## 2 Basic Concept of Integrated Waiting Area
### Organization of Traffic Flow at Intersection  
The basic design element of IWAs is the waiting area before the stop bar for left-turn, right-turn and through vehicles. The waiting area is for vehicles to utilize all the available lanes on the approach to clear the intersection, i.e., all available approach lanes can be used as through lanes, right-turn or left-turn lanes. The typical layout and traffic operation of the intersection with IWAs are shown in Figures 1 and 2. Right turns are combined with the through traffic at the rightmost lane since the influence of right-turn vehicles at the intersection is usually small [6; 7].  
Figure 2a illustrates the phase for the left-turn movement. When the pre-signal provides a green indication for the left-turn movement, the vehicles on the left-turn lanes at the approach can cross the pre-stop line and distribute to all lanes in the waiting area until the main signal lights for the left-turn turn green. They then enter the intersection and leave. At the same time, through and right-turn vehicles are held upstream by the pre-signal for those lanes. Figure 2b illustrates the same rules for the through/right-turn vehicles.  
Using all approach lanes for the integrated waiting area, as illustrated in this case (Figures 1 and 2), is ideal for fully utilizing intersection space. However, some considerations may prevent the use of all lanes for the integrated waiting area. For example, if right-turn traffic is relatively large in an intersection, the rightmost lane at the approach can be set as an exclusive right-turn lane with the right-turn special phase to control right turn traffic. Thus, leaving the right-turn flow out of the integrated waiting area, a 'partial-IWA' design can be adopted in such a situation, as shown in Figure 3d.  
Figure 3 illustrates the layout of the partial-IWA. As shown in Figure 3a, two leftmost lanes are utilized as an integrated waiting area, while rightmost lane is an exclusive right-turn lane and next to the rightmost lane is a through lane. In Figure 3b, the leftmost lane is an exclusive left-turn lane and  
Figure 2: Lane function conversion in an integrated waiting area. (**a**) Phase for the left-turn movement; (**b**) Phase for the right-turn/through movement.  
the rightmost lane is an exclusive right-turn lane. The two middle lanes are utilized as an integrated waiting area. Only one lane is utilized as an integrated area in Figure 3c.  
For example, in the design shown in Figure 3a, right-turn vehicles can utilize the rightmost lane without waiting for pre-signal indication. The same rule applies for the through movement vehicles to use the second lane on the right. The pre-signal will control the timing for left-turn/through vehicles to enter the two leftmost lanes, i.e., IWA. Though these designs in Figure 3 cannot fully engage all the approach lanes, they do provide solutions where the available lanes downstream of the intersection are limited or space does not allow several simultaneous turning movements.  
With the periodic changes of the main and pre-signal lights at the approach, the function of all lanes in the integrated waiting area can be switched between left-turn and through movement [26]. More particularly, the traffic in the integrated waiting area has undergone the process of \"entering--waiting--proceeding--clearing\".  
To make full use of the integrated waiting area at the signalized intersection, it is necessary to discuss and optimize a control method based on the general form of the integrated waiting area proposed above.