Many public goods may at times be subject to excessive use resulting in negative externalities affecting all users; for example air pollution and traffic congestion. The closeness of the people while interacting with other people in the public utilities also has appeared to cause negative impact to people. The result of this is a faster and increased spread of infectious diseases such as SARS and COVID-19. Public goods problems are often closely related to the "free-rider" problem, in which people not paying for the good may continue to access it. Thus, the good may be under-produced, overused or degraded. Public goods may also become subject to restrictions on access and may then be considered to be club goods; exclusion mechanisms include toll roads, congestion pricing, and pay television with an encoded signal that can be decrypted only by paid subscribers.