If one cycles beside cars, it is just a question of time for something to go wrong.
Also, when you exercise, you open the alveoli in the lungs so that you can use more oxygen, but if you are beside cars, you will absorb more pollution.
The only way that I recommend cycling is in rural areas, away from traffic.
And always wear a helmet.
Cycling indoors can be good if not overused and in low intensity.
I hope it helps.
In case of doubt of not knowing what to do in terms of exercise, go for a long walk.
Very dangerous and not that good for you, I would say.