Journal of Advanced Transportation, Vol. 31, No. 1, pp, 95-109 Special Provisions for Left Turns at Signalized Intersections to Increase Capacity and Safety Bu-Yong Shin Heavy left-turns at intersections are not only difficult to accommodate but also often cause traffic accidents. Such problem can be reduced by adopting an exclusive left-turn signal phase, but in this case the cycle length needs to be extended and the overall intersection capacity reduced. This problem is particularly important in such countries as Korea where unprotected left turns are rarely used. Innovative intersection designs and operational methods for avoiding these problems are presented and their effects of increasing capacity and reducing vehicle delay are analyzed. They rule out the requirement for exclusive left-turn phase while accommodating a large volume of left-turns quite safely. Why special treatment At an intersection a vehicle may go straight, turn right or left. A vehicle turning right can do so without interfering other movements, except for pedestrians. Therefore, right turns are normally allowed during all signal phases and seldom cause capacity problems. So, not much provision is required for right turns. Straight-ahead traffic of east-west bound and north-south bound intersects each other. Left-turns from any approach are unique in that they interfere with traffic in both east-west and north-south directions. Therefore, in order to facilitate a left-turn, traffic on both directions should yield or stop. Since left-turn volumes are normally smaller than that of straightahead movements, often, only one or no lane is assigned exclusively for left-turns. This means that for only one lane of left turn flow, all though lanes on three approaches should be interrupted. This will of course reduce the operation rate of the intersection. i.e. the ratio of operating lanes out of the total approach lanes. For this reason, traffic engineers Bu-Yong Shin is the Director of the Transportation and Environment Research Institute Ltd., Korea Received December 1992; la revized form April 1994.
96 B.Y. Shin a. Right turns do b. Straight-through c. Left-turn interfere not interfere with movements interfere with traffic on both other vehicular with other straight- directions. movements. through moments. Figure 1.1 Interference among directional movements at an intersection I 7 i Q a. A left-turn is made at a unsignalized intersection. b. A left-turn is made on a gap in the opposing traffic at a signalized intersection. Figure 2.1 Typical cases of unprotected left-turn.
Special Provisions for kji Turns at... 97 try to allow as little signal time as possible for left-turn movements. Unprotected left turn is one solution, but, sometimes, it is either undesirable or not applicable. Grade separation offers another solution, but this time, it can be too costly or physically difficult. Some special provisions that can accommodate left-turns requiring neither exclusive turning phase nor grade separation are introduced in this paper. Normal Practice of Treating Left-Turns There are several ways in treating left-turns. Among these the most common way is the "unprotected left-turn" in which no signal time is given for left-turns. The opposite case is adopting exclusive left-turn phase, which is a common practice in Korea and elsewhere. Unprotected Lefl-Turn A left-turn made through a gap in the opposing traffic is called "unprotected" whereas a "protected" left-turn is one that made on an exclusive left-turn signal phase. All left-turns made at unsignalized intersections are therefore unprotected left-turns. Figure 2.1 shows these cases. In order to make a unprotected left-turn at a signalized intersection, the vehicle should advance into middle of the intersection and wait for gap so that the gap on the opposing traffic stream that can be utilized swiftly. If the vehicle can not find any gap in the opposing traffic it should complete its turn during the amber period. In reality, most unprotected left-turns at a signalized intersections are made during the amber period. Normally 2 or 3 vehicles can make their left turns during an amber phase. Therefore, where a 60-second cycle is adopted, it is expected that at most 200 left turns can be made per hour per approach. However, in directionally unbalanced flow such as one seen in the morning rush hour near CBD, the traffic in one direction can be very light and offer a lot of gaps for the predominant direction. This selfadjusting left-turn service volume is a notable merit of the unprotected left-turn system. Unprotected left-turn system is not free of problems. In order to make a left turn, one has to decide whether to take or reject the gaps in the opposing traffic. This is not an easy task especially for inexperienced drivers, and sometimes causes collisions with the opposing
