路桥工程专业英语第四课

第四课Freeway and its Accessory Facilities高速公路及其附属设施Text: Freeway Design Reading Material: Freeway Accessory Facilities Text Freeway Design Freeway are the highest form of arterials and have full access control. The full control of access is need for prioritizing the need for through traffic over direct access. A freeway’s primary function is to provide mobility, high operating speed, and level of service, while land access is limited. Access connections,where deemed necessary, are provided through grade separated interchanges.The major advantages of access controlled freeways are high capacity of the facility, high operating speeds, efficiency of the facility, and safety to all highway users. The major differences between freeway and other arterials include the following elements: grade separations at cross roads and streets; the grade separated cross road connections between the freeway and crossroad are accomplished through exit and entrance ramps; and full control of access. Expressways can be designed with both freeway and non-freeway design elements. The use of jughandle style interchanges and use of right turn channelization is not considered freeway design, but can be used in expressway design. Urban freeways generally have more travel lanes and carry more traffic than rural freeways. Urban freeways can be eigher depressed, elevated, at ground level, or a combination of the above. Urban freeways usually have a narrower median than rural freeways and tend to have more connections than rural freeways. Rural freeways are generally similar in concept to urban freeways, except that the horizontal and vertical alignments are more conservative in design. This level of design is normally associated with higher design speeds and greater accessibility to right of way. Due to the rural nature, right of way is typically more available and less expensive. This allows for a wider median which improves the safety of the facility. In addition to the increase in safety of a rural freeway, the higher design speeds in a rural setting allow for greater capacity, a higher level of mobility, and potencially a reduced need for multiple lanes. Rural freeways are normally more comfortable from a driver perspective, and generally have lower maintenance costs. The sections below discuss the different design elements of urban and rural freeways. 1 Design Speed In general, the design speed of freeways should be similar to the desired running speed during off peak hours, keeping in mind a reasonable and prudent speed. In some urban areas, with populations under 50,000, the posted freeway speed is 65 mph. In other more densely populated urban areas (over 50,000), the posted speed is 55 mph. Because of the different posted speeds, the design speed chosen may vary. In many urban areas the amount of available right of way can be restricted and achieving high design speeds can be very costly. In balancing the need for safety and providing a high speed facility with consideration for right of way costs, the design speed for urban freeways shall be a minimum of 60 mph. For those sections of freeway, in which a 60-mph design speed is recommended by the Project Team, approval by the Roadway Engineering Manager is required. In mountainous terrain it can be very difficult to achieve high design speeds due to horizental and vertical limitations of the terrain. It is important, though, to design freeways, urban and rural, that provide a consistent roadway for drivers. Due to terrain limitation the design speed of rural freeways in mountainous terrain shall be a minimum of 60 mph. Rural freeways outside of mountainous terrain generally have higher design speeds. Normally right of way is more available in rural locations allowing for more liberal horizenal and vertical alignments. These higher design speeds allow for increased volumes and capacity while providing a safe facility and a more comfortable driving environment. Increased capacity leads to improvements to the level of mobility standards and a facility that will operate longer than a lower design speed urban freeway. For rural freeways, a minimum design speed of 70 mph shall be used. 2 Alignment and Profile Because of the rural terrain and high design speeds, rural freeways should have very gentle horizontal and vertical alignments. In rural areas, the designer should be able to create a safe and efficient facility while taking into consideration the aesthetic potential of the freeway. Most freeways are constructed near ground level and the designer should take advantage of existing topography to create not only a functional freeway, but also one that both looks and drives well and fits into the existing landscape. 3 Shoulders The shoulder width of urban and rural freeways is dependent upon the number of lanes of the facility. The right side shoulder for both urban and rural freeways shall be 10 feet. This wideh allows for emergency parking of vehicles on the right hand shoulder. The left side shoulder is dependent on the number of freeway lanes. When there are two lanes in each direction on the freeway, the left side shoulder shall be 6 feet wide. When the freeway consists of three or more lanes in each direction, the left side shoulder shall be 10 feet. This wide left side shoulder on a multi-lane section allows for vehicles in the left lane to use the left side shoulder in an emergency instead of crosing two lanes of traffic to find refuge in the right side shoulder. The standard shoulder widths also apply to bridge shoulderwidths, including any shy distance requirements. When right side roadside barriers are used, the normal right side shoulder width shall be increased to provide a 2 foot “E” offset or “shy” distance. When a roadside barrier is used on the left side shoulder of 10 feet or more in width, the left side shoulder shall also provide the 2foot “E”distance. Exceptions to the 2 foot “E”widening may be approved by the Roadway Engineering Manager when the additional shoulder widening is not practical. 