This paper presents a model of comfortable distance that captures the factors and conditions known to affect personal space in human-human and human-robot interactions, as well as any identified relationships between them. In the first known human-robot interaction (HRI) survey on approach distance, 19 papers were reviewed and the comfortable distance model was synthesized with three distinct types of inputs: environmental conditions, personal factors, and agent factors. Five environmental conditions (lighting, ceiling height, indoor/outdoor, room size, and barrier height) and seven personal factors (gender, age, mood, personality, pet ownership, robot experience, and sitting/standing) structure the model, with four agent factors (angle of approach, height of agent, speed of approach, and gaze) used as tuning parameters to produce behaviors with appropriate distances. Currently, HRI researchers generally focus on one factor at a time (e.g., approach angle or approach speed), without considering the previous work in adjacent fields, such as psychology and other social sciences. This has resulted in environmental factors being ignored by the HRI community. The comfortable distance model is a new tool for HRI researchers and is expandable so that it can incorporate new factors as they are identified. This survey will inform researchers about factors which had been previously overlooked in the field of HRI and will allow future researchers to consider the impact of identified variables to create more complete experiments.