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Show LHsrcj ks-tuJ 13 Feci Decidedly Tco Nir-ro-.v to Provide Proper Clearance. il'ri-.;ii,-d by llu- fnUM HuOi-a D, l.h 1 1 MK at ut AKrlcullurL-.t Do you know that you drive your car or truck by Instinct? You do, for engineers of the bureau of public roads have been watching yol). They have made observations to see how-far how-far from the edge of the pavement you drle and what you do at curves and on down grades. Having marked off pavements .Into one-foot sections, the engineers, by watching the right rear wheels of passing M-lili les. lind that on straight and level p'lols of various widths trom H to HI f'-et. with shoulders In l.ili condition, pavsenger car drivers habit ually maintain a distance of from 1 '-j to A feet between the outer wheel and the edge of the pavement Truck drivers operate somewhat nearer the edge, bat prefer not to approach clos er than P-i foot. Drivers will sacrl the clearance between their own and p:.ssing vehicles rather than drive clo-er to the ede than they instinctively instinct-ively f-el Is safe. Pavements Too Narrow. The bureau's observations indicate that paM-ments less H, an IS feet wide are decidedly loo narrow slice tlcy provide no clearanee for passenger cars or truek-j operating in the uual paths. While the lf.iot width Is up parently great enough for passenger cars In two-lane trulhc. It is not quite wide e r-'h for trucks. The 2'i-foot width gives ample clearance for trucks and Is not excessive for automobiles. In moving down nill on light grades, tialiic moves slightly toward the cento cen-to r cf the road. Light down grades do not sucgest reduction of speed hence tratlic lakes the precaution of moving slightly away from the edge of the pavement. No such tendency was observed on heavy grades where the speeil is reduced, and the fear of the pavement's edge is lessened. In rounding horizontal curves, traffic. traf-fic. In general, shifts toward the inside in-side edge, but tiie trucks shift courses toward the Inside of lire curve less than passenger vehicles. Under all circumstances, truck drivers are found to adhere more closely to the edges of the pavement than operators of passenger vehicles. Traffic Shifts Its Course. Trallic moving on ' the outside of the curve shifts its course farther in the directiou ol the inside than trallic moving in the opposite direction, which is limited In its choice of a course by the proximity of the edge of the pavement. I'nless, therefore, the pavement Is widened on the curves, the normal straight road clearance clear-ance between tne two lines of vehicles is reduced The used width of a pavement may frequently be considerably less than its apparent width, the observations disclosed. On straight roads, as well as on curves, the outer foot of the surfaced section is sometimes totally ineffective because of a bad shoulder A closely set guardrail, a steep crown, a- bad gutter, or an uneven, bumpy condition of the surface near the edge will cause the driver Instinctively to seek the center of the road. In one rase, a 2-t-foot pavement was found to have an effective width of not more than 20 feet because of the abutments of an overhead railroad bridge which were crowded close to the edge of the pavement. Smooth, white concrete shoulders at the edge of a block surface seem to lure the traffic toward the side. Center Cen-ter lines on straight roads, as well as on curves, exert a marked separatory influence. |