Billboards aren't printed at 300 DPI.
DPI is a printer spec. It’s how many printed dots per inch. This has nothing to do with a digital image. Any image can be printed at any DPI no matter how large the image is because one digital pixel can be represented by any number of dots on a paper.
Yeah your viewing it from super far away Not looking at them inches away. Zero need for 300dpiHow big of a billboard do you want?
Straight off the bat I can't imagine any billboard needing 300DPI. You could be looking at 1/10th of that or less depending on the viewing distance.
At 30dpi you're looking at a native 3.4m by 2.6m.