United States Public Roads Administration

“ Unlike roads, bridges are affected by the gross weight of vehicles as well as by the load on each axle. The effect of gross weight, however, is not that of a weight applied at a single point, but rather that of a weight distributed over a significant length—the length of the over-all wheel base—and applied to the bridge within that length at points separated by the distances between axles. Some of the stresses generated in a bridge are responsive to separate axle loads; others are determined in their amount by the magnitude and spacing of all the axle loads applied by a vehicle. ”

“ Highways are avenues of transportation. Within limits of feasibility they should be adjusted in the various elements of their design to the justifiable needs of traffic. Failure to effect such adjustments in highways previously built, because of a deficiency of traffic information and knowledge of road-traffic relationships, have been the cause of much of the present highway obsolescence. ”

“ The fact that the law of a State fixes particular limits upon the sizes and weights of vehicles that may be legally operated on the roads of the State is a matter to be reckoned with by the highway authorities, if for any reason it appears that highways should be designed to accommo-date vehicles of maximum size and weight either larger or smaller than the existing law allows. ”

“ If there were some roads which were, in greater degree than others, important from the viewpoint of national defense, it was obvious that they should be included in the system then in process of designation, because such inclusion would assure their earlier improvement. ”

“ Proper highway design requires that the length of highway ahead visible to the driver of a vehicle be sufficiently long at all times to enable a driver to bring his vehicle to a safe stop in advance of an object unexpectedly appearing on the road surface. In accomplishing such a stop the driver must perceive the object, react to his decision that a stop is necessary, and apply the brakes. Following these actions the vehicle requires a period of time to come to rest, the needed time depending on the speed of the car and the condition of the braking system. ”

“ For the various special-purpose vehicles for which the Army regulations permit designs exceeding in dimensions and weight the limits prescribed for vehicles intended for regular highway use, considerations of combat effectiveness are paramount in the determination of size and weight. Their occasional regulated use of roads in times of peace will present no serious traffic problems and all lesser considerations of convenience, necessity, and economy must yield to their exigent use in war. ”

“ The average age of all surfaces on rural sections of the system is 12 years. Many, of course, are much older. The roadways on which these surfaces are laid are nearly half again as old, averaging about 17 years of age. In other words, the average mile of rural highway on the interstate system consists of a surface that was last improved in 1937 and a roadway upon which the last major improvement of alinement and grade was made in 1932. These are most significant facts in view of the increases in traffic and the demands of safe operation that have occurred during these intervals. ”

“ Comprehensive studies have shown that it is uneconomical to design the average highway for a greater hourly volume than that which is exceeded during only 30 hours each year, and little will be saved in the construction cost and a great deal lost in expediting the movement of traffic if the highway is designed to accommodate fewer vehicles than the volume exceeded during the 50 highest hours of the year. ”

“ A substantial part of the street network of our cities and much of the rural road mileage improved during a period of 40 years past, is seriously obsolescent. Traffic has grown faster than the responsive improvement of the street and highway facilities. Of the entire street and highway network, the interstate highway system, its most important segment, is by and large the most seriously obsolescent part. In the general lifting of arterial highway standards that is now needed and overlong delayed, the routes comprising the interstate system should be among the first considered for improvement. ”

“ Presuming that highways throughout the Nation should be built to standards uniformly adequate in all States, the variety of size and weight laws persisting, highway authorities of many States are unavoidably confronted with the necessity of justifying any decisions they may reach as to road standards that appear inconsistent with the existing State law. ”

“ Commercial vehicles occupy a greater road space and influence other traffic over a larger area of highway than do passenger cars because they are larger and generally travel at lower speeds, especially on grades. When a highway is operating at its capacity the total number of vehicles is therefore less, if there are any commercial vehicles, than if traffic is composed entirely of passenger cars. ”

“ A third consideration, taken in recognition of the tendency of efficient practice in vehicle design and operation to move toward the supply and usage of vehicles of greater load-carrying capacity, was the adoption of limits which, while affording sufficient protection for existing roads and bridges, would allow for a reasonable enlargement of the vehicles in prevalent use, and serve as the basis of the advanced standards desirable of adoption for highways and bridges to be built in the future. ”

