To estimate the capacity of roundabouts more accurately, the priority rank of each stream is determined through the classification technique given in the Highway Capacity Manual 2010 (HCM2010), which is based on macroscopical analysis of the relationship between entry flow and circulating flow. maximum possible throughput under predetermined conditions, and the capacity of a roundabout provides the premise and foundations for traffic quality research at roundabout intersections. Nowadays, the capacity models of roundabouts divide into two main categories: the regression analysis model and the gap acceptance model. The former model is typically established through the regression relationship between the entry capacity and the conflict volume, relying on field data [1]. Meanwhile, the latter model is built by analyzing the entry capacity by means of gap acceptance theory [2, 3]. The former relies on large amounts of local data, leading to poor transferability [4]. Meanwhile, the latter can depict the complex relationships among traffic streams through gap acceptance theory but becomes more complicated and has worse applicability under certain traffic conditions (e.g., with limited priority or priority reversal) [5]. Brilon and Wu [5] solved the capacity at two-way-stop-controlled (TWSC) intersections via the additive conflict flow (ACF) method, which originated from conflict theory, and the results showed that the method reflects the actual traffic conditions better under high volumes. Ma et al. [6] also used this technique to analyze the capacity of T-type unsignalized intersections. Since the driving rule of circulating vehicle priority applies at modern roundabouts, the circulating vehicles have priority over the entering vehicles, which is similar to the priority control at major roads. Considering the structural characteristics of roundabouts, they are actually equivalent to a combination of N T-type intersections, which have priority and unidirectional control on the major road. Moreover, the T-type intersections interact with the upstream and downstream approaches and do not stand alone. Thus, combining the operating characteristics of roundabouts with the ACF method, the entry capacity of a roundabout can be determined in this paper. In this paper, based on an analysis of the relationships of traffic flow characteristics for the two different types of roundabout and using the ACF method derived from conflict theory, the relationship of streams in a conflict zone is modeled using probability theory. Then, the entry capacity model is derived, and sensitivity analysis is conducted on the key model parameters. On this basis, the entrance delay model is built using queuing theory. In addition, the model proposed in this paper is compared with other classic models, namely, the Wu model, the recommended procedures from the Highway Capacity Manual 2010 (HCM2010), and the recommended practical model in HCM2010. This provides a ADAM8 preliminary reference for traffic management and control in roundabout systems. 2. Traffic Operations Analysis at Roundabouts Differences in geometric characteristics lead to different operating characteristics on single and double lane roundabouts, leading to their capacities usually being analyzed separately [7C11]. Selecting the four-leg roundabout as the research object in this paper, the correlation between the entry flow and the circulating flow is analyzed for the single and double lane roundabouts, respectively. The two types of roundabout are defined as A-type and B-type, respectively, buy 444722-95-6 and the geometric diagrams are shown in Figure 1. In order to analyze the traffic streams more clearly, following the definition mode in HCM2010 [12], the entry flow is divided into three movements according to the pattern (i.e., left turn, through, and right turn). Assuming that U-turn is not allowed in this study, we define some parameters as follows: ?: the flow rate of (veh/h); { {(veh/h);? is a function of the conflict flow rate (i.e., = = = < > = 0.95, this entry approach will tend toward instability [13], where is the degree of saturation of approach {is the flow rates of buy 444722-95-6 approach is the capacity of approach and are the probabilities of through vehicles entering the inner and the outer circulatory buy 444722-95-6 lanes, respectively, then + = 1. Then, for the entry.