Determining the size of the Grid World

[WinteringAsch]

1. the Top and Bottom of the Grid World is 'Cliff', which implies the guardrail.
2. there should be some distances between the outstanding vehicle and following vehicle.
   2-1. after outstanding vehicle has lost control, the distance between two should gradually decrease.
3. following vehicle will get minus reward if it hits the guardrail, or get too close to the outstanding one.
   --this is to consider the size of the vehicle.