98 B.Y. Shin traffic. Conflicts with pedestrian crossing is also a problem. Protected LeJt-ntrn Left-turns being made with a signal phase specifically directing such movements are called "protected". Normally protected left-turn signal phase is indicated by an arrow pointing to the turning direction. Such arrow signals are given either for both opposing directions at the same time or synchronously with straight-through movements as illustrated in Figure 2.2. Problems associated with protected le#-turns To allow protected left turns for all directions at a normal 4-leg intersection, a signal cycle should into 4 phases. this will roughly halve the capacity of an intersection that adopts a 2-phase signal cycle with unprotected left-turn. The adverse effect on vehicle delay is much more serious. To illustrate the capacity and delay problems associated with 4-phase signals, let assume an imaginary intersection and a signal phase as shown in Figure 2.3. To make the presentation simple the amber period is eliminated from the illustration and the starting delay is neglected. For further simplicity, vehicles are assumed to arrive uniformly throughout the cycle and leave the stop-line at 2-second intervals. Then during a cycle of 120 seconds, 10 left-turns and 40 straight-through a. Concurrent with b. Synchronous with straightopposing direction through movements Figure 2.2. Signal phasing for protected left-turn
Special Provisions for Lefr Turn at.... 99 Figure 2.3. An Intersection operations with 2 and 4 phase signal timmg vehicles can be released from each stop-line using their green times of 20 and 40 seconds, respectively. If unprotected left-turn are allowed at the same intersection and a 60-second cycle divided into 2 phases of 30 seconds each is adopted, the
100 B.Y. Shin approach can process the above service volume with some extra capacity. Assuming, the same traffic volume as above occur, 5 left-turns and 20 straight-through vehicles or 10 vehicles per lane will arrive during a cycle time of 60 seconds. Since the through vehicles will require 20 seconds of green time, the approach will offer in average 10 seconds of gap to the opposing traffic. This means that 33% of extra capacity for through traffic has been created by adopting a 2-phase signal. This extra capacity will be used up to some degree by the left turns in the opposing traffic. Since 2 or 3 vehicles can make left turn during an amber period, the rest of left-turns (2.5 vehicles in average) will use up approximately 5 seconds out of the 10 seconds gap. Therefore, the half of the gap will be remained and the resulting surplus of capacity after accounting the left turn movements will be 16.5%. The effect on vehicle delay is also analyzed below using a simple method. As assumed above, 10 left-turns and 40 through vehicles (20 per lane) arrive uniformly per a 120-second cycle at each approach. Figure 2.4 shows how left-turn and straight-through vehicles queue up and are released at the stop line as the signal phases advance in the 2 and 4-phase signal systems. In the above presentation, which shows the 4-phase case, left turns queue up to 9 vehicles and straight-through Figure 2.4. Vehicle queuing in 2 and 4-phase operations.
Special Provisions for Lefr Turns at... 101 movements up to 14 vehicles on each lane before being released. The total vehicle delay accumulated during one hour at all four legs will amount to 68.27 vehicle-hours or 41.0 seconds per vehicle. This compares with the 2-phase case which will have maximum queue accumulation of 3 vehicles for left-turns and 5 vehicles for straightthrough movements. The total vehicle delay will be 18.25 vehicle-hours or 11.O seconds per vehicle. The above analysis illustrates clearly that a 4-phase signal system will increase the maximum queue length by more than 2.15 times and vehicle delay by almost 4 times. This fact is and being witnessed in many places in Korea where multiphase signal timing is adopted. It is obvious from this analysis that when such multiphase signal timing is replaced by a simple 2-phase system, the long queues and congestions at the intersection will be more than halved under the same traffic conditions. A simple phased signal will of course benefit pedestrians. For example a pedestrian who missed a green phase will wait for one phase before catching a green time in a 2-phase signal, but have to wait for three phases in a 4-phase system. Multi-phase signal timing is accompanied inevitably with more frequent lost times, thus reducing further intersection capacity. furthermore, it is much more difficult with multiphase signal timing to have large green bands or satisfactory progression. It is also argued that when the cycle length is long, drivers become overly anxious to pass through the intersection quickly, thus causing various trafic violations and, often, collisions. In fact in Korea, the rate of rear-end collision at intersection stop-lines are extremely high, possibly due to speeding and sudden stop near stop lines. Special Provisions for Left-Turns Several special provisions that accommodate left-turns without exclusive left-turn phase are introduced. These provisions, if successfully adopted will increase intersection capacity and reduce of vehicle delay as shown above. The principle of these provisions is similar to that of a unprotected left-turn. That is, left-turn vehicles find their haven somewhere while waiting for a turning chance. The provisions being introduced in the following find different type of havens depending on the cases.
