In Toronto -- a city that is often choked by smog -- there are motorized billboards trolling the streets. I saw one tonight on the way home from work. For a moment, I had to breathe its exhaust. In a world with limited petroleum resources -- and, arguably, a world whose survival is threatened by the continued burning of said resources -- using them to power a moving advertisement is the height of arrogance and environmental short-sightedness.
In fairness, the company that runs the billboard I saw tonight claims that some of its fleet is powered by biodiesel. They point out that biodiesel can be made from soybean oil, canola oil, or recycled cooking oil. What they don't mention is that biodiesel often contains regular diesel in a blended format. And I don't care if the truck I was behind tonight was burning 100% biodiesel or 20% biodiesel / 80% diesel... it still stank, it still needs petrochemicals for its lubrication/construction/distribution, and it still takes up valuable space on downtown streets. Until you can power a truck with rainbows and pixie dust, there will always be an environmental impact.
Save the biodiesel for public transportation.