Traffic merge on express-ways creates bottlenecks for traffic flow that potentially lead to traffic jams, especially in dense traffic. Cooperative speed harmonization (CSH), where vehicles are grouped and associated to virtual speed waves for group-wise joining at intersections, is proven to be efficient for on-ramp traffic merge. Based on CSH and considering variations of traffic density from joining roads, an adaptive wavelength adjustment (AWLA) mechanism is proposed in this paper. AWLA extends CSH by dynamically adjusting the length of segments in virtual waves according to the traffic densities of the joining roads. Therefore, roads with denser traffic may have larger segments to carry more vehicles through the intersection, which may improve the overall performance. Simulation results have shown that AWLA can achieve lower CO2 emissions and shorter travel time compared to CSH with static segment settings.