organizational structure a key enabler of the republic way operating model is our organizational structure that fosters a high performance culture by maintaining 360-degree accountability and full profit and loss responsibility with local management , supported by a functional structure to provide subject matter expertise . this structure allows us to take advantage of our scale by coordinating functionally across all of our markets , while empowering local management to respond to unique market dynamics . our senior management evaluates , oversees and manages the financial performance of our operations through two field groups , referred to as group 1 and group 2 . group 1 primarily consists of geographic areas located in the western united states , and group 2 primarily consists of geographic areas located in the southeastern and mid-western united states , and the eastern seaboard of the united states . each field group is organized into several areas and each area contains multiple business units or operating locations . each of our field groups and all of our areas provide collection , transfer , recycling and landfill services . see note 14 , segment reporting , to our consolidated financial statements in item 8 of this form 10-k for further discussion of our operating segments . through this operating model , we have rolled out several productivity and cost control initiatives designed to deliver the best service possible to our customers in an efficient and environmentally sound way . fleet automation approximately 75% ( 75 % ) of our residential routes have been converted to automated single-driver trucks . by converting our residential routes to automated service , we reduce labor costs , improve driver productivity , decrease emissions and create a safer work environment for our employees . additionally , communities using automated vehicles have higher participation rates in recycling programs , thereby complementing our initiative to expand our recycling capabilities . fleet conversion to compressed natural gas ( cng ) approximately 20% ( 20 % ) of our fleet operates on natural gas . we expect to continue our gradual fleet conversion to cng as part of our ordinary annual fleet replacement process . we believe a gradual fleet conversion is the most prudent approach to realizing the full value of our previous fleet investments . approximately 13% ( 13 % ) of our replacement vehicle purchases during 2018 were cng vehicles . we believe using cng vehicles provides us a competitive advantage in communities with strict clean emission initiatives that focus on protecting the environment . although upfront capital costs are higher , using cng reduces our overall fleet operating costs through lower fuel expenses . as of december 31 , 2018 , we operated 37 cng fueling stations . standardized maintenance based on an industry trade publication , we operate the seventh largest vocational fleet in the united states . as of december 31 , 2018 , our average fleet age in years , by line of business , was as follows : approximate number of vehicles approximate average age .
what is the ratio of the number of vehicles for the residential line of business to large-container
onefleet , our standardized vehicle maintenance program , enables us to use best practices for fleet management , truck care and maintenance . through standardization of core functions , we believe we can minimize variability .