Get first item of every sublist of list in Python

I have the following string which is a list of lists:

"ID":[[310.5,1.1612828],[310.0,15.0],[310.0,2.9755309],[309.5,30.0]]

I want to grab all the first values of each list into a variable, so the output should be in this case

print variable

expected output:

310.5
310.0
310.0
309.5