Is CNG a safe gas? 

Yes, it is absolutely safe as a gas. Being lighter than air, in case of a leak it rises up and disperses into the atmosphere. This property precisely makes it a safe fuel.

What is the cost of conversion from petrol to CNG ?  
Can I run my automobile on Petrol once the CNG in my fuel tank gets consumed ? 
What is the capacity of the CNG cylinder, and mileage from one fill ? 
How do I get to know the quantity of CNG left in my cylinder ? 
Does CNG have any harmful effects on my car engine? 
Can a diesel vehicle be converted to CNG ? 
How does a large-scale usage of CNG benefit my country?