NEW BOSTON, Texas—Testimony began Wednesday morning in the trial of a man allegedly high on meth when he crossed from his lane of traffic into the path of a mule cart hauling four young passengers.

“We’re saying to you that his meth use caused this accident,” said Michael Shepherd, Assistant District Attorney for Bowie County, in his opening remarks to jurors. He said the state would prove that Samuel Lyn McMillon’s drug abuse left him unable to control the white Dodge pickup he drove the night of April 10, 2005.

McMillon’s defense attorney Paul Hoover told the jury thi...