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Highway Types All state highway systems and most of the local highway and street systems encompass several type or classes of highway .At one extreme are high-speed ,high-volume facilities carrying though traffic , with no attempt made to serve abutting property or purely local traffic. At the other are local rural roads or streets that carry low volumes ,sometimes at low speeds ,and with a primary function of land service. Definitions for the various types of highways (and for many other highway term ) were prepared in 1968 by an (then) AASHO Special committee on Nomenclature .These are published as AASHO Highway Definitions .some of these are as follows: Expressway- Divided arterial highway for though traffic with full or partial control of access and generally with grade separations at major intersection. Freeway- Expressway with full control of access. Parkway- Arterial highway for noncommercial traffic , with full or partial control of access .and usually located within a park or a ribbon of parklike development . Control of access Condition where the right of owner or occupants of abutting land or other persons to access .light ,air ,or view in connection with a highway is full or partially controlled by public authority. Full control of access means that the authority to control access is exercised to give preference to though traffic by providing access connections with selected public roads only and by prohibiting crossings at grade or direct private driveway connections.Partial control of access means that the authority to control access is to give preference to though traffic to degree that .in addition to access connections with selected public roads ,there many be some crossings at grade and some private driveway connections. The other highway types lack the feature of access control .they include: Major street or major highway Arterial highway within intersections at grade and direct access to abutting property , and on which geometric design and traffic control measures are used to expedite the safe movement of though traffic .Though street or though highway Every highway or portion thereof on which vehicular traffic is given preferential right of way ,and at the entrances to which vehicular traffic from intersecting highways is required by law to yield right of way to vehicles on such through highway in obedience to either a stop sign or yield sign ,when such signs are erected.Local road Street or road primarily for access to residence , business, or other abutting property .Precise definition for other highway types such as arterial ,belt bypass , radial, frontage also are given ,In general the label explain the function .It should be pointed out here ,however ,that the freeway , as typified by the Interstate System represent the highest type highway facility, for all other classes certain important advantages must be sacrificed .In capsule form .these include the following .Capacity On freeway the absence of intersections or crossings at grade the elimination of major friction through access control permit unrestricted ,full time use moving vehicles ,rather than restricted ,part time flow .Reduced travel time On freeway , time losses from stopping and waiting at intersections are eliminated .In addition ,most of the conflicts that contribute to accidents are eliminated except under unusual circumstances ,Drivers normally can and travel at higher and sustained speeds.Safe operation On freeway ,elimination of conflicts at intersections and along both margins of the roadway and the barring of pedestrians from the right of way usually bring substantial reductions in accidents .Permanence access control along freeway prevents the growth of businesses or other activities along the roadway margin ,Without access control ,these activities generate unordered traffic and parking ,In a short time ,capacity is reduced and accident potential is substantially increased.Reduced operating coat, fuel consumption, air pollution, and noise-smoother operations and fewer stops reduce fuel consumption and other operating costs Reduce fuel consumption in turn reduces air pollution. Smoother operation with fewer stops also greatly reduce noise, particularly that from trucks . Highway Cross Section The principal elements of a highway cross section consist of the travel lanes , shoulders ,and medians (for some multilane highways) .marginal elements include median and roadside barriers ,curbs, gutters ,guard rails , sidewalks, and side slopes .Travel lane widths usually vary from 10ft to 12ft .most arterials have 12-ft travel lanes since the extra cost for constructing 12-ft lanes over 10-ft lanes is usually offset by the lower maintenance cost for shoulders and pavement surface , resulting in the reduction of wheel concentrations at the pavement edges. On two-lane, two-way rural road. Lane widths of 10ft or 11ft may be used ,but two factors must be considered when selecting a lane width less than 12 ft wide , the first is that it has been shown that when pavement surface are less than 22 ft , the accident rates for large trucks tend to increase, Second , the capacity of a highway significantly decreases as the lane width is educed from 12 ft .Lane widths of 10 ft are therefore used only on low- speed facilities. Lanes that are 9 ft wide are used occasionally in urban areas if traffic volume is low and there are extreme right-of-way constraints.The shoulders is contiguous with the traveled lane and provides an area along the highway for vehicles to stop, particularly during an emergency .Shoulders are also used to laterally support the pavement