What chemical regularly transported by road in the UK would cause serious injury/loss of life if it escaped in a road accident? I'm writing a story where a serious multi-vehicle pileup happens in the winter during foggy weather. I need the fallout from this to include the escape of a substance that will cause significant injury/loss of life to those immediately involved in the accident.

What chemical can I use that is regularly transported by road in the UK that could cause this to happen?
Organophosphates in general would be a great idea as you can go from potent insecticides to chemical warfare as it fits the drama of your story.