# Highway Needs of the National Defense/Appendix 2

Layout 2
Washington, DC: United States Government Printing Office, pages 98–112
Appendix II.—TRAFFIC DETERMINANTS OF INTERSTATE HIGHWAY STANDARDS
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.
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. These roadway features relate to the alinement and profile, the plan of intersections, the clearances, and the horizontal dimensions of the highway cross section.
Many of the recent studies have been devoted to the dynamics of highway movement; and the results of these studies, combined with the evidence of traffic growth and distribution and other information resulting from the State highway planning surveys, form the basis of the geometric highway standards, previously described, which have been employed in rating the deficiencies of the interstate highway system, and estimating the cost of its adequate improvement.
WIDTHS OF SURFACES AND SHOULDERS
No features of a highway have a greater influence upon the safety and comfort of driving than the width and condition of the surface. On two-lane roads the width of pavement is most critical when vehicles meet, as it is then that drivers must allow sufficient clearance between vehicle bodies to assure complete safety, while at the same time maintaining a comfortable margin of safety from the pavement edge. The clearances necessary for safety at the speeds that vehicles are operated, added to the combined widths of the two vehicle bodies, constitute the needed pavement width.
Lane widths
Most of the freight-carrying vehicles now using public highways are 8 feet wide. The results of investigation of the transverse positions of vehicles in motion show that lanes 12 feet wide are the minimum that will provide ample clearances between commercial vehicles as they meet one another. This lane width will also be adequate in case the width of commercial vehicles is increased to 8.5 feet. It is only where the traffic streams are composed almost entirely of passenger cars that lane widths of 11 feet are satisfactory.
Roadside obstructions
These investigations also reveal that any objects such as retaining walls, bridge trusses, or headwalls adjacent to a roadway constitute a safety hazard and are an obstruction to the free movement of traffic, unless they are 6 feet removed from the normal pavement edge. If they are 3 or 4 feet away, however, their effect will not be critical enough to justify greater clearances from the normal pavement edge on long bridges and underpasses.
Shoulders
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.
A shoulder width of 6 feet is required to insure that lateral obstructive objects will not decrease the effective pavement width; and an ​additional 4 feet, or a total shoulder width of 10 feet, is required to permit and encourage drivers of disabled vehicles to stop clear of the traffic lanes so as not to constitute a traffic hazard.
Effect of vehicle length and off-tracking on curves
When a vehicle makes a turn, the rear wheels follow a path having a shorter radius of curvature than the path followed by the front wheels. The distance between the two paths is known as the amount of off-tracking. 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. In some cases, it establishes the minimum curvature which it is practical to employ for the design of intersections at grade and at interchanges. The off-tracking of the larger vehicles that are likely to use the interstate highway system is not sufficient to require lane widths in excess of 12 feet on any curve up to 9 degrees, which is the absolute maximum permitted in the design standards for the system. It is significant that on the sharper curvatures which may be employed in the vicinity of intersections and interchanges, the 35 feet for single-unit trucks, the 45 feet for tractor-semitrailer combinations, and the 60 feet for truck-trailer combinations proposed as limits of lengths by the American Association of State Highway Officials, off-track approximately the same amounts on correspondingly sharp curves. The off-tracking of these vehicles varies from about 0.8 foot on a curve with 500-foot radius to about 7 feet on a 50-foot radius.
SPEED AND NUMBER OF TRUCKS DETERMINE GRADE LENGTHS AND HIGHWAY CAPACITY
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.
Effect of trucks on capacity
Added lanes
One means available to the highway designer for elimination or lessening of the congestion that is created by heavy vehicles crawling up hills is the construction of added lanes or truck lanes on the uphill side of the grades. This method has been employed by several States with excellent results.
When the traffic volume on a two-lane road does not exceed 300 vehicles per hour the congestion on grades resulting from slow-moving vehicles in numbers normally found on main highways is not sufficient to justify the construction of a third lane, regardless of the alinement and profile, for grades up to 7 percent.
Where the alinement and profile are such that sight distance does not restrict the performance of passing maneuvers, added lanes are not justified for hourly volumes below 500 vehicles. But this is seldom the case, because sight distance is commonly restricted at the crests of hills. With the sight-distance restrictions that are normally present on sections that include grades, an added truck lane is justified under the conditions shown in table 12.
