They are interchangeable. Just as you can say "On the street" or "In the street". Personally I prefer "On the road" to "In the road" but "In the street" to "On the street". Maybe it is the way I was brought up! I speak "English English" and wonder if those whose first language is English from different parts of the world have different preferences.

I remember this in the street vs on the street discussion came up a couple of years ago. As an British English speaker I would say ' park on the road' (as opposed to in the driveway, on the footpath, in the garage etc). As I recall, an American respondent was surprised by this and said he would say 'in the road'.

Each version is correct, but each expresses a slightly different meaning. "It's dangerous to walk on the road" might be used to indicate a hazard associated with the road in general, such as the existence of gangs of thieves who rob pedestrians. "It's dangerous to walk in the road" is somewhat more specific, suggesting a distinction between walking along the edge of the road vs walking in the middle. It also has associations with related phrases such as walking in traffic. So it would be dangerous to walk ON the road because you might get robbed, or struck by lightning during a storm, etc, while it is dangerous to walk IN the road because you might get hit by a car.

How can I teach Tamil for Sinhala or English speaking guys in Italki

Exactly the phase should be you do not have a garage on the road you can park on the shore of the road