An understanding of the relationship between user acceptance and system effectiveness is necessary for designing collision warning systems for cars and for accurately estimating their safety benefits. To better understand this relationship, an exploration was conducted into the results of a recent naturalistic field operational test of an integrated vehicle-based safety system installed into a fleet of passenger vehicles. Correlation analyses were conducted for system effectiveness, which was measured in terms of near-crash reduction, and for user acceptance, which was measured using responses to a questionnaire that drivers filled out after completing the study. The key findings were as follows: (1) although remaining generally positive overall, user acceptance decreased slightly for drivers as system effectiveness increased. This pattern appears to be due primarily to older drivers (aged 60 to 70), for whom it was strongest; (2) older drivers rated the system higher and were more likely than younger drivers to want more alerts; (3) a possible contributing factor to the above results is false alerts, as older drivers experienced a smaller proportion of false alerts than younger drivers. These results are discussed in the context of existing evidence and recommendations are made for future experiments and safety benefits estimates.