=Destructive Factors.=--From the above it appears that there is a general opinion that there should be some common measure for the destructive effect of vehicles upon road surfaces. As yet no unanimity of opinion has crystallized. While density of traffic influences the surface wear of the road crust--considerably in the case of earth and gravel, less for macadam and asphalt, and still less for brick and concrete--the actual weight of the wheel load seems to have a much greater destructive effect. The impact due to speed and irregularities of the road surface, the resiliency of the tires, the proportion of sprung to unsprung weight, and the shoving forces of the wheels all have their effects which are usually in some way connected with either the weight or the speed, or both, of the vehicle. The many experiments now being carried on by the United States Bureau of Public Roads, and the several states may furnish data from which a practical measure will some day be devised. Mr. Older, Chief Highway Engineer of the State of Illinois, under whose direction the comprehensive investigational and endurance tests under way in that state are being carried on, recently stated to a party of visitors, of which the author was one, that in his opinion weight, including impact, is the prime factor in the destruction of a pavement. Wear is of very minor importance, temperature and weather is of considerable importance. Road surfaces must be considered as bodies acted upon by forces. Some day the stresses produced by these forces will have been analyzed, then will it be possible to standardize the importance of the several vehicle loads. At present it is known that the weight of the load and the weight of the pavement itself are under some circumstances sufficient to produce cracks in the pavement and disruption of the road crust. Bearing tests and bending tests are being devised to measure the effects of such loads. Road crusts, earth, gravel, macadam, asphalt, brick, concrete, are to varying degrees elastic bodies and when loaded they give, as an elastic band stretches, a spring shortens, or a bow bends, until the internal stresses reach a limiting point where the crust is broken or permanently distorted. It is well known that the effect on an elastic body of a suddenly applied load is twice as destructive as the same load gradually applied. And when the action is an impact the destructive effect may be very great indeed, depending on the physical properties of the impinging bodies. But however the load is applied, whenever the internal stresses reach the limiting strength of the material of which the road crust is composed it will go to pieces. The sudden application of the load by fast driving is a sort of impact. The stresses produced by this impact are now being studied. Much good is expected to come toward the solution of the problem of destructive vehicle influence from these researches.