4 Medians Freeway medians provide a nontraversible separation between the travel ways of opposing traffic. Medians provide a sense of security and convenience to the operators of motor vehicles. The wider the median the more comfortable the driver becames with the facility. The width of urban and rural freeway medians is dependent upon available right of way. Because urban freeways have high speed and high volume traffic, the median should be as wide and flat as possible. A wider median on an urban freeway can provide for future transit, rail, HOV (high occupancy vehicles) , HOT (high occupancy toll) or travel lanes. Many times the width of medians arre restricted due to the highly developed and expensive right of way. For urban freeways the minimum median width for a freeway with two lanes in each direction and a concrete barrier is 18 feet between edge of travel lanes. This allows for 6 foot shoulders, a 2 foot “E” distance, and a 2 foot concrete barrier. For urban freeways with three or more lanes in each direction and a concrete barrier, the median shall be 26 feet wide between edge of travel lanes, This distance allows for 10 foot shoulders, a 2 foot “E”distance, and a 2 foot concrete barrier. The designer should be considering future needs of the facility when dealing with minimum median designs, particularly accommodating future lanes or transit. The desirable median width in an urban and rural area is 76 feet (inside edge of travel lane to inside edge of travel lane). This allows for a median that has the flexibility of allowing additional lanes in the future. In areas where the right of way is inexpensive the edge of travel lane to edge of travel lane distance should be increased to 126 feet. Median widths ranging from 76 to 126 feet (inside edge of travel lane to inside edge of travel lane) are very commen for rural freeways. The median width allows for future widening, installation of a raised mound median, or drainage facilities. In areas of steep topography, the use of a wide median allows for the designer to use independent profiles and proper sides lopes. 5 Lane Widths Due to the high speed, high volume traffic, and the need to provide for safe facilities, the travel lane width for both urban and rural freeways shall be 12 feet. A design exception is required for lanes less than 12 feet. 6 Cross Slope The cross slope for four lane (two lanes in each direction) urban and rural freeways is 2%. When an urban or rural freeway consists of three or more lanes in each direction, the cross slope shall be increased to 2.5% for the outside lanes and is applicable to the outside shoulder cross slope. The two inside lanes shall retain a cross slope of 2%. 7 Grades Generally grades on urban and rural freeways are very similar. In urban and mountainous areas, increased grades are allowed due to terrain. Care should be taken in urban areas to minimize the use of steep grades due to the close spacing of interchanges and the multiple speed changes needed in an urban area. In an urban environment, the driver must process large amounts of information in short periods of time. Steep grades make it more difficult for lane changes and other maneuvers to be made. The maximum grade for rural flat, rural rolling, rural mountainous or urban freeways are 3%, 4% , and 5% respectively. 8 Vertical Clearance The vertical bridge clearance on all new urban and rural freeway structures shall be a minimun of 17 feet. The clearance shall be from the top of the pavement to the bottom of the structure and includes the entire roadway width including the usable shoulder width. The clearance requirements for transmission and communication lines vary considerably and must comply with the National Electrical Safety Code. Clearance information should be obtained from the Railroad/Utilities Engineer. To accommodate future resurfacing, an allowance of 6 inches should be added to the vertical clearance of certain structures, such as sign trusses, pedestrian overpasses, and through-truss structures, because of their lesser resistance to impacts. The minimum railroad clearance to be provided on crossings shall conform to OAR 741. Additional clearance may be required and should be determined individually for each crossing. Words and Expressions Arterial:干线 prior:优先;priority:优先权;Prioritizing:使……有优先权; through traffic :过境交通;Direct:直接的,顺的,直通的; operating speed:运行车速;Deem:认为、相信; interchange:互通式立体交叉;capacity:容量,通行能力; facility:简易、灵巧,设备、便利(复数);Efficiency:效率; crossroad:十字路;ramp:弯坡,匝道; Expressway:快速路;Jughandle:单方面的;(壶把) Channelization:渠化;lane:小巷,通道; Depressed:沮丧的,降低的;Median:中央分隔带; Connection:=connexion:联系、联系物;conservative:保守的、守旧的; Accessibility:易接近的、可到达的;right of way:通行权、用地、筑路权; Potencially:潜在地;multiple lanes:多车道; normally :正常地;generally:一般地、综合地; perspective:透视、透视图,眼力,观点;Maintenance:保养、维护、养护; design speed:设计(计算)车速;running speed:行驶车速; peak hours:高峰小时;Reasonable:合理的,适当的; prudent :谨慎的; posted:公告、宣布;posted speed :公示车速; amount:数量;restrict:限制; Consideration:考虑;approval:批准、赞成; recommend :推荐;Mountainous:多山的; terrain:地形;consistent:坚固的、连贯的、一致的; Liberal:自由主义的,慷慨的;allow for:考虑到、估及、体谅; volume :容积、体积;Profile:侧面,剖面、外形,纵断面; Aesthetic:美学的、审美的;topography:地形、地势、地形学; Functional:官能的、实用主义的;shoulder:路肩; refuge:避难所、安全岛;Shy:害羞的、畏缩的; barrier :栅栏、障碍;Offset:旁支、分支、支脉; Exception:除外、例外;nontraversible:不可穿越的; transit :运输、通行;Occupancy:占有、占领,占有率; HOV:高占用率车辆; HOT lines:HOV可以无偿使用,其他车辆交费可以使用的车道; Accommodating:乐于助人的,亲切的,随和的; Desirable:理想的、称心的;Flexibility:弹性; Installation:就职、设置、安装、设施;Mound:土堆、土墩、堆; Raised:高起来的、凸起的;Steep:陡峭的; proper:适当的、规矩的、固有的;sideslope :侧坡; Slope:斜坡、倾斜;cross slope:横坡; Applicable:可适用的、适当的;Grade:坡度; Minimize:将……减到最少;spacing:间距; Process:过程、经过、处置、工艺程序;Maneuvers:机动; Rolling:地形起伏的;respectively :分别; Clearance:清除、净空;usable:可用的; Transmission:传送;Communication:通信、通讯、交通; Considerably:重要的、相当多的;Comply:答应、顺从、遵守; Utility:效用,公共事业;resurfacing:重铺路面; sign truss:广告牌;pedestrian overpass:人行天桥; through-truss structures:直通桁架结构;Impact:碰撞; conform:符合、顺应;Individually:单独地;