As well as spending a lot of time reading up about goings on in the transport industry, unsurprisingly we're partial to the odd blog, article or book on advertising and marketing.
It keeps us on our toes and keeps us learning. Ad agency Sell! Sell! produced one such book a year or so ago, and this is a quote from it -
Unfortunately for clients. as the old saying goes the hammer always sees the nail but the problem isn't always a pointy piece of metal nor the solution a heavy knocking implement. There's no one-size-fits-all, one way to do advertising that's right for every product, brand, category and business problem.'
Granted, you might have a budget in mind that might sway a preference for one medium over the other, but surely it would make sense to see what your agency comes up with when they have a 'media neutral' mind, then see where it gets the best exposure.
Either way, it's best to let your agency get on with producing some cracking creative and then have a chat about what to do with it afterwards. Remember, if the ad's no good to start with, it doesn't matter where it gets placed.