You may travel by plane until you are close to delivery (35-36 weeks). On long flights, walk around the plane every few hours, stretch your legs, and make sure you are well-hydrated. After 36-38 weeks, it is more prudent to stay close to home unless there are extenuating circumstances.
During a car trip, stop every couple of hours to walk and move around. Always wear both the lap and shoulder belt. Buckle the lap belt low on your hip bones, below your belly; never secure the lap belt across your belly.