Speed-density relation model of congested traffic flow under minimum safety distance constraint

Based on the conforming change characteristic of speed and density for traffic flow under traffic congestion state, the reason that the average headway-distance of congested traffic flow was a fixed value was analyzed, and the speed-density relation model under traffic congestion state was put out combined with minimum safety distance constraint. The relation between the average reaction time of driver and the minimum headway-distance under traffic congestion state was discussed, and the parameter calibration of reaction time for speed-density relation model was carried out. The speed-density relations with different vehicle lengths, blocking parking distances and reaction times were analyzed, the traffic data of US-101 Highway and I-80 Highway in America were fitted by using the proposed speed-density relation model, Greenshields model, Greenberg model, Underwood model, Northwest model and Edie model respectively, and the fitting results and absolute errors were obtained. Analysis result shows that by using the proposed speed-density relation model, the change relation of speed and density under traffic congestion state can be explained in theory, and the discrete phenomena of speed-density data can be described. Compared with the other models, the absolute errors of fitting results for US-101 Highway and I-80 Highway are minimum by using the proposed speed-density relation model, and are 4.91, 7.50 veh·km^-1 respectively. So the proposed speed-density relation model based on minimum safety distance constrain reveals the substantive characteristic of congested traffic flow, and the fitting effect on actual data is better. 1 tab, 12 figs, 24 refs.