a.

Destruction of cement mortars and concrete through expansion and contraction. (A. H. White, Engineering Record, 64, 45, July 8, 1911.)

b. The expansion and contraction of concrete while hardening. (A. T. Goldbeck, Engineering Record 64, 73, July 15, 1911.)

C.

Shrinkage of concrete and conditions of curing. (F. R. McMillan, Engineering Record, April 17, 1915.)

d. The expansion and contraction of concrete roads. (Goldbeck & Jackson, page 27, Bulletin No. 532 U. S. Dept. of Agriculture.)

e.

f.

Test of the Sears-Roebuck Building, by D. E. Hooker. (Pacific Northwest Society of Engineers, Proceeding XV, 88-90, January, 1916.)

Shrinkage and Time Effects in Reinforced Concrete, by F. R. McMillan. (Bulletin No. 3, University of Minnesota, March, 1915.)

g. Concrete Roads, W. A. McIntyre. (The Cornell Civil Engineer, 24, 331, April, 1916.) h. In Engineering News-Record 83, 518-520, September 11, 1919, under the heading, "Does Rich Concrete in Roads Crack More Than Lean?" are answers to a questionnaire by: E. N. Hines, A. N. Johnson, Clifford Older, P. St. James Wilson, Duff Abrams, A. H. Hinkle, J. C. Pearson and W. D. Uhler.

THE REPORT OF THE COMMITTEE ON MODIFICATION OF LAWS RELATIVE TO WEIGHT AND SPEED OF VEHICLES

DAVID R. FARIES, General Counsel of the Automobile Club of Southern
California, Chairman.

WATT MORELAND, General Manager, Moreland Motor Truck Company.
C. H. RICHARDS, Engineer.

The results obtained from the engineering investigations conducted by the Automobile Club of Southern California demonstrate that California's present highways are not capable of withstanding the effects of the present traffic over them and that they will not be able to resist the effects of future traffic. The committee on the modification of laws relative to weight and speed of vehicles believes that all future highways constructed in California should be constructed in such a manner as to warrant the hope that they will survive the effects of not only present day traffic but of the greatly increased volume of traffic which will undoubtedly make use of our highways in the years to come.

While we believe that the alarming disintegration of our present highways is due in the main to the manner of their construction, it is nevertheless true that the overloading and overspeeding of motor vehicles is seriously injuring the 1,400 miles of state pavements already built which must be used as they are.

The abusers of the highway are chiefly responsible for the criticisms being leveled at all highway users. California has a vehicle act intended to regulate traffic on its highways both as to speed and as to weight. The speed provisions have, as to private passenger cars, been fairly well enforced, by the officers of the law; these same officers, however, have made little, if any, effort to enforce the provisions defining and prohibiting the overloading of vehicles. This lack of enforcement has caused many truck operators to remain in the grossest ignorance of the law while others have flagrantly violated its provisions. The census of traffic on our state highways as given previously in this report contains data amply supporting these assertions.

Public-spirited, law-abiding truck operators will, however, be found in great numbers ready to aid in the careful enforcement of the laws regulating the weight and speed of their vehicles, as they realize that these laws have been carefully drawn and that the legal operation of their trucks is the efficient, economical and profitable method of operation.

Overloading is not only a violation of the laws of the state but is also, in most cases, contrary to the wishes and recommendations of truck manufacturers. Manufacturers and dealers, realizing that overloading shortens the life of the vehicle they manufacture or sell and thereby brings it into undeserved ill repute, and realizing that public sentiment will not long endure an abuse of the highways constructed at public expense, are urging rational loading and the restriction of speed on the part of truck operators.

This committee believes that restrictions imposed by law as to the size and width of vehicles, the maximum loads to be carried therein, the relationship between the maximum load of vehicle and contents and width of tire and the relationship between weight and speed are engineering problems. We believe that the legislature should simply put into the Vehicle Act the engineering conclusions to be deduced from the investigations conducted by the Automobile Club of Southern California and the California State Automobile Association. We also believe that once this is done the law should be rigidly enforced with a view to preserving our highways as long as possible and in as good condition as possible. We believe that the California Vehicle Act as it now stands is an excellent piece of legislation, but should be modified in certain particulars. The modifications, we suggest, are as follows:

Maximum Weight

The present Vehicle Act provides for a maximum weight of vehicle and load of 30,000 lbs. It is the opinion of the engineers for the Automobile Club of Southern California, in which this committee concurs, that this maximum should be lowered to 28,000 lbs.

From the schedule of state laws attached to this report, it will be seen that of the states having vehicle load restrictions, none permit loads exceeding California's gross load of 30,000 lbs. with the possible exception of Ohio, which has a limit of 800 lbs. per inch of tread, and Indiana, with a capacity of load of 20,000 lbs. exclusive of truck weight.

Weight on One Axle or Wheel

The weighing in connection with this investigation by engineers of the Automobile Club of Southern California of a number of trucks as disclosed in the truck census contained in this report, shows that about 75 per cent of the load of a truck is carried on its rear wheels. This practical demonstration corresponds with the theory of manufacturers of trucks on the subject.