102 B.Y. Shin ljpe 1: Signalized Rotary This method can be adopted where the intersection has wide space so that it can be redesigned as a rotary with storage areas for left-turns. The intersection depicted in Figure 3-a is located outskirts of Seoul and presents a typical case. There is no shortage of such intersections in Korea. The distance from the stop lines to the center of the intersection is almost 50 m. Presently, a cycle of 150 seconds divided into 4 phases is being employed. Extensive queue is common at all approaches. It is not difficult to judge from the analysis as given above that when this intersection is redesigned as Figure 3-b and operated in a 70-second 2-phase signal, the queue length and the vehicle delay will be halved. A capacity analysis showed that the left turn storage areas created by the proposed redesign can accommodate more than 3 times of the current left-turn volumes. In order to ensure that the left-turn queues not to lock up the intersection, vehicle detectors can be laid at key locations to control the traffic to flow smoothly. As a rule of thumb, signal timing can be redesigned to advance to the next phase if the left- I I a. Current situation b. Proposed redesign Figure 3.1. A typical example of the signalized rotary type.
Special Provisions for Ley? Turns at..,. 103 turn storage is completely filled. Dpe 2: Using side ma& as lejt-turn stomge This method utilizes the space of side roads located up-stream of the intersection as a haven of the left-turns. Figure 3.2 illustrates this system. Left-turns from the N-S approach enter the side roads during the E-W green phase and complete their turns with the following N-S green. These left-turns do not interfere with any straight-through movements nor with pedestrians. E-W green phase N-S green phase a. Left-turns enter the side road. b. Left-turns complete the turns Figure 3.2. Type 2 Intersection: Using side roads as left-turn storage. Figure 3.3. Type 2 Intersection: Accommodating left-turns from E-W Road.
104 B.Y. Shin In order to accommodate left-turns from the E-W road, the same side roads can be used. In this case the left-turners manoeuvre as shown in Figure 3.3. Naturally, good signing will be required. When turning movements are very light even one set out of the two such side roads as shown above can accommodate left-turns from all directions. This is illustrated in Figure 3.4. In this case the side roads need to have at least 4 lanes and require signalization at the junction with the main roads. Signal operations at this junction will become obviously complicated, therefore, this application should be planned with careful analysis. When this arrangement is adopted on an existing road system, all left turns will be diverted to the side roads and traffic volumes will increase substantially which might be unacceptable to the neighbourhood. The problem will be more serious where there is large volume of heavy vehicles. Figure 3.5 presents a road system in which a pair of one-way roads is included in each block, so this type of left-turn provision can be employed without causing much neighbourhood problems. In this way to make all intersections be operated by 2-phase signals even intersections formed as arterials crossing each other will not require grade separation. As can be seen in the Figure 3.2, the opening on the main road near the stop line should be kept clear of vehicles so that the left turns from the side road can pass through when they make turns. In order to make this certain, thus eliminating conflicts between the traffic on the main road and left-turns, additional signal heads should be installed at the opening. Prohibiting right-turns during red may be required to further reduce traffic conflicts. From the capacity point of view, this type of intersection should perform better than one with unprotected left-turns because the whole approach width is utilized for through movement. Left-turn capacity will be also increased because the turns are made without any conflict during the whole green period of the crossing street. This will of course eliminate the possibility of collisions between left-turns and straightthrough movement from the opposing direction. The safety of pedestrians crossing the street is also enhanced because the normal conflict with left-turns are eliminated in this provision. ope 3: Using Downstream Medians At intersections where a wide road with more than say 10 traffic
Special Provisions for Left Turns at.... 105 -q _ - - - Figure 3.4. Type 2 variation: with one set of side road lanes or where such space as shown in Figure 3.6 can be provided, leftturn phase can be eliminated and adopt a left-turn bay at down stream of the intersection. Such wide roads are also quite common in Korea. Left-turns stored at this bay can make a U-turn then a immediate right turn during the E-W green phase utilizing the wide space specially provided at the approach. Left-turns from E-W road advance with a green light and make a wide right-turn immediately followed by a U-turn to complete their leftturn manoeuvre. In this case straight-through flow should stop at the stop line adopted upstream to provide sufficient empty space for these turning movements. A separation of around 70 m between these two stop lines can provide sufficient space for this manoeuvre in most cases. The basic difference of this design from the ordinary ones is whether the left-turn bay is located at upstream or downstream of the intersection. Since this design employs 2-phase signal timing instead of 4 phase, the left-turn storage space can be reduced substantially, or more than halved as explained in Figure 2.4. Compared with operations with unprotected left-turn, turning capacity will be obviously increased and the normal left-turn conflicts will be eliminated.
