Calculate percent of roads (by length) where condition is >=15
There will be other solutions - but how about ...

select 
  length_15_og
, total_length
, round( length_15_og / total_length * 100, 2 ) as percent_15_og
from (
  select 
    ( select sum( length ) from road_condition where condition &gt;= 15 ) as length_15_og
  , ( select sum( length ) from road_condition ) as total_length
  from dual
) ;

-- result
LENGTH_15_OG  TOTAL_LENGTH  PERCENT_15_OG  
135           275           49.09 


Dbfiddle