structure .the shoulder width is known as either graded or usable, depending on the section of the shoulder being considered . pavement to the intersection of the shoulder slope and the plane of the side slope, The usable shoulder width is that part of the graded shoulder that can be used to accommodate parked vehicle. The usable width is the same as the graded width when the side slop is equal to or flatter than 4:1 , as the shoulder break is usually rounded to a width between 4 ft and 6 ft , thereby increasing the usable width .When a vehicle stop on the shoulder , it is desirable for it to be at least 1 ft and preferably 2 ft from the edge of the pavement .Based on this ,AASHTO recommend that usable shoulder width of least 10 ft and preferably 12 ft be used on highways having a large number of trucks and on highways with high traffic volumes and high speeds. However, it may not always be feasible to provide this minimum width ,particularly when the terrain is difficult or when traffic volume is low. A minimum shoulder width of 2 ft may therefore be used on the lowest type of highways .but 6-to8-ft widths should preferably be used .The width for usable shoulder within the median for divided arterials having two lane in each direction. However may be reduced to 3 ft ,since divers rarely use the median shoulder for stopping on these roads, The usable median shoulder width for divided arterials with three or more lane in each direction should be at least 8 ft, since drivers in difficulty on the lane next to the median often find it difficult to maneuver to the outside shoulder. It is essential that all shoulders be flushed with the edge of the traveled lame and be sloped to facilitate the drainage of surface water on the traveled lanes. Recommended slopes are 2 percent to 6 percent for bituminous and concrete-surfaced shoulders . and 4 percent to 6 percent for gravel or crushed-rock shoulders.A median is the section of a divided highway, that separates the lanes in opposing directions, The width of a median is the distance between the edges of edges of the inside lanes , including the median shoulders the functions of a median include: (1) providing a recovery area for out-control vehicles ,(2) separating opposing traffic ,(3) providing stopping areas during emergencies,(4) providing storage for left-turn and U-turning vehicles,(5) providing refuge for pedestrians, (6) reducing the effect of headlight glare,(7) providing temporary lanes and crosss-overs during maintenance operations .medians can either be raised ,flush, or depressed. Raised medians are frequently used in urban arterial streets because they facilitate the control of left-turn traffic at intersections by using part of the median width for left-turn-only lanes. Some disadvantages associated with raised median include possible loss of control of the vehicle by the driver if the median is accidentally stuck and the casting of a shadow from oncoming headlights, which results in drivers having difficulty seeing the curb.Flush medians are commonly used on urban arterials. They can also be used on freeways. But with a median barrier. To facilitate drainage of surface water , the flush median should be crowned. The practice in urban areas of converting flush medians into two-way left-turn lanes is rather popular ,since this helps to increase the capacity of the urban highway while providing some of the features of a median.Depressed medians are generally used on freeway and are more effective in draining surface water . A side slope of 6 : 1 is suggested for depressed median , although a slope of 4 ; 1 may be adequate.Median width vary from a minimum of 2 ft to 80 ft or more , median widths should be as wide as possible but should be balanced with the other element of cross section and the cost involved . In general , the wider the median, the more effective it is in providing safe operating conditions ,AASHTO recommend a minimum width of 10 ft for four-lane urban freeways, which is adequate for two 4 ft shoulders, and a ft median barrier. A minimum of 22 ft ,preferably 26 ft ,is recommended for six or more lanes of freeway.Median width for urban collector streets, however, vary from 2 ft to 40 ft , depending on the median treatment . for example ,when the median is a paint-striped separation, 2-to 4-ft median are required. For narrow raised or curbed areas ,2-to 6 ft median are required, and for curbed sections, 16-to 40 ft medians are required .the large width is necessary for curbed sections because it provided space for protecting of freeway. Sidewalks are usually provided on roads in urban areas, but vary seldom are they provided in rural areas .however ,the provision of sidewalks in rural areas should be evaluated during the planning process to determine if any sections of the road require them ,such as areas adjacent to schools, industrial plants ,and local businesses ,Generally, sidewalks should be provided when pedestrian traffic is high along main or high-speed roads in either rural areas , when no shoulders are provided on arterials ,sidewalks are necessary even when pedestrian traffic is low .In urban areas sidewalks should also be provided along both side of collector streets that serve as pedestrian access to schools , parks, shopping .centers, and transit stops ,and along collector streets in commercial areas . Sidewalks should have a minimum clear width of 4 ft in residential areas and a range of 4 ft to 8 ft in commercial areas.To encourage pedestrian use of sidewalks during all weather conditions ,they should have all-weather surface , otherwise ,pedestrians will tend to use the traffic lanes.The right of way is the