106 B.Y. Shin c r I 1 C c I i Figure 3.5. Typical road network that accommodate Type 2 operations. Although some straight-through vehicles on the main road may have to stop twice before clearing from the intersection of this type, traffic signal timing can be designed so that the majority of the traffic can pass through without stopping twice. ope 4: Using LejZ-Turn Gate Where the previous types of provision cannot be adopted, yet the capacity problem is very severe. Figure 3.7 offers a method as a last resort. At this type of intersection, straight-through movements should stop at the stop line adopted upstream while left-turners advance to the front area of this stop line. A short exclusive left turn phase say less than 10 seconds will be given for these left turns to clear the intersection, then a straight-through phase follows. It is emphasized that this provision should be considered where no alternative is available. For example,
Special Provisions for Lefr Turns at.... 107 Figure 3.6. Type 3 Intersection: Using Downstream Medians Figure 3.7. Type 4 Intersection: Using Left-Turn Gate
108 B.Y. Shin if a special occasion occurs at a remote area and traffic volumes jump suddenly, this type of operation may work as a temporal measure until a more permanent solution takes place. Thorough police guide accompanied with some temporary markings and other necessary provisions will be essential. Examples of Actual Use Some intersections employing similar concepts to those introduced in this paper can be found in reality. The rotary at Hyewha-dong, Seoul, Korea, has a viaduct over the main street. The intersection under this structure is being operated as a signalized rotary showing similar vehicle movements as shown in Type 1. The author has proposed a design as shown in Figure 3.1 to the City of Seoul which has been accepted. However, this intersection will be equipped with a underpass, running east-west, while adopting the proposed Type 1 design on the surface. Operations similar to the Type 2 intersections can also be seen in Seoul around some major intersections where left-turning is prohibited. Actually, when left-turning is prohibited, it would not be difficult to find similar movements occurring naturally around the area. Type 3, "using downstream medians", has not been found, in Korea, but there are numerous places where U-turns are allowed downstream of the intersection. However, the location of such U-turn is at a quite distance from the crossing, and so the concept is different from that presented in this paper. Type 4, left-turn gate is not known to be existent in practice. However, in Germany, a "bus-gate" is employed similar to the left-turn gate, at upstream of the stop line to hold other traffic while allowing buses to proceed through the gate. Conclusion Special provisions for left-turns at signalized intersections that can accommodate large volumes of left turns without requiring an exclusive left turn phase are introduced. Although such provisions are not free of problems their performance in the capacity and vehicle delay is the same or better than that with unprotected left turn. Furthermore, by eliminating traffic conflicts which occur in unprotected left-turn, the safety for the left-turning vehicles as well as pedestrian is obviously enhanced.
Special Provisions for Left Turns at... 109 To implement these special provisions, some redesign of intersection geometries and signal system will be required and drivers will have to familiarize themselves to the new situation. Careful signing and police guidance especially in the early period of implementation will be essential. Despite these drawbacks, it appears that the effect of such provisions in reducing congestions and vehicle delay is too great to ignore, particularly, when compared with the case with 4-phase signal control. Therefore, it is recommended that consideration be given to these provisions to solve intersection capacity problems, especially in such countries as Korea, Malaysia and Israel where unprotected left-turns are not generally employed. References Shin, Bu-Yong, 1992, The Road Efficiency and Left-Turn Treatments in Korea. Journal of the Korean Society of Civil Engieers, Vol. 40, No. 5. Shin, Bu-Yong, 1992, The Effect of Left-Turn Treatments on the Intersection Capacity. Journal of the Korean Society of Civil Engineers, Vol. 40, No. 6.