A 20-year Hilmar man sustained serious head injuries after his vehicle collided with a big rig on Highway 99 Thursday morning. The collision occurred about 10:15 a.m. Thursday on Highway 99 near Westside Boulevard. The 20-year-old was driving southbound on the highway in a Dodge Neon when he suddenly lost control of his vehicle and collided with a big rig that was also traveling southbound.