It is the recommendation of the Club's engineers that the maximum weight to be allowed by law to rest upon any one wheel of a vehicle be not more than 10,000 lbs. This recommendation is joined in by the members of this committee with the exception of Mr. Moreland, who advocates that a gross weight of 11,200 lbs. be allowed on any one wheel or 22,400 lbs. on any one axle. This recommendation of Mr. Moreland conforms to the recommendations of the proposed Uniform Vehicle Act adopted by the joint committee on uniform vehicle laws which issued a pamphlet dated January 20, 1920, containing a proposed uniform vehicle law. This joint committee was composed of representatives of the American Association of State Highway Officials, the National Automobile Chamber of Commerce, the American Automobile Association and the Federal Highway Council.

The majority of the committee believe that Mr. Moreland's recommendation might prove very desirable in states where the highways constructed are sufficiently strong to resist such weights. It is the opinion, however, of the engineers of the Automobile Club of Southern California that California's highways cannot resist the wear of loads in excess of 10,000 lbs. on any one wheel. The majority of the committee admit that such a restriction may prevent the use of 7-ton trucks when loaded to full capacity, but believe that such a limitation is made necessary by the highway conditions in this State. The majority of the committee also call particular attention to the weight restrictions contained in the table annexed to this committee's report and direct attention to the fact that Illinois and Iowa limit the gross weight on one wheel to 8,000 lbs. notwithstanding the fact that these states have eight-inch concrete pavements on their main roads.

Weight on Tires

The present California Vehicle Act provides that the load of a vehicle shall not exceed 800 lbs. per inch per width of tire when solid rubber tires are used. The engineers of the Automobile Club of Southern California and this committee believe that this restriction should be kept in the law but re-written so as to be made to definitely express the intentions of the framers of the law. The law should be made to definitely state that a limitation of load includes the weight of both vehicle and load. The width of the tire should be measured at the base of the tire rather than at the point of contact with the highway. The reason for this statement is that solid rubber tires as usually manufactured and sold are, when new, narrower at the point of contact with the highway than at the base. A new tire is much more desirable because of its thickness and resiliency than a tire which has been worn down by long use, but a law which requires measurement of the tire at the point of its contact with the highway penalizes the use of new tires and discriminates in favor of the tire which is practically worn out. The effect on the road of old worn tires is shown on page 120.

A glance at this table will show that the speed of 14.6 miles per hour, with a drop of two inches, in the case of a three-ton truck with a 42-ton load produces an impact with an old worn solid tire of 26,500 lbs. as compared with 18,700 lbs. with a new solid tire and 8,300 lbs. with a pneumatic tire.

The effect of this impact on the truck is, of course, in proportion to that on the highway, so whether we are trying to protect the truck or the highway or both, we should favor a statute requiring the measurement of the tire at its base at the rim rather than at the point of contact with the road.

Permit for Excessive Size and Weight

This committee recommends that the present provisions of the Vehicle Act authorizing the granting of permits by the Highway Commission for the transportation over the highways of vehicles of excessive size or weight be further restricted as to prevent the granting of such permits, unless the vehicle whose use is thus permitted is equipped with at least one inch of tire width for every 800 lbs. of the weight of such vehicle and its load.

Thickness of Rubber on Solid Tires

This committee also favors the inclusion in the law of a provision prohibiting the use of any solid tire which is, on account of excessive use or for any other reason, less than one inch thick where its width is five inches or more and prohibiting the use of new tires less than three-quarters of an inch thick where the tire is less than five inches wide.

Cushion Wheels

The type of wheel known as the cushion wheel is now in use, which has not been taken into consideration in the preparation of vehicle laws. Users of these wheels claim that the vibration of a truck equipped with them is much less than that of one equipped with solid tires. The proponents of this type of wheel ask that it be considered in the law and this committee represents that while no favor be shown this type of wheel over the solid tire as to size or weight restrictions, that the speed limits imposed upon solid tired vehicles be modified in favor of cushion wheel vehicles so as to permit them to travel at a rate of speed 25 per cent in excess of the speed limits imposed upon solid tired vehicles of the same weight and load capacity, provided that such increases in speed shall in no case permit the cushion wheel vehicle to exceed the speed limit fixed for pneumatic tired vehicles.

Speed of Freight Vehicles

This committee is advised by the engineers for the Club that tests show that vehicles equipped with pneumatic tires do a great deal less damage to the roads than vehicles equipped with solid tires. We are also advised that the damage done by a vehicle to the highway is caused not by the weight of the vehicle alone, but by a combination of the weight and of the speed. We are told that the force of impact of a solid tired vehicle increases approximately with the square of its speed-that is, that a solid tired vehicle traveling at twenty miles per hour produces four times as great an impact on the highway as the same vehicle traveling at ten miles per hour, and nine times as much impact when traveling at thirty miles per hour as when traveling at ten miles per hour. Granting this, it will readily be agreed that there must be definite speed limits on heavy vehicles considerably slower than those allowed the ordinary passenger vehicle. In considering the recommendations it should make on this subject, this committee has carefully considered the speed laws of every State in the United States and various proposed uniform vehicle acts including the one already referred to herein which was prepared by the joint committee on uniform vehicle laws, the International Traffic Officers' Association and other organizations. We have also had the