Daytona Prototypes are a product of the Grand-Am Rolex Sports Car Series, and offer a different interpretation of the prototype theme. DPs, as they are often called, are closed-cockpit, purpose-built racing machines which are less expensive and (deliberately) somewhat slower than Le Mans Prototypes, which were becoming dangerously quick on the Daytona oval and prohibitively expensive for smaller teams to run. Compared to the LMPs, DPs are severely limited in terms of approved technology; for instance, they are required to be constructed of steel tube frames with carbon-fibre skins, rather than being carbon-fibre monocoques, and must use production-based engines. In addition, contrary to their European counterparts who continuously alter and develop a vehicle to increase performance as a season progresses, DPs are restricted to their original conception of the car from the start of the season. For these reasons, the category being labeled as a "prototype" has occasionally been criticised as misleading and being more in line with traditional "spec" race series prevalent in the United States. The intention of the DP formula was to provide a class in which tight technical regulations encouraged close competition and where budget would be relatively unimportant. DP chassis are subject to a franchise-like approval system in which only approved constructors are eligible, with rules stability enforced for several years at a time, although this led in 2007 to established constructors like Lola and Dallara entering the 2008 series by taking over the rights of existing constructors (Multimatic and Doran respectively).