This is a slightly modified update to Daniel Kornev's excellent post which goes into more detail about why building from source is necessary on 18.04. His post was missing a few dependencies that I didn't have installed namely cmake, opencv and pkg-config. The following steps should get you a working build of openalpr on a fresh install of Ubuntu 18.04.
If you're not working with a fresh install, this might be a good time to clean and update your system before proceeding. (Optional)
$ sudo apt update #fetch list of available updates
$ sudo apt upgrade #install updates – does not remove packages