One of the reason of slow adaption of bitcoins at the groceries store is the slow transaction time of the bitcoins. Sometime bitcoin tx takes more than 20-20 mins for a single confirmation and consider that you went to a grocery store, send the payment through bitcoins and standing in the counter for the tx to confirm. This is not a practical approach.
Still this concerned really preventing businesses to take the adoption, doubts and worries with that long waits for confirmation.
Most of the time customers are in such hurries and they can't take that long time to wait, online transaction though are already
workable as there are businesses who caters bitcoin as part of their online payment process.