The drink-driving campaign is running for two weeks

The four Welsh forces launch their summer anti-drink driving campaign in Tenby, west Wales, on Tuesday.

This year, they are asking motorists to consider "What's in a drink?".

Police have been concerned that summer activities such as barbecues and garden parties mean people were often consuming large amounts of unmeasured alcohol.

They are asking people to consider how much alcohol is actually in a glass of wine or beer and to remember that home-poured measures can be much higher than the units of alcohol served in pubs.

But the real message remains the same - if you are going to drive, do not drink at all.

The campaign's launch at Tenby harbour in Pembrokeshire will see officers displaying drinks of varying types and strengths, and asking members of the public to guess how many units each contain.

The launch is being supported by Primary Substance Misuse (PriSM), Mid and West Wales Alcohol and Drug Advisory Service, British School of Motoring and drinks industry body the Portman Group.

It is impossible to tell what the concentration of alcohol is in your body, without taking a breath test Police drink-drive campaign

The campaign is trying to highlight that even government advice about how much motorists can drink and still drive can be questionable.

In a statement, forces said: "Much of this advice refers to fixed units of alcohol, which is a very unreliable way of calculating the level of alcohol within your body, as everyone's body reacts in different ways.

"It is impossible to tell what the concentration of alcohol is in your body, without taking a breath test.

"Additionally, the strength of beers varies tremendously and a home-measured glass of wine varies considerably to that in pubs."

Police said most people were sensible about making travel arrangements when drinking, but there were still a few that the message had not reached, and some who would still drive after drinking too much.

The campaign is running from 12 August to 26 August.

Last year's summer campaign showed that 3% more drivers tested positive for excess alcohol compared to a similar campaign run at Christmas.