Headway or interarrival time (IAT) data of successive vehicles in free-flowing traffic ahead of work zones at six different Ohio freeways (two with two lanes in one direction, three with three lanes, and one with four lanes) for three days were measured and analyzed. Approximated cumulative IAT distributions were generated as a function of the traffic volume for each lane and relationships between traffic volumes and approximate cumulative IAT distributions were established allowing a direct conversion from hourly traffic counts to corresponding cumulative IAT distributions. Based upon the validation of these cumulative IAT distributions using IAT data collected in an independent study, this conversion method produces fairly accurate cumulative IAT distributions for selected hourly traffic volumes. A set of cumulative IAT distribution spreadsheets may be downloaded at http://webce.ent.ohiou.edu/orite/cumulativeIATdistributions.html. It was also found that the same approximated cumulative IAT distribution can be used to model and simulate the free-flowing traffic at other freeway locations in Ohio. It is not known whether or not these universal approximated IAT distributions would apply for freeways in other states.