Formal pairs suit Writing Task 2. Neutral ones work anywhere. Informal ones belong in Speaking.
1.heavy traffic
neutrala lot of vehicles on the road
Heavy traffic in the city centre doubles journey times at rush hour.
Heavy also goes with rain, smoker and workload.
Avoid: strong traffic → heavy traffic
2.heavy rain
neutrala lot of rain falling
Heavy rain caused flooding across the region.
Heavy, not strong or big.
Avoid: strong rain → heavy rain
3.a strong argument
neutrala convincing reason for an opinion
There is a strong argument for making public transport free.
Compelling and powerful are alternatives.
Avoid: a hard argument → a strong argument
4.a growing concern
formala worry that more and more people share
Online fraud is a growing concern for banks.
Mounting concern is a stronger version.
5.widely accepted
formalbelieved or agreed by most people
It is widely accepted that smoking causes cancer.
Widely also goes with believed, known and used.
6.highly effective
formalworking very well
Vaccination has proved highly effective against measles.
Highly also goes with unlikely, skilled and competitive.
Avoid: very much effective → highly effective
7.deeply rooted
formalfirmly established and hard to change
Gender roles are deeply rooted in many cultures.
Deep-rooted before a noun: deep-rooted beliefs.
8.a major cause of
neutralone of the most important causes
Poor diet is a major cause of heart disease.
A leading cause of is a formal alternative.
9.a significant increase
formala large rise that matters
There has been a significant increase in remote working.
Also useful in Task 1 for data. A big increase is fine in Speaking.
10.a sharp decline
formala sudden large fall
The graph shows a sharp decline in bird numbers after 1990.
A Task 1 favourite: a sharp rise, a sharp fall.
11.a key factor
neutralan important reason or influence
Cost is a key factor in choosing a university.
A crucial factor is stronger.
12.a vital role
formala very important part in something
Teachers play a vital role in shaping young minds.
Play a vital role in.
13.serious consequences
neutralresults that are bad and important
Ignoring climate change will have serious consequences.
Far-reaching and severe also collocate.
14.a heated debate
neutralan argument where people feel strongly
The proposal sparked a heated debate among parents.
Spark and provoke collocate with debate.
15.compelling evidence
formalevidence that is very convincing
There is compelling evidence that exercise improves mood.
Strong and clear evidence are alternatives.
16.a wide range of
neutralmany different kinds of something
Cities offer a wide range of jobs and services.
Broad range is also correct.
Avoid: a big range of → a wide range of
17.strongly disagree
neutraldisagree completely
I strongly disagree that university should be only for the wealthy.
Strongly also goes with believe, oppose and recommend.
18.highly unlikely
formalvery improbable
It is highly unlikely that cars will disappear from cities soon.
Highly likely is the opposite.
19.a pressing issue
formala problem that needs urgent attention
Housing is the most pressing issue for young adults.
Pressing also goes with need and problem.
20.a common misconception
formala mistaken belief that many people hold
It is a common misconception that bilingual children learn more slowly.
Popular misconception is also used.