Our new restaurants, once opened, may not be profitable, and may adversely impact the sales of our existing restaurants.
Historically, many of our new restaurants have opened with an initial ramp-up period typically lasting 24 months or more, during which they generate sales and income below the levels at which we expect them to normalize after the restaurant has built a customer base, and during which costs may be higher as we train new employees and adjust our food deliveries and preparation to sales volumes and peak-hour trends. If we are unable to build the customer base that we expect for new restaurant locations or overcome the higher fixed costs associated with new restaurant locations, new restaurants may not have results similar to those of our existing restaurants and may not be profitable. Our new restaurant sales volumes since the fourth quarter of 2015 have also been negatively impacted by the food safety issues described elsewhere in this report and other adverse publicity, and as a result, the effect of new restaurants on our average restaurant sales over the past two years has been of greater magnitude than we have seen in the past. This trend may continue into 2018 and beyond.
We have also opened restaurants in nearly all major metropolitan areas across the U.S. New restaurants opened in existing markets may adversely impact sales in previously-opened restaurants in the same market, as customers who frequent our established restaurants begin to visit a newly-opened restaurant instead. This impact could worsen as we open additional restaurants, and could make it more difficult for us to increase comparable restaurant sales and profitability. Existing restaurants could also make it more difficult to build the customer base for newly-opened restaurants in the same market, and could limit our growth potential if we determine that one or more of our nearby restaurants makes an otherwise viable new restaurant site unattractive to us.
In addition, in the event we are not able to contain increases in our average restaurant development costs, which could result from inflation, an increase in the proportion of higher cost locations, project mismanagement or other reasons, our new restaurant locations could also result in lower returns on our investment in new restaurants.
Finally, our new restaurant development activity has broadened recently to incorporate trade areas or types of restaurant sites in which we have little or no prior experience, including smaller or more economically mixed communities, highway sites, outlet centers, and restaurants in airports, food courts, or on military sites. These types of sites may become more important to our restaurant growth strategy as we find fewer opportunities to open in traditional sites, given our past growth. Many of these site types may involve additional costs that we do not incur in our more traditional restaurant sites, which will adversely impact the profitability of restaurants in these types of sites. The risks related to building a customer base and managing development and operating costs in some or all of these types of trade areas or restaurant sites may also be more significant than in our traditional sites, which could result in unexpected negative impacts on our new restaurant operating results.
We may continue to be negatively impacted by food safety incidents, and further instances of food-borne or localized illnesses associated with our restaurants would result in increased negative publicity and further adverse impacts on customer perceptions of our brand.
During late October and early November 2015, illnesses caused by E. coli bacteria were connected to a number of our restaurants, initially in Washington and Oregon, and subsequently to small numbers of our restaurants in as many as 12 other states. During the week of December 7, 2015, an unrelated incident involving norovirus was reported at a Chipotle restaurant in Brighton, Massachusetts, which worsened the adverse financial and operating impacts we experienced from the E. coli incident. As a result of these incidents and related publicity, our sales and profitability were severely impacted throughout 2016. In July 2017, cases of norovirus associated with a Chipotle restaurant in Sterling, Virginia had a further adverse impact on our sales, particularly throughout the mid-Atlantic and Northeast regions. The significant amount of media coverage regarding these incidents, as well as the impact of social media (which was not in existence during many past food safety incidents involving other restaurant chains) in increasing the awareness of these incidents, may continue to negatively impact customer perceptions of our restaurants and brand, notwithstanding the high volume of food-borne illness cases from other sources across the country every day. As a result our sales may not return to levels we were achieving prior to late 2015.