Turner Yarbrough drove his 18-wheeler into Dallas one afternoon in August 2004, hauling a load of medical waste. A 15-year veteran of the road, he was a crack cocaine user with a long criminal record. At an exit on LBJ Freeway, he plowed into a line of cars. Three people were hurt, including a 76-year-old man who later died. The trucker, tests showed, had cocaine in his system. Mr. Yarbrough may not be the typical trucker on Texas highways, but he is hardly unique. Faced with a shortage of experienced