VARIATIONS OF TRAFFIC VOLUME AND CRITICAL HOURLY VOLUME DETERMINE REQUIRED HIGHWAY CAPACITY
Certain general facts pertaining to the volume and pattern of traffic movement are widely known. The daily pattern is one with which everyone is familiar; and the morning and evening rush hours, with intervening lulls, are accepted facts of the traffic day. Also, the fact that traffic on certain days of the year is much heavier than that on other days is a matter of common knowledge. The manner in which these variations in flow are related to the traffic-carrying ability of a roadway facility is not so readily apparent.
| Gradient | Traffic volume exceeding— | Traffic volume exceeding— | Length of grade |
| 3 percent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 550 vehicles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | Over 1,100 feet. |
| 4 percent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 500 vehicles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | Over 800 feet. |
| 5 percent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 400 vehicles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | Over 600 feet. |
| 6 percent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | $\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$ | 400 vehicles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | Over 500 feet. |
| 6 percent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | $\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$ | 350 vehicles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | Over 4,000 feet. |
| 7 percent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | $\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$ | 350 vehicles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | Over 500 feet. |
| 7 percent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | $\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$ | 300 vehicles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | Over 4,000 feet. |
Investigations of the fluctuations in traffic movement show that a highway planned to accommodate the traffic demand during the peak hour of the average day of a year will be congested, on an average, during 160 days of the year. Also, if the road is planned to take care of the traffic during the average hour of any day the demand during the peak hour of that day will be more than double the capacity of the highway. It is apparent then, that the roadway should be designed to accommodate the traffic that moves during most of the heavier hours of the year, but that it is uneconomical to plan for the infrequent extremes that occur from time to time throughout the year.
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.
For the interstate highway system the thirtieth highest hour of the year has been selected as the criterion for decision upon the adequacy of existing facilities and for determinations of geometric features of the highways, such as the number of traffic lanes, maximum gradients, and curvatures, that are required to accommodate traffic using this system.
STOPPING DISTANCES DETERMINE MINIMUM SIGHT DISTANCE REQUIRED
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. The distance that the car travels during these periods is termed the stopping distance. The distance within which all but the few vehicles with brakes in poor condition can stop when traveling at the maximum reasonable speed which the highway should be capable of accommodating is the needed sight distance.
The minimum sight distances in table 13 are sufficient for emergency stops where vehicle speeds do not exceed the design speeds as shown in the table. For safe operation sight distance at least as great as these minima must be provided at all points on a highway, and at intersections the driver who is about to enter the highway must. have a clear view in either direction, right and left, equal to the safe stopping distance for the design speed of the highway.
Provision of these adequate stopping sight distances places certain limitations on many of the geometric features of the highway, such as maximum curvature, rates of change in gradient at crests, width of cuts, clearances to lateral obstructions along the traveled way, and the combination of many of these features with one another.
| Design speed | Stopping sight distances for 2-, 3-, and 4-lane highways | Passing sight distances for— | Passing sight distances for— | Passing sight distances for— | Passing sight distances for— |
| Design speed | Stopping sight distances for 2-, 3-, and 4-lane highways | 2-lane highways | 2-lane highways | 3-lane highways | 3-lane highways |
| Design speed | Stopping sight distances for 2-, 3-, and 4-lane highways | Desirable | Absolute | Desirable | Absolute |
| | Feet | Feet | Feet | Feet | Feet |
| 30 miles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 200 | 600 | 500 | . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
| 40 miles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 275 | 1,100 | 900 | . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
| 50 miles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 350 | 1,600 | 1,400 | 1,100 | 900 |
| 60 miles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 475 | 2,300 | 2,100 | 1,500 | 1,300 |
| 70 miles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 600 | 3,200 | 2,900 | 2,000 | 1,800 |
RELATION OF TRAFFIC VOLUME, OPERATING SPEED, AND PASSING SIGHT DISTANCE TO HIGHWAY CAPACITY
The maximum possible capacity of a highway is attained when the lanes are completely filled with vehicles following each other at a minimum distance. Under this condition all vehicles will be moving at the same speed, which is established by the speed of the slowest driver on the road.
Effect of traffic volume
The high degree of restraint placed upon every motorist when the possible capacity has been reached, or even approached, results in ​driving conditions that are wholly intolerable. Drivers are entitled to exercise some freedom in their selection of speed and their ability to maneuver. The extent to which this liberty may be exercised is dependent primarily upon the volume of traffic using the highway and also, for two-lane roads, upon the portion of the highway where sight distances are adequate for overtaking and passing. Other elements such as narrow lanes and narrow shoulders also provoke congestion, but on highways built to the adopted standards for the interstate highway system these inferior features are not present.
