When it rains, the impermeability of hard surfacing causes more rain water to run-off. This causes greater fluctuations in amount of water going into the roadside storm drains and from there to local streams and rivers. This in turn leads to:
As it runs off, rain water picks up oil and heavy metals from the hard surfaces beside and close to roads, plus pesticides, herbicides and other chemicals used in gardens. This leads to:
More run-off means less rainwater percolating through the soil.
Heat, Noise and Dust
Hard surfaces have several other characteristics which create problems.
Hard surfacing front gardens for parking leads to a further set of problems.
Increased numbers of pavement crossovers (dropped kerbs) to allow front garden parking causes a corresponding reduction in the number of on-street parking spaces available. That's because people generally don't park in front of a pavement crossover, thus effectively reserving that section of the road for the sole use of the dwelling with the crossover.
Plants, Trees & Wildlife
Hard surfacing of front gardens often means that most or all of the garden's vegetation is removed. This creates yet a further set of problems.
Hard surfacing of front gardens changes the aesthetics and character of roads and neighbourhoods.
Hedges and Fencing
The removal of boundary structures (hedges, fencing etc.), common when front gardens are hard surfaced for parking, is also problematic.
Finally, the sourcing of hard surfacing materials used can cause problems.