There are two principal functions of medians specifically located at intersections:
Another important benefit of a median in an urban area is that it offers a green space for trees and lowgrowing plant material. Careful consideration is needed, however, to select the proper location and type of plantings. Particularly in narrow medians, plantings can create maintenance problems, and trees can cause visual obstructions if not carefully located.
Field studies and accident analysis provide similar findings on the operational and safety effects of the median width at intersections.' At rural unsignalized intersections, accidents and undesirable driving behavior decrease as the median width increases. In contrast, at suburban signalized and unsignalized intersections, accidents and undesirable driving behavior increase as the median width increases. 1 Median Intersection Design, NCHRP Report 375, Transportation Research Board, National Research Council,
National Academy Press, Washington, DC, 1995.
Left Turn Lane Warrants and Design
Leftturn lanes may provide added safety and efficiency at both unsignalized and signalized intersections. At signalized intersections, leftturn lane warrants are based on the magnitude of turning movements, accident experience, and general capacity relationships. The design values for leftturn approach tapers, turn bay tapers, and storage lane lengths are based on deceleration in the lane, storage in the lane, or a combination of both. At signalized intersections, the required length of storage bay is a function of signal cycle length.
An example of a simple safety improvement is the addition of a painted leftturn lane at a rural intersection. This action not only reduces the potential for yearend accidents, but also provides drivers with a comfortable way to make a left turn. However, as is discussed in the Issues section of this chapter, the addition of a left turn lane can also affect resources along the side of the road or change the character of the road corridor. These are tradeoffs for designers to consider.
RightTurn Lane Warrants and Design
Depending on rightturn traffic volumes, accident history, highway speed, and availability of rightofway, rightturn lanes may be appropriate for some intersections. As with leftturn lanes, the taper and storage length design is based on deceleration, storage requirements, or both.
Corner Radius Design
The design for an intersection corner radius is based on the selection of a reasonable design vehicle for the specific location. Design vehicles can range from large (tractortrailer combinations) to small (private autos). There are a number of tradeoffs involved in this decision. Designing the corner radius for large vehicles requires more open intersections, and increases cost, and such intersections are more difficult to mark, signalize, and operate. In addition, the larger the dimensions of the radius, the greater the distance across the intersection from one side of the street to the other. This can make crossing the intersection much more difficult for pedestrians, particularly people who are elderly or have mobility impairments. Conversely, designing the corner radius for small vehicles can create operational problems should a significant number of larger vehicles have to use the intersection. Table 8.1 presents some general guidelines to assist in the selection of the appropriate design vehicle for various highway types.
Source: Intersection Channelization Design Guide, NCHRP Report 279, 1985. Note: WB50 =semitrailer combination, large; WB40 =semitrailer, intermediate; SU30 =single unit truck; B40=single unit bus; P=passenger car.
The actual radius or curb return design can be accomplished in one of four ways. Simple circular radius designs are the most commonly encountered design on lowspeed collector and local streets and in downtown areas. Alternative design methodologies include the use of symmetrical threecentered compound curves, asymmetrical threecentered compound curves, or simple radius curves with tapers. These designs better fit the paths of turning vehicles, thereby providing more efficient operations.
Table 8.2 illustrates some of the operational characteristics associated with a range of intersection corner radius dimensions for simple radius curves. This can be used as a guide in determining the appropriate radius design.
Traffic islands, or channelization, represent one of the most important tools in the design of intersections. Islands can either be painted directly on the roadway surface or they may be raised. Painted or "flush" channelization may be used on highspeed highways to delineate turning lanes, in constrained locations, or where snow removal is a concern. Raised islands, with appropriate channels or curb ramps to accommodate users of wheelchairs or other related devices, should be used where the primary function of the island is to shield pedestrians, locate traffic control devices, or prohibit undesirable traffic movements.
There are two basic types of traffic islandscorner islands that separate rightturning vehicles and median or divisional islands that separate opposing traffic flows on an intersection approach. Although islands in general provide a safe refuge for pedestrians, corner islands that separate rightturning vehicles in particular may make crossing intersections more difficult for pedestrians. These islands tend to widen the crossing distance. They can also make it more difficult for pedestrians to maneuver through the intersection, see oncoming traffic making right turns, and know where to cross, if the islands are not clearly delineated.
Traffic Control Devices
Traffic control devices are installed to designate rightofway at intersections and to provide for the safest and most efficient movement of all traffic, including pedestrians and bicyclists. The standards established in the latest edition of the Manual on Uniform Traffic Control Devices for Streets and Highways (MUTCD), published by the FHWA, must be followed to determine proper intersection control.
NEW INTERSECTION DESIGN CONCEPTS
In recent years, a new intersection design concept has evolved to provide an alternative to the traditional T, fourleg, and multileg intersections. This design concept is called a roundabout.