Practical working capacity
The maximum volume of traffic that will permit a reasonable degree of freedom from congestion may be termed the practical working capacity of the facility. Its magnitude depends in large measure upon local conditions. For rural areas operating conditions are satisfactory so long as drivers who so desire may travel at average speeds of 45 to 50 miles per hour without undue hazard. On roads having a lane width of 12 feet and excellent alinement, such speeds can be attained with the traffic volumes shown below, for highways of different numbers of lanes:
    Two-lane highways: a total of 800 vehicles per hour.
    Three-lane highways: a total of 1,400 vehicles per hour.
    Four- or more-lane highways: 900 vehicles per hour per lane for the lanes in the heavier direction of travel.
These volumes include a normal percentage of trucks. The average speed of all traffic under these volume conditions will be about 40 miles per hour. Where a lower average speed is satisfactory, as is the case on multilane expressway facilities in urban areas where flow is uninterrupted, a volume of 1,350 mixed vehicles per lane per hour is practicable. The average speed at this volume will be about 30 miles per hour but drivers having a desire to do so would be able to average 35 to 40 miles per hour in safety.
Passing sight distance
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.
Where sight distances within the range of 1,500 to 2,000 feet are not continuously available throughout the length of a two-lane highway, practical capacities are reduced as shown in table 14, where operating speeds of 45 to 50 miles per hour are desired.
These observed facts of the relation of the degree of continuity of passing sight distance to highway capacity are the basis of the sight-distance standard adopted for the interstate highway system. The standard requires that 1,500 feet of sight distance shall be available on percentages of the length of highway sections rising from 16⅔ to 100 percent as the volume of traffic to be served increases from 500 to 800 vehicles in the thirtieth highest hour of the year.
| Percentage of total length of highway on which sight distance is restricted to less than 1,500 feet | Practical capacity for operating speed of 45–50 miles per hour |
| 0 percent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 800 vehicles per hour. |
| 20 percent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 770 vehicles per hour. |
| 40 percent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 715 vehicles per hour. |
| 60 percent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 645 vehicles per hour. |
| 80 percent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 555 vehicles per hour. |
| 100 percent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 450 vehicles per hour. |
VERTICAL CLEARANCES OF BRIDGES
The design standard providing that all bridges on the interstate system shall have a clear height above the surface of not less than 14 feet, and of not less than 12½ feet above the outer edges of the shoulders, is based on an allowable height of 12½ feet for vehicles plus a reasonable clearance between the vehicle and the bridge.
Data obtained by Nation-wide planning surveys indicate that the occurrence of loaded trucks and combination units of 12½ feet and over and of 14 feet and over is as follows:
| | Percentage | Percentage |
| | Over 12½ feet | Over 14 feet |
| Single units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 0.3 | 0.03 |
| Truck-tractors and semitrailers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 2.4 | .2 |
| Trucks and full trailers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 6.7 | .5 |
| All trucks and combinations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 1.1 | .1 |
The above tabulation shows there is little demand for bridges to accommodate vehicles higher than 12½ feet and practically no demand for a height that will accommodate vehicles over 14 feet high. The demand for vehicles over 12½ feet actually involves only a few commodities. The principal demand for vehicles over 12; feet stems from auto carriers which, when loaded in the most efficient manner as now practiced, have a total height of 13½ feet.
The tolerance of 1½ feet between the allowable height of 12½ feet for vehicles and a 14-foot bridge height is necessary to provide for:
* The resurfacing of the pavement, which over a period of years may result in several inches of increased thickness. ​
* Accumulation of ice and packed snow.
* Emergency movement of special construction and industrial equipment.
* The springing action of vehicles.
AXLE LOADS DETERMINE REQUIRED ROAD-SURFACE STRENGTH
So far as vehicular effects are concerned, the load carried on a single axle is the principal determinant of the required load-supporting capacity of roads. Road load-supporting capacity is a composite result of the composition and depth of the subgrade or foundation, base, and surface or pavement provided. The supporting capacity of any combination of foundation, base, and surface is affected by variations of water or moisture content in any of the three elements of the structure. Supporting capacity of some of the materials employed, particularly portland cement concrete, is also affected by variations of temperature. The numerous combinations of variables involve complexities of relation and result which are imperfectly understood.
Static loads
It is known that the effect of two loads statically applied upon a road surface is not greater than the effect of either of the loads singly if the two loads are separated by not less than about 40 inches. Hence, for the estimation of vehicular effects the axle-load determinant is considered to be the total load transmitted to the road by all wheels the centers of which are included between two parallel transverse vertical planes 40 inches apart, extending across the full width of the vehicle.
Whether, and in what degree, the road effect of closely spaced loads dynamically applied differs from the known effect of static application, is not established.
Effect of weather
Roads which in certain conditions (especially conditions of moisture content) will support a particular axle load, under more adverse conditions will fail when subjected to considerably lighter axle loads; hence the common necessity to reduce axle loads in the spring when, as a result of the melting of frost, elements of the road structure are heavy with moisture.
Warping of concrete
Concrete pavements warp or bend downward at their edges when, as commonly on a warm day, the temperature of the upper surface is higher than that of the bottom of the slab. Conversely, when the temperature of the upper surface is lower than that of the bottom of the slab, as often it is at night, the edges of the slab bend or warp upward to such an extent that they are no longer in firm contact with the foundation. Hence, a given load applied at the edge of a concrete road slab may differently affect the slab according as the slab at the time is or is not in intimate contact with its foundation.
Action of repetitive loads
A road which will support a particular axle load in single or very infrequent application may fail or rapidly deteriorate under frequent ​application of the same or even lesser loads. Some of the road effects of vehicular load are cumulative.
Applied loads may cause slight movements of the stone particles composing a so-called nonrigid road surface. If, by reason of the manner of load application, these movements tend to occur in the same directions under successive load applications, the condition of the road surface may rapidly deteriorate. In this manner corrugations are formed in gravel road surfaces.
Photo by Ansgar Johnson
Tests of beams made of portland cement concrete have shown that such a beam can be broken by frequent and rapid application of a load barely more than half as great as the load required to cause rupture in single application. So, it is known that concrete is a material which, like many others, suffers fatigue under repeated stress. Concrete pavements undoubtedly suffer such fatigue under the repeated application of vehicular loads. Without question, the load that a pavement will withstand in frequently repeated application is not as great as the heaviest load it would support once or infrequently applied. Precisely the degree of such fatigue of the road slab, i. e., what load repeated in what frequency on a slab of given dimension will rupture the slab, has not been determined, and the desirable eventual determination will be extremely difficult. Meanwhile, the great variety of conditions under which concrete roads exist and are built precludes even an approximate generalization of the effects of fatigue.
Cracking of concrete
It is a matter of common observation that cracks form progressively in concrete pavements throughout their life. Many of these cracks result from causes other than vehicular loads. Some, undoubtedly, are caused by loads, but a load-induced crack is rarely noticed and may be unnoticeable at the moment of its formation. Hence it is practically impossible to impute observed cracking to particular load causes. The most that can be said with assurance is that concrete pavements of presently designed dimensions are cracked by vehicular loads, that they are probably cracked by loads less than those which theoretically they should support in single application, and that ordinary prudence requires safe allowance for the possible effects of repeated application in the limitation of axle loading.
Joint pumping
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. If, then, there is free water in the vicinity of the bottom of the joint, the downward pressure of the bending slabs tends to force it upward and out of the joint onto the surface of the pavement. If the subgrade soil is of such character that it enters into suspension in the water that collects, some of the soil will be pumped out with the water. By such pumping action, frequently repeated, portions of the subgrade may be gradually removed, leaving the pavement slab without subgrade support in the vicinity of the joint. Lacking such support, the pavement may crack near the joint under a passing load heavy enough to overtax its unsupported strength.
Joint pumping, as will be seen from this description, is not caused by vehicle load alone. Detrimental pumping occurs only in the ​presence of water and subgrade or foundation material that the pumped water can carry with it. On the other hand the pumping action does not occur in the absence of some loading adjacent to the joint. From widely distributed observations the conclusion has been reached that detrimental pumping is generally associated with a substantial frequency of axle loading in excess of 14,000 pounds. Moreover, loads of that magnitude, frequently repeated, are likely to overtax the strength of pavement slabs of the usual thickness when long continued pumping has deprived them of subgrade support.
The fact that joint pumping has been observed to occur under certain conditions associated with axle loading in excess of 14,000 pounds is not a sufficient reason to suggest the limitation of all axle loads to 14,000 pounds as a maximum. It is, however, a definitely observed fact associating a particular type of road damage with a particular magnitude of axle loading. In association with the conclusions drawn from the long experience of many highway authorities, it suggests that the limitation of axle loading should be fixed at no greatly superior weight. Among the other lessons of experience are those gained from many observations of the difference in damage occurring on heavily and lightly loaded lanes of the same pavements, from the quick occurrence of more extensive damage to pavements suddenly subjected to heavier axle loading, and from the differences in maintenance expense for similar pavements lightly and heavily loaded.
Adherence to 18,000-pound limit necessary
These lessons of experience have brought highway officials almost unanimously to the conviction that the axle-load limit of 18,000 pounds presently fixed by law in 34 States should not be exceeded, but rather should be more rigidly enforced as a prudent measure of existing road preservation.
If the 18,000-pound limit should be enforced for the protection of existing roads, the need for such limitation will certainly continue for many years during the normal life of the existing roads.
The question then arises: Should new roads henceforth built be designed to support heavicr axle loads? If so, they will have to be made stronger than the great majority of existing roads. If new roads are built of such greater strength, what assurance is there that future vehicle-operating practice will not demand still greater strength? It is an incontestable fact that the highway system cannot be efficiently designed and administered to serve an uncertain and increasing axle loading.
Additional evidence supporting 18,000-pound limit
Impressed strongly by this fact, and holding the conviction that 18,000 pounds is the heaviest axle load the generality of existing roads will safely support, highway officials have also the evidence of their frequent vehicle weighing to convince them that axle loading in excess of 18,000 pounds is not an essential concomitant of greater pay loads for vehicle operators. Additional gross weight and pay load can be readily accommodated if the number of axles is increased. The evidence to this effect is extensive. Its gist is sufficiently conveyed by the following comparison of the relation between the average axle loading and gross weight of all loaded heavy trucks and combinations ​weighed during 1947 in two States in which different transportation practices have developed in the presence of materially different laws:
| | | New Jersey | California |
| Average gross weight of all loaded vehicles weighed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | pounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 45,513 | 54,075 |
| Average number of axles, all loaded vehicles weighed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | Average number of axles, all loaded vehicles weighed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 2.94 | 4.77 |
| Average axle load of all loaded vehicles weighed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | pounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 15,482 | 11,347 |
It is apparent that in California, where the law prescribes an axle load limit of 18,000 pounds, a maximum gross vehicle weight of 76,800 pounds, and a maximum length of 60 feet, the average gross vehicle weight of vehicles in use is greater, while the average axle load is less than the corresponding weights of vehicles in use in New Jersey where the law prescribes no limit of axle load, a maximum gross vehicle weight of 60,000 pounds, and a maximum length of 50 feet.
Since an axle-load limit of 18,000 pounds is now, and for years to come will be, essential for the protection of existing roads; since a large mass of evidence exemplified by the above comparison indicates clearly that such an axle-load limitation need not unduly limit either the gross weight or pay loads of vehicles; and since the necessities of efficient design and administration of the highway system require fixation of axle loading, the conclusion is strongly supported that 18,000 pounds should be adopted as the maximum axle load permis- sible under the laws of all States, that this limit without future in- crease should be rigidly enforced, and that highways built in the future should be designed for the normal support of axle loads of that magni- tude in the frequency of their probable occurrence.
AXLE-LOAD COMBINATIONS AND SPACINGS DETERMINE REQUIRED BRIDGE DESIGN
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. Stresses generated by single axle loads are critical mainly in the floor system of the bridge; stresses generated by groups of axle loads are critical in the larger supporting fabric, such as the trusses or main girders. In the latter parts, vehicles of widely different total weight may generate identical stresses if the lengths and axle spacings of the vehicles differ appropriately in relation with their weights.
Distribution of weight important
So, for the design of a bridge it is not sufficient to know the maxi- mum gross weight of the vehicles that are expected to use it. Some assumption must also be made of the manner in which the gross weight will be distributed and applied through axles of definite spacing. Similarly, whereas an existing bridge may be adequately protected ​against the danger of overstressing by the posting of a single weight limit, if the limit posted corresponds to the load that may be safely carried on the shortest vehicle, such posting will prohibit the use of the bridge by longer vehicles of substantially heavier weight that could safely use it.
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.
Conventional design loadings
In the design of highway bridges certain conventional vehicular loadings are assumed. The magnitude of the assumed loading is described by a designation involving the letters H or H and S and numerals expressive of weight. The combination expresses loading equivalent to that of a vehicle or combination of vehicles of definite weight and length. Thus, H15 loading is equivalent to the weight of a single vehicle of 15 tons total weight carried on two axles, 14 feet apart, one loaded with 12 and the other with 3 tons. H20-S16 loading is equivalent to the loading of a tractor-semitrailer combination of 36 tons total weight carried on three axles, separated by distances of 14 feet and loaded with 4, 16, and 16 tons, respectively. An H20-S16 bridge is a bridge designed for H20-S16 loading. An H20 bridge is a bridge designed for H20 loading.
But the fact that an H15 bridge is designed to support a vehicle of 15 tons or 30,000 pounds total weight does not mean that it will, with equal safety, carry every vehicle of 30,000 pounds total weight nor that a vehicle of 30,000 pounds total weight is the heaviest vehicle it will safely support. Such a bridge would be overstressed by a 30,000-pound vehicle having a wheel base of less than 14 feet; it would carry without any overstress a combination vehicle weighing 60,000 pounds having an over-all wheel base of 42 feet and axles appropriately loaded.
Fallacy of gross-weight control
Herein lies the fallacy of laws aiming to control the gross weight of vehicles by specified fixed limits of weight corresponding to certain classes of vehicles, such as 30,000 pounds for two-axle vehicles, 40,000 pounds for vehicles of three or more axles. Such limits are unnecessary for the protection of roads which, as previously explained, are affected by axle loading rather than gross load. They permit the operation of vehicles that will generate in some bridges stresses greater than those contemplated in their design, and more frequently, if they are enforced, they will prohibit the operation of many heavier but safer vehicles.
For example, a State in which there are many bridges of H15 design standard, if its laws limit gross weight at 30,000 and 40,000 pounds for vehicles of two and three or more axles, respectively, will permit the overstressing of its H15 bridges by vehicles of 30,000 pounds gross weight and wheel base less than 14 feet; and it will, supposedly at least, prohibit the operation of many vehicle combinations of the greater ​lengths which would be less onerous in their demands upon the bridges despite the fact that their gross weights might materially exceed 40,000 pounds.
Interstate system design loading
These are the reasons that underlie the recommendation of the American Association of State Highway Officials which proposes to limit the gross weight of vehicles by the limitation of constituent group axle loads conforming to a table of weights corresponding to a range of axle group spacings. The values in the table have been calculated in such amounts as to permit the operation of vehicles of various lengths and corresponding weights, all safe in respect to the capacity of H15 bridges.
Bridges of H15 design now so greatly predominate among existing bridges, on even the principal highways, that no higher limits of vehicle gross weight can be reasonably permitted. The design standard proposed for bridges on the National System of Interstate Highways is H20-S16. Bridges of this standard are designed to support loading equivalent to that of a tractor-semitrailer combination of 28-foot over-all wheel base, weighing 72,000 pounds. Such a bridge would be equally safe for a combination vehicle weighing 100,000 pounds if its over-all wheel base length were 54 feet and the constituent axles were appropriately loaded. The statement that it would be equally safe means that it would support such a vehicle without stress in excess of the design assumption, which involves an ample factor of safety to allow for uncertainties of material and construction and for the effects of fatigue induced by repetition of stress. In war or other emergency such H20-S16 bridges will support without danger the infrequent application of vehicles of appropriate length and weight distribution grossing from 160,000 to 200,000 pounds, depending upon the vehicle wheel base, the length of bridge span, and the number of load repetitions.
PROTECTION OF ROADS AND BRIDGES AGAINST OVERLOADING
For reasons stated in the preceding pages it is proposed to build road foundations and surfaces on the interstate highway system for the support of 18,000-pound axle loads, and to build bridges of H20-S16 design. Costs of improving the system have been estimated for roads and bridges conforming to these standards.
If heavier loads are to be permitted, the contemplated design of roads and bridges should be strengthened and the estimated costs should be correspondingly increased. The roads and bridges actually built should be designed for the safe support of loads to be permitted. When so built, they must be protected against overloading by appropriate laws adequately enforced. Unnecessary and costly damage will result from failure to heed this injunction.