Chapter 4 DESIGN CONTROLS.docx

1、Chapter 4 DESIGN CONTROLSthere are a number of design controls that affect the geometry of a highway. (Rt. 71, IL) Chapter 4DESIGN CONTROLS Refer to Chapter II of the AASHTO Green Book BACKGROUND In order to design the basic elements of a highwayincluding its alinement and cross sectionthe designer

2、must have an understanding of the basic design controls and criteria associated with the highway. One of the most important, highway functional classification, was discussed in Chapter 3. Other important design controls include, but are not limited to the following: The design speed of the facility

3、The acceptable degree of congestion (i.e., the designyear peakhour level of service) on the facility The physical characteristics of the design vehicle (i.e., the largest vehicle that is likely to use the facility with considerable frequency); in virtually all instances, the highway design vehicle i

4、s an overtheroad tractortrailer The performance of the design vehicle (particularly important in terms of accommodating heavy trucks in mountainous terrain or buses and recreational vehicles in areas subject to high levels of tourist activity) The capabilities of the typical driver along the facilit

5、y (i.e., local residents using lowspeed neighborhood streets versus interstate travelers on rural freeways) The existing and designyear traffic demands to be placed on the facility (e.g., daily and peakhour traffic volumes, the mix of passenger cars and trucks on the facility) Two of the most import

6、ant of these factors are design speed and peakhour level of service. This chapter will focus on these two important criteria. The peakhour level of service, however, only serves as a controlling factor for a small number of highways. For most highways, after the functional classification and associa

7、ted design speed for a particular highway facility have been established, the degree of flexibility available to the designer is significantly limited. Speed limits are not the same as the design speed. They are regulatory limits enforceable by law. Design Speed Design speed is defined by the AASHTO

8、 Green Book as: .the maximum safe speed that can be maintained over a specified section of highway when conditions are so favorable that the design features of the highway govern. Table 4.1 Relationship Between Design Controls and Design Features Design ControlsDesign FeaturesFunctional Classificati

9、onTraffic DataTerrain LocaleDesign SpeedLane width, ruralXXXLane width, urbanXXRural shoulder width, typeXXUrban shoulder width, typeXXGuiderail offsetXXDegree of curveXGradesXXXBridge clearances (horizontal & vertical)XXStopping sight distanceXSuperelevationXWidening on curvesXRural design speedsXX

10、XUrban design speedsXXSource: Location and Design Manual, Vol. 1, Roadway Design, Ohio DOT. All geometric design elements of the highway are affected in some way by the selected design speed. Some roadway design elements are related directly to and vary appreciably with design speed. These include h

11、orizontal curvature, superelevation, sight distance, and gradient (see Table 4.1). Other elements are less related to design speed, such as pavement and shoulder width and clearances to walls and traffic barriers. The design of these features can, however, affect vehicle operating speeds significant

12、ly. As a result, more stringent criteria for these features are generally recommended for highways with higher design speeds. Conversely, less stringent criteria for these features may be more appropriate on roadways with lower design speeds. The selection of a particular design speed is influenced

13、by the following: The functional classification of the highway The character of the terrain The density and character of adjacent land uses The traffic volumes expected to use the highway The economic and environmental considerations. Typically, an arterial highway warrants a higher design speed tha

14、n a local road; a highway located in level terrain warrants a higher design speed than one in mountainous terrain; a highway in a rural area warrants a higher design speed than one in an urban area; and a highvolume highway warrants a higher design speed than one carrying low traffic volumes. (Left)

15、 Lower design speeds are appropriate for city streets. (Right) Higher volume rural highway facilities should use higher design speeds. As discussed in Chapter 3, most States and localities have adopted a range of acceptable design speeds for each of the major classes of highways and streets (i.e., f

16、reeway, other arterial, collector, and local). Table 4.2 illustrates typical minimum design speeds for various types of highways located in level, rolling, and mountainous terrain. FreewaysDesign SpeedsTerrainRural Urban Flat70-8070Rolling60-7060-70Mountainous 50-60 50-60 Arterial HighwaysTerrain Ru

