our environmental site activity was as follows : 2013 2012 2011 .
what was the net change in the number of environmental sites from 2011 to 2012?
the environmental liability includes future costs for remediation and restoration of sites , as well as ongoing monitoring costs , but excludes any anticipated recoveries from third parties . cost estimates are based on information available for each site , financial viability of other potentially responsible parties , and existing technology , laws , and regulations . the ultimate liability for remediation is difficult to determine because of the number of potentially responsible parties , site-specific cost sharing arrangements with other potentially responsible parties , the degree of contamination by various wastes , the scarcity and quality of volumetric data related to many of the sites , and the speculative nature of remediation costs . estimates of liability may vary over time due to changes in federal , state , and local laws governing environmental remediation . current obligations are not expected to have a material adverse effect on our consolidated results of operations , financial condition , or liquidity . property and depreciation 2013 our railroad operations are highly capital intensive , and our large base of homogeneous , network-type assets turns over on a continuous basis . each year we develop a capital program for the replacement of assets and for the acquisition or construction of assets that enable us to enhance our operations or provide new service offerings to customers . assets purchased or constructed throughout the year are capitalized if they meet applicable minimum units of property criteria . properties and equipment are carried at cost and are depreciated on a straight-line basis over their estimated service lives , which are measured in years , except for rail in high-density traffic corridors ( i.e. , all rail lines except for those subject to abandonment , yard and switching tracks , and electronic yards ) for which lives are measured in millions of gross tons per mile of track . we use the group method of depreciation in which all items with similar characteristics , use , and expected lives are grouped together in asset classes , and are depreciated using composite depreciation rates . the group method of depreciation treats each asset class as a pool of resources , not as singular items . we currently have more than 60 depreciable asset classes , and we may increase or decrease the number of asset classes due to changes in technology , asset strategies , or other factors . we determine the estimated service lives of depreciable railroad property by means of depreciation studies . we perform depreciation studies at least every three years for equipment and every six years for track assets ( i.e. , rail and other track material , ties , and ballast ) and other road property . our depreciation studies take into account the following factors : f0b7 statistical analysis of historical patterns of use and retirements of each of our asset classes ; f0b7 evaluation of any expected changes in current operations and the outlook for continued use of the assets ; f0b7 evaluation of technological advances and changes to maintenance practices ; and f0b7 expected salvage to be received upon retirement . for rail in high-density traffic corridors , we measure estimated service lives in millions of gross tons per mile of track . it has been our experience that the lives of rail in high-density traffic corridors are closely correlated to usage ( i.e. , the amount of weight carried over the rail ) . the service lives also vary based on rail weight , rail condition ( e.g. , new or secondhand ) , and rail type ( e.g. , straight or curve ) . our depreciation studies for rail in high density traffic corridors consider each of these factors in determining the estimated service lives . for rail in high-density traffic corridors , we calculate depreciation rates annually by dividing the number of gross ton-miles carried over the rail ( i.e. , the weight of loaded and empty freight cars , locomotives and maintenance of way equipment transported over the rail ) by the estimated service lives of the rail measured in millions of gross tons per mile . rail in high-density traffic corridors accounts for approximately 70 percent of the historical cost of rail and other track material . based on the number of gross ton-miles carried over our rail in high density traffic corridors during 2013 , the estimated service lives of the majority of this rail ranged from approximately 15 years to approximately 30 years . for all other depreciable assets , we compute depreciation based on the estimated service lives .