John left his home state of missouri early one afternoon to drive to new mexico. eventually the sun set, but john continued on. hours later, while driving 70 mph on an interstate highway east of amarillo, he was pulled over by the texas highway patrol. the last speed limit sign john had seen had been in oklahoma and had read 70 mph. as she issued the ticket, the officer noted that the speed limit in texas on interstates is 70 during the day but drops to 65 at night. john protested he was unaware of the change. was he still guilty of speeding?