What would happen if there was a lot of traffic?

Many things -

If you are talking about the city roads -

1. A lot of accidents

2. A waste of time if you stuck in that traffic.

3. A waste of fuel as it would take more time(and so fuel) to reach somewhere nearby place.

4. Lot of air polution due to many vehicles at one place.

5. A noice polution due to a lot vehicles gathering at one place.

6. Waste of your time if you are travelling at such place and it can ruine both your whole day's plan and your partner's mood. ;)