We have the relation between gallon and liter
1 gallon = 3.78541 liter
So 30000 gallon means 113,562.3 liters
So there is water of 113,562.3 liters in 1 kilometer
Which means 56,781.15 liters of water in 0.5 kilometer
Accidents occur when the water reaches 30,000 liters per 0.5 kilometer
Here 56,781.15 > 30,000
So the cars are in danger.