The right use of irony?

Today I went to watch the Kids` Festival at Zenrinkan. All the children`s classes did a presentation of something they had learnt in class. It was all in English and some of them were so good. The little 3 and 4 year olds doing "head, shoulders, knees and toes" was adorable! Will put a photo up soon.

On the way there I saw a scene that made me chuckle to myself. There was a group of people protesting against traffic congestion. They had a big loud speaker and they were telling people they should be walking instead of using their cars. A very worthy cause indeed. However, the loud speaker they were using happened to be mounted to a large white van that was blocking most of the road and churning grey smoke out of its exhaust. I wonder if they could spot the irony?