I've been thinking about this more, and I think the 'cost/ease/laziness' issues aren't just applicable to car use.
I work in London 3 days a week (bare with me, this is dull to make a point), because I have to use the tube for 4 stops each way, the cheapest way for me to get to work is to buy a monthly rail card with zones 1-6 with an extension to my local town. This means at lunchtime, I'll think nothing of jumping on the tube to go 1 or 2 stops to go shopping, however colleagues who get the train to Victoria (therefore can walk to the office so only have rail only tickets) and have oyster cards for any tube/bus journeys think it's odd I wouldn't walk (as it's about 20 minute walk to Oxford Street). I think, well, it takes less of my lunchbreak more childless shopping time and it's "free" as I pay for the travel card just for the commute, I don't calculate against any 'extra' trips I take.
In the same way, I could walk to Tescos from my house, and I will for little bits, but even though it's only a mile away, I'll drive for the convience of not having to walk back with shopping - the cost is rather irrelevant as I've already paid to learn, paid for my car, paid tax and insurance and it uses so little petrol to go so short a distance in my tiny car, it's not worth worrying about.
For those who use public transport, once you have a travel card/paid a flat sum, are you sure every journey you make is really necessary? Do you ever hop on a tube/bus that really, if you had a bit more time, you could walk? It's the same mentality as car usage, most people are lazy unless it's going to be expensive/complex.
Therefore, a good way to massively reduce the small journeys is to find a way to charge for them. So if insurance wasn't a flat rate, but a 'pay as you go' same as car tax, if you could be taxed just on the miles you did, it would make people think more about leaving the car at home. For small engined cars like mine, the cost in petrol for a short journey is so tiny it's not worth worrying about, but if I had to think "well, it'll be a fiver to go to tescos, am I only buying things I could carry?" I might walk. unless it's raining and i've straightened my hair