Door to door canvassing has almost ceased because so many of those whose doors are knocked on are so unpleasant to the canvassers and canvas returns are no longer the reliable guide to voting intentions that they used to be.
Also most parties have a far smaller group of active workers to call on in each constituency, regardless of party and they just do not have enough people to do door to door canvassing.
I am not sure that placards of themselves encourage people to vote for a particular party, at least not consciously, but I think when there is a sea of placards in a constituency of one particular party, I think people who are undecided, may well choose to vote for someone who they had been thinking of voting for, because they feel their vote will count.
This particularly applies to smaller parties like the Lib Dems. If you are thinking of voting Lib Dem but do not want to 'waste' your vote, seeing lots of placards for them may convince you that they are in with a fighting chance in that constuency so that it is worth voting for them.
Casdon I have noted exactly the phenomena you have noticed. All those fields that in the past always voted Conservative are now all voting for the Lib Dems.
So far travelling from Somerset to Cambridgeshire and DS having travelled down from York. The only signs you see are Lib Dem. I have seen them to many to count. Apart from that I have seen 2 Green Party placards and 1 Labour placard.