“ In mountainous areas the standards call for an effective shoulder not less than 4 feet wide. Shoulders effectively 8 feet wide are needed to provide an extra emergency lane into which to turn to avoid a collision. They are needed also as an area in which to stop for the changing of tires or the making of other emergency repairs, and for other similar purposes related to highway safety and the avoidance of obstruction of free traffic flow. More than 71 percent of the rural mileage of the system, 22,684 miles, is without shoulders as wide as 8 feet. ”

“ Since both the weight and length of vehicles in combination determine their effects upon bridges and since the weight-length relation varies so widely among all the vehicles that will use a bridge—and since, further, vehicles of different weights will have exactly the same effect if their lengths are varied in proper relation to their weights, there is no single answer to the question as to what weight of vehicle a particular bridge will safely support. ”

“ It is apparent that the rural interstate system, in its present state of improvement, is about as safe as, but certainly no safer than, the average of all rural roads, and since its traffic is far above the average in volume, it accounts for an exceptionally large proportion of the annual highway death toll. ”

“ As a result of off-tracking, vehicles negotiating horizontal curves occupy a greater width of roadway than they do in following a straight course.
The amount of off-tracking increases with the length of vehicle and varies inversely with the radius of the curve. The off-tracking of passenger cars is insignificant on modern highways except at intersections and on the ramps at interchanges. The off-tracking of trucks and truck-trailer combinations, however, is much greater than for passenger cars and governs the amount by which the normal lane widths must be widened on the sharper curves. ”

“ Sight distances that restrict passing maneuvers are a serious detriment to the capacity of two- and three-lane roads. Where passing sight distances are inadequate, drivers are restricted in their freedom of movement in much the same manner as when the lane used for passing is filled with oncoming vehicles. The reduced capacity resulting from short sight distances can be determined by using as a criterion the percentage of the total highway on which sight distances are insufficient to permit passing maneuvers to be performed safely. ”

“ Excessive military damage to highways is that caused by the movement of military vehicles or units when such movement is for the direct defense of the United States, or if in peacetime, use of the highways or streets has been authorized by the responsible State or other government agency at the request of responsible military authorities, and which does visible damage in excess of that caused by normal military and civilian vehicle use. ”

“ Significant advance has been made in the last 10 years in the development of improved highway design standards as a direct result of more exact information concerning road usage, motor-vehicle performance, and driver behavior in the presence of various traffic conditions and roadway characteristics. This information is used as a base for determining the features of road design that are most directly affected by the increase in volume and speed of traffic, features in which there has been the highest degree of obsolescence in the older highways. ”

“ It seems probable, therefore, that in a future struggle all forms of motor-vehicle transportation, and the roads and streets over which they travel, will be more essential to the war economy than in the preceding one. ”

“ Improvement of the principal secondary roads, long deferred, remains a present need whatever may be the provision for the primary roads, and these improvements also will contribute to a state of highway readiness for the national defense. The need for these other improvements is inferior only to needs on the interstate system. ”

“ Another phenomenon associated with concrete pavement, to which the axle loading of vehicles is known to contribute, is the occurrence known as joint pumping. Some jointing of concrete pavements is unavoidable. If the joints are omitted, cracks form as a result of temperature contraction. Where either joints or cracks exist, surface water may penetrate to the soil subgrade, or water may reach the sub- grade at the joints in other ways. When a vehicle passes over the pavement at a joint the edges of the slabs forming the joint are bent slightly downward by the load. ”

“ It is essential to permit a distribution of the heavier of probable highway freight loadings consistent with reasonable standards of road, bridge, and vehicle design. There is substantial warrant for the belief that such a length will be required and efficiently used in future civil transport operation, as it is already equaled and exceeded in the design of military vehicles. No confirmation has been found in extensive traffic observations of the fear that vehicles of such length will be unduly difficult or dangerous to pass. ”

“ More serious than its effects upon highway design policy, however, is the effect of this variety of State vehicle regulatory laws upon the flow of interstate traffic. When different dimensions and weights are fixed as the largest permissible in each of two States, vehicles that can legally be operated in one can be operated in the other only in violation of law. ”

“ In relation to highway capacity, one commercial vehicle has approximately the same effect as two passenger cars in level terrain and four passenger cars in rolling terrain. In mountainous terrain the effect of one commercial vehicle may be as great as eight passenger cars. On individual grades trucks affect the safe and efficient flow of traffic because they go so slowly upgrade that long queues of passenger cars are formed behind them and then go so fast down the other side that none can pass. ”

