The average person is exposed to more than 2000 messages per day. ‘Message’ being defined as an advertisement. People are being subjected to more and more information every day and are getting less time to absorb it.
The way to be effective with a :15, :30 or :60 message is this – repeat the same points over and over. Do not pretend that your dealership ad will win any kind of award for creativity; the award comes in the form of traffic and you benefit from the profit. Keep your message simple and repetitive and you will win the mental shelf space that your product has with your customer.