Double parking has many negative effects on the traffic indicators, such as travel time, road capacity, traffic congestion, traffic flow speed, and safety of traffic flow. Double parking activities are considered as drivers\\\\\\\' driving behaviour. This study defines an index for estimating the likelihood of committing double parking called the NPC index. This parameter indicates a driver\\\\\\\'s visual search distance to find a vacant on-street parking space based on the number of cars parked on the curbside of the street. The metropolis of Mashhad has been selected as the case study of this research. The selected statistical sample consisted of 275 drivers who had violated the traffic rules by committing double parking in the central areas of Mashhad. The recorded values of the NPC index were clustered using the K-means clustering method, and also, other variables were classified. The factors affecting the NPC parameter were identified using the Mann-Whitney U and the Kruskal-Wallis statistical tests. Based on the results, the variables of educational degree, trip purpose, presence or absence of the driver, and parking duration were identified as the factors affecting the visual search distance to find a vacant on-street parking space. The categories of each of these factors were also ranked using the Mann-Whitney U and the Kruskal-Wallis statistical tests.

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