1、PDF外文：http:/ A 外文翻译 A.1 原文 CONSISTENCY IN DESIGN FOR LOW-VOLUME RURAL ROADS3 By Clarkson H. Oglesby, H. M. ASCE (Reviewed by the Highway Division) ABSTRACT: The 2,000,000 miles of low-volume rural roads in the United States are different than the high-volume roads and should
2、be designed differently. Traffic volumes on them are low, averaging about 110 vehicles/day or about one vehicle entering a given mile from both ends every three minutes during peak hours. This contrasts with one vehicle every four seconds at capacity. Geometries on many of these roads have not chang
3、ed since they were built in the 1920s and 1930s. Today, road improvements should be based on designs that are consistent and safe, but economical, because needs are great and funds are scarce. Present-day design practices for high volume roads require that each of their features meet a stipulated de
4、sign speed set by modern surfaces and vehicles. This practice does not fit the low-volume situation since, whenever possible, drivers will exceed any affordable design speed. They must be slowed down when situations warrant it. A consistent approach to design which realizes cheap but safe improvemen
5、ts to low-volume roads is proposed. It involves integrating geometric design and positive guidance approaches. Positive guidance employs striping, signing, and other devices and strategies to mobilize drivers' senses so that they will drive sensibly. Selecting the less costly between geometry an
6、d positive guidance techniques will produce safer roads more cheaply. INTRODUCTION There are approximately 3,200,000 mile (5,100,000 km) of rural roads in the United States. A rough estimate places some 2,000,000 (3,000,000km) of these in a low-volume category； this commonly includes those with aver
7、age daily traffic less than 400 vehicles/day in both directions. On most of these roads volumes are considerably lower. One estimate places this average at 110 vehicles/day or a possible 20 in the peak hour. This means one vehicle every three minutes entering a given mile from both ends. In contrast
8、, a major two-lane road, operating at capacity, will carry possibly 1,800 vehicles/hour so that a vehicle will enter a given mile every four seconds or 90 times as often. The money available to those responsible for high-volume roads is on the order of ten times as great per mile as for low-volume r
9、oads. It follows that strategies for new construction, upgrading, or maintenance of low-volume rural roads will be entirely different than for higher-volume roads, if the very limited money available for these purposes is to be used wisely. Given the uniqueness of the low-volume road problem, it see
10、ms appropriate to examine and possibly redefine what is meant by "consistency in design" for them. This paper attempts that task by examining the following topics as it applies to them: 1. The origin and current status of local rural roads. 2. How "consistency" in present-day geo
11、metric standards for new construction or renovation of low-volume roads has developed. 3. Factors that have impinged on design standards for low-volume rural roads. 4. Conclusions. ORIGIN AND CURRENT STATUS OF LOCAL RURAL ROADS For the purpose of this paper, local rural roads are those that provide
12、access to and thereby support activities on rural lands. These include farming, ranching, recreation, and access to forests or other natural resources. This definition excludes those roads, once rural or near towns, that are now in suburbia. Relatively little mileage has been added to low-volume rur
13、al systems in the last 50 years. They were developed when the aim was "to get the farmer out of the mud." They are often characterized by narrow roadways and rights-of-way. In the middle west and west, where much of the land had been laid out in sections one mile square, rights-of-way were
14、 66 ft (20 m). This width was dedicated to land access along the edges of adjacent sections. In the eastern states, many rights-of-way were narrower, often 33 ft (10 m) or less. In rolling or mountainous country, tortuous alignments were fitted closely to the contour of the ground. Today these often
15、 restrict speeds to 30 mile/hr (48 km/h) or less. In these earlier years, travel was mainly in horse-drawn vehicle. Even in the 1930s, when the last of these land-access roads were being constructed, speeds were low because neither vehicles nor road surfacings permitted fast travel. For reasons such
16、 as these, the concept of design speed did not exist. Today, the performance of motor vehicles is far different and the sizes and weights of trucks have increased dramatically. Furthermore, for possibly two-thirds of this low-volume rural mileage, gravel or earth surfaces have been paved, surface tr
17、eated, or otherwise made relatively smooth and free of corrugations or dust. Presently, then, drivers expect to travel at higher speeds and only slow down when forced to do so by intersections or restricted vertical or horizontal alignment. On higher volume roads, many of which have been successivel
18、y improved, this slowing is seldom required. And when it is, elaborate measures are taken to alert drivers. But this matching of improvements with speed has been far less frequent on low-volume rural roads because money has been scarce. Of that available, more than two-thirds (in 1978) has gone to m
19、aintenance and other purposes, leaving little for new construction or betterment. It would be untrue and unfair to say that those responsible for low-volume rural roads have done nothing to overcome this mismatch between driver expectations regarding speed and the roads. Through strategies such as s
20、pot improvements and scrounging money from their budgets and higher governmental levels for rebuilding certain roads, they have done much. But the gap still remains large. This, of course, applies not only to road geometry, but to surfacings and bridges as well. How CONSISTENCY IN PRESENT-DAY GEOMET
21、RIC STANDARDS FOR NEW CONSTRUCTION OR RENOVATION OF LOW VOLUME RURAL ROADS HAS DEVELOPED As noted, most of the need for and geometries of low-volume rural roads developed fifty or more years ago to fit specific situations. Unless altered by maintenance, betterment, reconstruction, or complete replac
22、ement, they have changed little since. At that time, main rural roads were built to meet the same conditions and their geometry was not an issue. For example, as late as 1940, a leading highway engineering text book, by T. R. Agg (1), devotes only 22 pages to the entire subject of geometric design.
23、In it Agg stated that "considerable latitude is allowable in adapting the design to the particular situation (which may be topographical, financial, or political) as long as the design does no violence to basic principles." Agg calls for "the exercise of originality and good engineeri
24、ng judgment that does not necessarily follow stereotyped standards." It was at about this same time (1937) that AASHO (now AASHTO) created a Committee on Planning and Design Policies. Its aim was to incorporate, in practice, highway design features that would result in maximum safety and utilit
25、y. From this effort, in the period 1938-1944, came seven policy statements on geometric design that were adopted by AASHO. These were consolidated without change in 1950 into a single volume, Policies on Geometric Highway Design (3). A reworking resulted in a 1954 document called A Policy on Geometr
26、ic Design (4). This document, commonly called the Blue Book, was redone again and published in 1965 under the same title (5). In 1969, a publication applying more specifically to low-volume rural roads was issued (2). Since that time the appropriateness of these policies, which set standards for all
27、 aspects of geometric design, have been under almost continuous review and a comprehensive revision is under preparation. From the beginning, those responsible for developing standards for geometric design have been attempting to keep pace with changes in the characteristics of motor vehicles and th
28、e expectations of drivers. This has led to a substantial raising of design controls or features. FACTORS THAT HAVE IMPINGED ON GEOMETRIC STANDARDS FOR LOW-VOLUME RURAL ROADS In tracing the development of geometric standards and their application over the years in terms of their impact on low-v
29、olume rural roads policies, several factors can be seen. These include the following: 1. Low-volume road engineers or administrators have made few direct inputs into geometric standards. They have been developed by specialists in geometric design, most of them in the Federal Highway Administration.
30、They were adopted after review by geometric-design specialists in the state highway agencies working through AASHTO. Because these agencies deal primarily with high-volume situations, it is claimed that their representatives are not sensitive to the low-volume road situation. For example, the standards for low-volume as well as those for high-volume roads were based on the "design speed" concept, which has been defined as "The maximum