Modern roundabouts are increasingly being recognized as design alternatives to the use of traditional traffic signals for intersections for arterials. They improve both safety and efficiency for pedestrians and bicyclists, as well as motor vehicles. So far, roundabouts have been built in such States as California, Colorado, Maryland, Nevada, Florida, and Vermont. These roundabouts are different from rotary or traffic circles that have been used in the United States for a number of years to give entering traffic the rightofway and encourage higher design speeds.
The modern roundabout is designed to slow entering traffic and allow all the traffic to flow through the junction freely and safely. Unlike the older rotary design, entering vehicles must yield the rightofway to vehicles already in the circle. A deflection at the entrance forces vehicles to slow down. Traffic signals are not used, and pedestrians cross the streets at marked crosswalks.
The average delay at a roundabout is estimated to be less than half of that at a typical signalized intersection. Decreased delay may mean that fewer lanes are needed. Signalized intersections often require multiple approach lanes and multiple receiving lanes, which leads to a wider road.
Perhaps the greatest advantages of roundabouts are their urban design and aesthetic aspects. Roundabouts eliminate the clutter of overhead wires and signal poles and allow signage to be reduced. They can be distinctive entry points into a community or mark a special place. The central island offers an opportunity for a variety of landscape designs, as well.
Each of the various components of intersection design can cause conflicts between the need for a safe and adequate design, on the one hand, and the need to minimize impact to the surrounding physical and human environments, on the other. In addition, the need to accommodate pedestrians and bicyclists can sometimes cause conflicts with the need to provide an efficient operating environment for vehicular traffic.
The safe and efficient accommodation of pedestrians at intersections is equally important as the provisions made for vehicles. Pedestrian movements should be provided for and their locations controlled to maximize safety and minimize conflicts with other traffic flows. Too often, pedestrians are a secondary consideration in the design of roadways, particularly at intersections in suburban areas.
For all but a few exceptions, pedestrian crosswalks should be located at intersections, should have appropriate curb ramps for accessibility, and should be clearly marked. Two parallel painted lines generally are not enough of a distinguishing marking. Often motorists confuse these lines with the stopping line and pull right up to the edge of the crosswalk. At a minimum, some type of striping or painting inside the crosswalk area is recommended to improve safety. Many cities and suburban areas have gone beyond this and added aesthetic treatments to their crosswalk designs, including use of the following:
Pedestrian signals should be used in conjunction with vehicular traffic signals at all signalized intersections where pedestrians are likely to be present. Push buttons can be used at isolated intersections or where vehicular demand warrants maximizing the time for vehicular movement through the intersection. Fixedtime traffic signals with relatively short cycle lengths are more appropriate in urban or downtown areas.
Appropriate Corner Radius Design
As mentioned earlier, there are many tradeoffs involved in the selection of the appropriate type and dimension of radius designs. Issues arise when all of the factors involved in the design decision are not considered. For example, if the primary intent of the intersection design is to move traffic through as quickly as possible, a higher corner radius would be selected. The dimensions of the corner radius send a message to drivers entering residential neighborhoods regarding the speed they can drive and should be designed with this in mind. Encouraging fast speeds around intersection corners into residential areas will undermine efforts to lower operating speeds within the neighborhoods themselves. In addition, faster speeds create an unsafe environment for pedestrians.
Addition of LeftTurn Lanes
A common conflict arising from the use of channelization, or separation of traffic into definite paths of travel by traffic islands, medians, or pavement markings, is the addition of leftturn lanes. While there is no doubt that this can create a smoother flowing intersection, especially on twolane roads, the addition of a leftturn lane can significantly widen the width of the roadway, unless there is a median. This can change the character of an area, affect adjacent development or resources, and cause the road to be out of scale with its surroundings.
In cases where a leftturn lane is truly needed to improve safety and operational efficiency in a constricted rightofway, there may not be an easy solution to this issue. Sometimes the addition of leftturn lanes depends on new growth and development along the corridor. If the scenic, historic, or cultural resources are such that any additional widening would affect these resources, it may be that decisions made at the land use stage of planning should be reconsidered. Limiting development along the corridor will limit traffic volumes and the need for additional leftturn lanes. Another option is to lower traffic volumes on the roadway through other means, including creating or widening alternative routes.
The most important point is that the necessity of the leftturn lane must be carefully determined. If it is truly needed, designers can use the flexibility available to them in the applicable geometric design standards to try to minimize any additional widening of the traveled way and limit the impact on adjacent resources to the fullest extent possible.
Aesthetics of Traffic Signals and Sign Hardware Supports
Traffic control devices are beneficial in improving the safety and efficiency of intersections. In addition, there are many alternative design treatments that meet MUTCD and crashworthy standards and can be used to fit the design into the surrounding context.
Traffic control device supports should be designed to be safe and compatible with their surroundings. This may be a particular concern for designers in scenic or historic settings. Options for the designer include: