acquire operations and facilities from municipalities and other local governments , as they increasingly seek to raise capital and reduce risk . we realize synergies from consolidating businesses into our existing operations , whether through acquisitions or public-private partnerships , which allow us to reduce capital and expense requirements associated with truck routing , personnel , fleet maintenance , inventories and back-office administration . operating model the goal of our operating model pillar is to deliver a consistent , high quality service to all of our customers through the republic way : one way . everywhere . every day . this approach of developing standardized processes with rigorous controls and tracking allows us to leverage our scale and deliver durable operational excellence . the republic way is the key to harnessing the best of what we do as operators and translating that across all facets of our business . a key enabler of the republic way 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 . we have rolled out several productivity and cost control initiatives designed to deliver the best service possible to our customers in the most efficient and environmentally sound way . fleet automation approximately 74% ( 74 % ) 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 18% ( 18 % ) of our fleet operates on natural gas . we expect to continue our gradual fleet conversion to cng , our preferred alternative fuel technology , as part of our ordinary annual fleet replacement process . we believe a gradual fleet conversion is most prudent to realize the full value of our previous fleet investments . approximately 36% ( 36 % ) of our replacement vehicle purchases during 2016 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 , 2016 , we operated 38 cng fueling stations . standardized maintenance based on an industry trade publication , we operate the eighth largest vocational fleet in the united states . as of december 31 , 2016 , our average fleet age in years , by line of business , was as follows : approximate number of vehicles approximate average age .
what is the approximate number of vehicles that have been converted to compressed natural gas ( cng )
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