When Toby Gale and I talked to people in Manchester as part of the neighbourliness review we did a couple of years ago, we came across a man living in a semidetached house in a relatively affluent area, who had only spoken once to his next-door neighbour in 15 years, and that was on the day he moved in.
He put this down in part to the layout of the long front garden, with cars parked at the top near the house. And whatever the length of the garden, hard standing for cars outside the front door is a surefire way of minimising contact with neighbours.
But making roads out of front gardens has other implications as well, and in Ealing they're taking it seriously. The Ealing Front Gardens project has just set up a website with a couple of reports and a tabular presentation of detrimental effects.
Apart from the implications of loss of vegetation, there are various other concerns such as the reduced amount of rainwater percolating through soil, detrimental effects on water quality and on wildlife, 'heat island' effects, and reduction in gardening activity. Among other negative implications listed are: "Reduction in community cohesion" due to changed appearance/aesthetics; and "Adverse effect on neighbour relations" due to loss of demarcation.