Off-street minimum parking requirements for new developments have been around for quite a while. While parking requirements were implemented in cities at differing times, most cities established minimum parking requirements in and around the 1950’s. Minimum parking requirements were brought into effect in an attempt to avoid a tragedy of the commons with regards to the low quantity of on street parking spaces available when compared with the rapidly increasing amounts of vehicles attempting to use them. City planners decided to implement bylaws that required new developments to provide a minimum amount of off-street parking. The amount of parking required generally depended on the purpose of the building and the size of the building. The ideal result was that each development would have enough off-street parking to completely fulfill the parking demand created by the new development, so that no on street parking spaces would need to be used.
Adding off-street parking requirements were also meant to help prevent a phenomenon known as cruising for parking. “If curbside (i.e., on-street) parking demand exceeds curbside parking supply, some drivers cannot immediately find a vacant parking space; thus cruising for parking will emerge, which imposes external costs on all drivers by increasing congestion.” (Inci 2015). Shen et al. in their study on Xi’an, China, finds that a lack of parking stalls determines parking congestion which results in cruising for parking and illegal double parking. (Shen, et al. 2020) Findings from their study imply that greater amounts of parking allow for a decrease in traffic congestion as illegal double parking and cruising around for parking will decrease. Vehicles are finding parking spots more quickly when there are ample off-street parking spaces. Off-street parking requirements also space buildings further apart and reduce the density of parking lots, which reduces the number of places where vehicles enter and exit roads, which reduces the number of vehicles interfering with the regular flow of traffic. (Shen, et al. 2020)
To identify how parking affects the flow of traffic, consider a simple model where one is trying to find the steady state of the flow of traffic, which is to say that the number of vehicles entering into traffic is equal to the number of vehicles exiting traffic and parking. To find this, we must consider a number of variables. Let V equal the number of vehicles, T equal the number of vehicles in traffic, P equal the number of vehicles that are parked, and τ equal the flow of traffic. In the model we are trying to find when dτ/dt = 0. Vehicles enter into traffic from being parked, so let η be the rate at which vehicles enter into traffic from parking. Vehicles exit from traffic to parking, so let φ be the rate at which vehicles find parking and exit traffic. Given the above variables, we know that V = P + T and τ = T/V.
Therefore, to find dτ/dt = 0, we know that it is the equivalent of (1) d(T/V)/dt = 0. Breaking the derivative, we find (2) dT/dt*1/V + dV/dt * T/V2 = 0. At this point, we assume that the change in vehicles over time, dV/dt, is equal to 0. Namely, that the stock of vehicles throughout the entire cycle remains constant. dT/dt is equal to the rate of vehicles entering traffic from being parked minus the rate of vehicles exiting traffic and finding parking. Therefore, (3) dT/dt = ηP – φT. Substituting equation (3) into equation (2) will result in equation (4) (ηP – φT)/V = 0. We can substitute V = P + T for P in equation (4) and break everything out into common denominators. This will give us (5) ηV/V – ηT/V – φT/V = 0. We know that V/V is simply just 1, and we can further simplify by taking both T/V terms and replacing them with τ to get (6) η – ητ – φτ = 0. Finally, by isolating τ we find the final solution for the steady state level of the flow of traffic to be equal to (7) τ = η/η+φ. What this formula states is that the steady state flow of traffic is dependent on the rate which vehicles enter traffic and the rate at which vehicles exit traffic to find parking. The easier it is to find parking, the higher the rate of vehicles exiting traffic will be, which makes a higher φ value, causing τ to become smaller.
Minimum parking requirements were put in place and continue to be in effect in most municipalities because they seem to offer a solution to two problems. They seem to help ease traffic congestion by making it quick and easy to park one’s vehicle; and they also help to ensure that on street parking spaces are sufficient to meet the on street parking demand by creating excess parking spaces that are closer to where people want to park.