Churumbeque is right and areing on caution.
If you do go down to the store I think its best to take a groom with you to keep a watchfull eye on your horse.
A halter would be the better option.
Also a bungie backed up with a lead rope just in casefrom the bit over the head and back through the bit with a bit more slack than the bungie.
Here is a bungie I have got for my new trailer. DSCF4134.JPG
But I would recomend a groom to acompaney the horse espeasherley on todays roads I think that's the safest way.
Or get your horse a nose bag so his mind is on eating his food whilst you shop that could be another idear whilst he/she is tied to a hitching post.