I've got a smallish diesel car. It's VED band A and has stop/start so if it ain't moving, it ain't emitting anything. It's exactly the sort of car the government have been encouraging us to buy with tax breaks and kind words until recently.
Given recent headlines, and to see if it would work for us, for a week before Xmas, me and Mrs ND left the car at home (I drop her at work and pick her up again as part of my commute so for part of the journey we are car sharing).
While there were no major issues, buses and trains running pretty much to time and we managed to get a seat apiece on every journey, it isn't an experiment I'll be running again any time soon.
Reasons? Firstly cost - even buying weekly "discounted" tickets it cost £50 for the two of us - around double the cost of the diesel the car would have used. The other and probably more important reason was the time penalty. Journey times were more than doubled - in the car the morning journey takes 45 - 50 minutes, bus, train and the 15 minute walk from the station to work took an hour longer. The return journey was the similar meaning that we were taking around two hours a day longer to get to and from work, which is ten hours a week and means that we're back in the car from tomorrow. Bear in mind too that the majority of buses we travelled on were a decade old, and the trains were built in the 80s so are unlikely to be 'green' by any current emissions standards.
I know there is the argument that there is more than the cost of diesel to consider when running a car, but given that the car is already bought and paid for, other ongoing costs are minimal.
One final thought - an electric car could probably work for us, but living in a terrace house we have nowhere at home to charge it and there are no public charging points anywhere near where I work that I could use, so until the charging issue is resolved, that won't happen either.