“ Shoulders capable of supporting all types of vehicles standing on them or passing onto them infrequently and in emergency at high speed during any weather conditions are essential for traffic safety. Adequate shoulders are also essential to realize the full capacity of the surface width. Without adequate shoulders, one disabled vehicle can reduce the capacity of both two-lane and multilane highways during peak periods by as much as 60 percent. ”

“ The 1 percent of the country’s total rural-road mileage, which forms the rural portion of the designated interstate system, served in the several States an average of 2714 percent of the travel on all rural roads in each State by vehicles registered in other States. The designated rural system is thus shown to be outstandingly of service to that portion of the country’s highway traffic that can be classed as interstate in character. ”

“ A road carrying 600 trucks per day in the earlier year would have had, on the average, eight axle loads daily weighing 18,000 pounds or more, three weighing 20,000 pounds or more, and only one weighing 22,000 pounds or more. Assuming an average traffic increase, this same road in 1948 would have been carrying 1,200 trucks per day and would have been subjected daily to 91 axle loads of 18,000 pounds or more, 40 of which would have weighed 20,000 pounds or more and 17 of which would have weighed 22,000 pounds or more. ”

“ Requests made to State authorities for occasional movement over highways of Military Establishment vehicles, designed for off-road operation in a combat zone, have been refused because of their excessive axle and gross weight. The fact that there are no Federal funds currently available for repair of the possible damage if the highway engineer does not give proper allowance for deterioration of surface or structures, is believed in large part responsible. ”

“ EFFECTS OF WAR ON MOTOR-FUEL CONSUMPTION
Although the gasoline-rationing program was directed primarily toward the conservation of rubber, it resulted in great savings of motor fuel, a commodity much in demand for military use in the United States, on the high seas, and in combat areas. The great reduction in civilian highway use of motor fuel during the war years is illustrated in the following tabulation:
During World War II the motortruck was part of the production line, often extending across many States. ”

“ Main roads are generally State highways, or primary State highways in cases where all or nearly all of the rural road mileage is under State jurisdiction. In 1948, nearly three-quarters of all travel on rural roads was on these main roads, as can be seen from the table, and 21 percent of this travel was by trucks. The discussion of truck traffic is confined to main rural roads because weight surveys have been made principally on this class of road since the 1936-38 prewar period. ”

“ An axle load of 18,000 pounds is the heaviest that can be allowed. Such a limit should be rigidly enforced for the protection of existing road surfaces, most of which will not carry heavier loads without damage and unreasonable shortening of their normal service lives. If future roads were designed to support heavier axle loads it would be years before the replacement of existing surfaces would permit vehicle loading to the higher axle limit. ”

“ The total length of the highways included in the strategic network is approximately 78,800 miles. Within a system of this extent, there naturally are differences of importance among the roads included, even while as a whole the system comprises the most important highways. In its review in 1941, the War Department indicated such distinctions among the routes included in the network. In general, it found that the routes of greatest importance coincided with those that had previously been chosen by the Bureau of Public Roads as desirable of inclusion in a system of interregional highways. ”

“ More than 70 percent of the workers in war plants depended on private automobiles for transportation to and from work. Congestion on the old roads leading to the Willow Run bomber plant in Michigan was later minimized by construction of an expressway. This is the record of motor-vehicle transportation in World War II. Should another such emergency arise, another record, differing in details but essentially of the same character, will be marked up. The decentralization of industry has continued during the postwar years ”

“ Generally, the State highway departments complied readily with requests for the use of their apportioned Federal-aid funds for purposes of study and the planning of access-road improvements. When it came to the construction of such roads, however, the remaining restrictions of the Federal law and similar provisions of State law often presented insurmountable obstacles. Many of the roads were included in neither the Federal-aid nor State highway systems, and many were of such character and location as to be ineligible for construction even with the available Federal-aid secondary road funds. ”

“ Fixed gross-load limits corresponding to classes of vehicles, still provided in the laws of some States, should be abolished as rapidly as possible and group axle-load limitation equivalent to that recommended should be substituted. Existing roads and bridges will be equally adequate for gross vehicle weight greater than the highest value allowed in the recommended table if vehicle length and number of axles are correspondingly increased and the 18,000-pound axle-load limit is faithfully observed. ”