Calculate the duration or expected drive time between two latitude, longitude points

I want to calculate the duration between two points. I know there is Google Maps API but I don't want to use it, I need an equation to do it. There is one calculate the distance I need one for the duration of drive time.

Well, take a guess how you can achieve that without the knowledge of

• roads
• speed limit
• traffic
• graphs

Pretty hard. I see 2 solutions:

Use the Google Distance Matrix API

https://developers.google.com/maps/documentation/distance-matrix/start

https://maps.googleapis.com/maps/api/distancematrix/json?origins=my_origins&destinations=my_destinations&departure_time=now

Estimate with Speed

Location loc1 = new Location("");
loc1.setLatitude(lat1);
loc1.setLongitude(lon1);

Location loc2 = new Location("");
loc2.setLatitude(lat2);
loc2.setLongitude(lon2);

float distance = loc1.distanceTo(loc2);

int speed=30;
float time = distance/speed;