Road sag sections cause approximately 60% of traffic congestion on Japan's highways. To help resolve this, Japan's National Institute for Land and Infrastructure Management is investigating adaptive cruise control, a technology for automatic control of following gap. In previous research, it has been hypothesized that maintenance of following gap is an effective measure for reducing congestion in road sag sections. To verify that hypothesis, we performed test runs on the Yamato sag section of the Tomei Expressway while maintaining speed and following gap between test vehicles at times of heavy traffic but before congestion began. This paper analyzes the feasibility, driver acceptance, and effects on other cars when driving in this manner, and reports the results obtained in regard to alleviating congestion in a traffic simulator.