This data is derived from manual traffic counts on major roads, so isn't necessarily going to capture any reduction in traffic on LTN roads.
It includes *all roads* that pass through the borough, so it's disproportionately influenced by trunk roads (A406, A12, etc) which aren't really going to be affected by local LTNs.
In fact, I've just downloaded the raw counts data. If you exclude the A406 and A12 the number of motor vehicles spotted drops from 6.6m to 3m. Obviously that's not the same as total vehicle miles.