Critical headway is an important parameter for roundabout design, particularly with regard to analytical modeling approaches. The models that have been developed over the past 30 years were typically based on data obtained from manually reduced video or field observations. This paper reports on the application of wireless magnetometers to collect point presence detection for calculating the rejected critical headways. Data were collected at a single-lane roundabout in Carmel, Indiana. Carmel has had more than 60 roundabouts in operation for several years: it represented a community that was highly experienced with roundabouts. More than 260,000 entering vehicles were observed at one of the single-lane roundabouts over a 2-week period with more than 45,000 rejected headways analyzed. For the roundabout studied, 75% of the rejected headways were found to be less than 3.0 s. The rejected headway values were somewhat lower than the values reported in NCHRP Report 572, perhaps because of the evolving driver familiarity with roundabouts. Although this community had a particularly large number of roundabouts, the rejected headway characteristics observed suggested that as roundabouts became more common throughout the United States, it was appropriate to revisit some of the basic traffic engineering parameters used for analysis, much as the traffic signal community did with saturation flow rate in the 1990s. The techniques presented in this paper could be scaled to several roundabouts with varying geometrics and traffic to diversify the data set necessary to update some of the values developed during the past decade before roundabouts were common in the United States.