50% isn't impossible
http://news.bbc.co.uk/2/hi/science/nature/4000225.stm
There's a catch. Use of oil is going up rapidly though cheap flights, more use of cars and globalisation. There are cases where bottle tops are made in one country, sauce in another, the ingredients in yet another and packaged in another. There is a lot of excess transport. Why is this? In short, it's cheaper. Cheap labour markets abroad, cheap oil, and cheaper methods of production have enabled this.
1. We could re-localise markets, but are people prepared to pay extra and just eat seasonal local food?
2. Are people prepared to fly less? There might be a possibility for further efficiency in aircraft design, but it's all back of the envelope stuff at the moment. And generally, in practice all it would mean is airlines would offer cheaper flights and people would fly more.
3. More efficient cars might be possible, but it's likely that people would just use them more as they can go further, creating congestion which wastes fuel and not really dropping the demand much. This can already be seen in statistics comparing the increased annual mileage of diesel cars with petrol ones.
So if you are prepared to drive less, fly less and buy more expensive local products then yes. Whether everyone else does is another matter, you saw how savage Roger Savage got with me on another thread.
Post peak, it might be possible to couple efficiency as the supply diminishes. However, the supply may drop away faster than changes can be implemented, in fact, with the average life of cars and planes, that’s highly likely. Demand may also rise quickly in places like China, India etc. Place your bets.