total land area acquired for the construction for the highway .Its width should be enough to accommodate all the elements of the highway cross section ,any planned widening of the highway and any public utility facilities that will be outside the right of way on easement areas , The right of way two-lane urban collector streets should be between 40 ft and 60 ft ,whereas the desirable minimum for two-lane arterials is 84 ft .Right-of-way widths for undivided four-lane arterials vary from 64 ft to 108 ft ,whereas for divided arterials ,they range from about 120 ft to 300 ft ,depending on the numbers of lanes and whether frontage roads are included , the minimum right-of-way widths for freeways depend on the number of lanes and the existence of a frontage road.Highway LocationThe basic principle for locating highway is that roadway elements such as curvature and grade must blend with each other to produce a system that the easy flow of traffic at the design capacity ,while meeting design criteria and safety standards . The highway should also cause a minimal disruption to historic and archeological site and other land-use activities. Environmental impact studies are therefore required in most cases before a highway location is finally agreed upon.The highway location process information.Reconnaissance survey.Preliminary location survey.Final location survey.Office study of existing informationThe first phase in any highway location study is the examination of all available data of the area in which the road is to be constructed . This phase is usually carried out in the office prior to any field or photogrammetric investigation. All the available data are collected and examined. These data can be obtained from existing engineering repots ,map, aerial photographs, and charts ,which are usually available at one or more of the state departments of transportation ,agriculture, geology, hydrology, and mining .the type and amount of data collected and examined depend on the type of highway being considered ,but in general, data should be obtained on the following characteristics of the area: Engineering , including topography, geology, climate, and traffic volumes. Social and demographic, including, land, use and zoning patterns. Environmental ,including type of wildlife ,location of recreational ,historic, and archeological sites ,and the possible effects of air ,noise, and water pollution. Economic ,including unit costs for construction and trend of agricultural ,commercial ,and industrial activities.Preliminary analysis of the data obtained will indicate whether any of the specific sites should be excluded from further consideration because of one or more of the above characteristics .For example if it is found that a site of historic and archeological importance is located within an area being considered for possible route location, it may be immediately decided that any route that traverse that site should be excluded from further consideration . at the completion of this phase of study , the engineer will be able to general areas through which the highway can traverse . Reconnaissance survey The object of this phase of the study is to identify several feasible routes ,each within a band of a limited width of a few hundred feet , when rural roads are being considered , there is often very little or no information available on maps or photographs , and therefore aerial photography is widely used to obtain the required information. Feasible routes are identify by a stereoscopic examination of the aerial photographs ,talking into consideration factors such as;Terrain and soil conditions Serviceability of route to industrial and population areas.Crossing of other transportation facilities, such as rivers, railroads, and other highways .Directness of route.Control points between the two terminals are determined for each feasible route. For example a unique bridge site with no alternative may be taken as a primary control point ,the feasible routes identified are then plotted on photographic base maps.During this phase of the study ,the position of feasible routes are set as closely as possible by establishing all the control points and determining vertical and horizontal alignments for each. Preliminary alignments are used to evaluate the economic and environmental feasibility of the alternative routes.Economic evaluation of each alternative route is carried out to determine the future effect of investing the resources necessary to construct the highway .The benefit-cost ratio method is used for this evaluation ,factors usually taken into consideration include road user costs , construction costs, maintenance costs. Road user benefits. And any disbenefits, which may include adverse impacts due to dislocation of families ,business, and so forth , the results obtained from the economic evaluation of the feasible route provide valuable information to the decision maker . for example, these results will provide information on the economic resources that will be gained or lost if a particular location is selected .this information is also used to aid the policy maker in determining whether the highway should be built .and if so ,what type of highway it should be.Construction of a highway at any location will have a significant impact on its surroundings. A highway is therefore an integral part of the local environment and must be considered as such , this environment include plant ,animal, and human communities and encompasses social ,physical, natural, and man-made variables . these variables are interrelated in a manner that maintains equilibrium and sustains the life style of the different communities .