'Heavy Traffic' or 'Lot of Traffic'?
Solution 1:
I think "Heavy Traffic" is quite commonly used and is totally appropriate
EDIT : On 2nd thoughts, i think they are both inter-changeble but it might depend on the formulation of the sentence - where one might suit better (depending on the overall style) as compared to the other.
Example:
I was delayed for my meeting due to heavy traffic on the M1
Solution 2:
Both "heavy traffic" and "a lot of traffic" are commonly used in the UK.
Solution 3:
As a traffic engineer, we use "heavy traffic" and "increased traffic volumes"