After providing information on their professional backgrounds and specific work in law enforcement in Colorado, they discuss the state's legislative history regarding the use of medical marijuana, which became legal in 2000, and recreational marijuana, which became legal in 2014. They did not conjecture as to whether the legalization of marijuana had directly increased the number of DUI cases in Colorado, largely because of the prevalence of drug mixtures in problem drivers' systems. They questioned the belief of some that marijuana ingestion improves alertness and driving skill. They recognize the need for reliable research on the effects of marijuana consumption on motor and mental abilities that could undermine safe driving. Absent a traffic crash that brings officers to the scene for an investigation, traffic stops are based on officer observations of unsafe driving patterns. A charge of DUI can be based on probable cause that a driver is under the influence of a substance, with a subsequent toxicology test providing further evidence. Knudsen and Davis also comment on Colorado's effort to conduct and update public information campaigns that provide reliable facts on the possible effects of marijuana in various dosages on driving capabilities.