Keep the headline to about seven words. Drivers and pedestrians glance at a billboard truck rather than reading it, so the message has to land in one look. If the headline needs a second line of explanation, it is probably two ads.
Use one strong visual. A single product photo, a face, or a bold graphic will carry further than a collage. Busy layouts turn into visual noise once the truck is a block away or moving through traffic.
Choose high contrast. Dark text on a light background, or light text on a dark background, reads in bright sun, at dusk, and from an angle. Thin fonts and low-contrast color pairs disappear at distance.
Make the next step obvious. A short web address or phone number in large type is easier to remember than a long URL or a QR code that only works for people standing still.
Finally, check the proof at real size. A layout that looks clean on a laptop can feel crowded on the side of a truck. Approving a proof sized to the actual display is the simplest way to catch that before your campaign runs.