If you suspect someone of driving impaired, call 9-1-1. Even if you're not 100% sure, it's better be safe than sorry. Juliane Porritt, media relations officer with the OPP says there are signs to look for.
She says impairment isn't always by alcohol. When you call make sure you provide where you are, which direction you're going, vehicle make and model, if you can identify the driver and a licence plate if you can obtain it safely.