17、ral Urban Flat60-7030-60Rolling40-6030-50Mountainous 30-50 30-50 Collector and Local RoadsTerrainRural Urban Flat30-5030-40Rolling20-4020-40Mountainous 20-30 20-30 Source: Traffic Engineering Handbook (Fourth Edition), Institute of Transportation Engineers, Washington, DC, 1992, p. 156. Note: 1 mile

18、/hr = 1.613 km/hr Table 4.2 Typical Minimum Design Speeds for Various Types of Highways (in mph) The values presented in Table 4.2 are minimum acceptable design speeds for the various conditions of terrain and traffic volumes associated with new or reconstructed highway facilities. Designers need to

19、 balance the advantages of a higher vehicle operating speed gained through the use of a higher design speed against the flexibility lost in design. It may be more important to retain the maximum possible flexibility, so that a contextsensitive roadway that is more in tune with the needs of a communi

20、ty is designed using a lower design speed. As used here, the term context sensitive refers primarily to the land use and environmental conditions adjacent to the highway. For example, for any particular highway other than a freeway or major arterial, as land use density increases, the design speed w

21、ould typically decrease. The design speed of an urban collector street passing through a residential neighborhood should be appreciably lower than that for a rural highway with the same functional classification. This also recognizes the fact that bicycles and pedestrians would be more likely to use

22、 a route located in an urban area. Similarly, in areas that have significant historic interest or visual quality, a lower design speed may be appropriate in recognition of lower average operating ,speeds and the need to avoid affecting these historic or aesthetic resources. The Green Book agrees wit

23、h this philosophy: Aboveminimum design values should be used where feasible, but in view of the numerous constraints often encountered, practical values should be recognized and used. Along arterial streets, the controlling factor of design speed applies to a lesser degree than on rural highways or

24、hightype urban facilities, such as freeways or expressways. On many of the arterial streets located in large urban areas, maximum vehicle operating speeds for several hours of the day may be limited to those at which the recurring peak period traffic volumes can be accommodated. Thus, speeds may be

25、governed by the presence of other vehicles traveling en masse both in and across the through travel lanes and by traffic control devices, rather than by the physical characteristics of the street. During offpeak periods of lowtomoderate traffic demand, vehicle operating speeds are governed by such f

26、actors as speed limits, midblock turns, intersection turns, number of driveways and entrances, traffic signal spacing, and signal timing. As a result, when arterial street improvements are being planned, the selection of the appropriate design speed must be balanced against such factors as speed lim

27、its, physical and economic constraints, and the probable running speeds that can be attained during offpeak hours. Although most States have adopted a range of allowable design speeds appropriate for each of the various functional classifications for use in the design of new or reconstructed highway

28、 facilities, situations may arise where even the use of the lowest typically acceptable value would result in unacceptably high construction or rightofway costs or unacceptable impact on adjacent properties. In such instances, the design exception process discussed in Chapter 2 can be employed. For

29、example, the reconstruction of a twolane rural arterial route through a relatively flat but environmentally sensitive area might need to employ a design speed of 80 km/h (50 mph) rather than the recommended value for this functional classification of 100 km/h (60 mph) shown in Table 4.2. PeakHour Le

30、vel of Service Once an appropriate design speed has been selected, the other basic defining elements of the highway (i.e., the number of lines and the basic configuration of junctions with other highway facilities) can be determined through application of the concept of acceptable peakhour level of

31、service. Level of service is a grading system for amount of congestion, using the letter A to represent the least amount of congestion and F to refer to the greatest amount. For a comprehensive treatment of this topic, refer to the Highway Capacity Manual. Table 4.3 presents a brief description of the operating characteristics associated with each level of service. As congestion approaches capacity, speed decreases. Level ofService DescriptionAFree flow with low volumes and high speeds.BReasonably free flow, but speeds beginning to