This paper deals with the development of a credible and valid simulation model of Collingwood circle in New Jersey for the afternoon peak period. The circle is modeled in PARAMICS simulation software. PARAMICS is amongst the limited number of off-the-shelf simulation softwares that can model unconventional traffic circles. However it is demonstrated in this paper that the default capabilities of PARAMICS are not sufficient to develop a valid simulation model of unconventional circles and roundabouts. It is also shown that the Application Programming Interface (API) feature of PARAMICS is required for a statistically valid simulation model of the study circle. Gap acceptance/ rejection of drivers is modeled using binary probit model in STATA statistical software based on field data. The probit model is then implemented in the simulation using PARAMICS API.