We've been trialling a customisable matrix board with National Highways for a while now and looking at the impact these have on server performance. This wouldn't be rolled out to all police, most probably restricted to RTPC, however that's under the assumption that the signs don't have a high impact on performance, as the experiment would need expanding to all their vehicles first, however we currently have more important projects to work on.
For now i'll be declining the suggestion as this is something we're trialling, however i cannot promise anything currently.