Driving in Scotland requires adherence to UK-wide traffic laws combined with an awareness of unique local road conditions. First and foremost, you must drive on the left-hand side of the road and give way to traffic coming from your right at roundabouts. Speed limits are strictly enforced and measured in miles per hour (mph): typically 70 mph on motorways and dual carriageways, 60 mph on single carriageways, and 20 or 30 mph in built-up areas. Scotland has a much lower drink-driving limit than the rest of the UK—50mg of alcohol per 100ml of blood—so the general advice is to avoid alcohol entirely if you plan to drive. You will frequently encounter "Single Track Roads" in the Highlands and islands, which require the use of marked Passing Places to allow oncoming traffic or faster vehicles to pass; it is customary to wave a thank-you to drivers who pull over. Be extremely vigilant for livestock and deer, especially at dawn and dusk. Also, note that while there are no toll roads in Scotland, many cities have introduced Low Emission Zones (LEZ) that restrict older, more polluting vehicles from entering city centers.