# The Effect of the Number of Right-Turn and Left-Turn Lanes on the Performance of Undersaturated Signalized Intersections

Authors: Omid Rahmani; Amir Saman Abdollahzadeh Nasiri; Iman Aghayan
Journal: Journal of Advanced Transportation
 (2023)

---

## Abstract

Vehicle traffic flow channelizing can significantly contribute to fewer collisions and smooth traffic flow in isolated signalized intersection design. Therefore, this study investigates the effect of changes in the number of right-turn lanes with splitter islands and left-turn storage bays for different volume approaches on intersection performance. This study aims to provide an accurate analysis to estimate the effect of such changes on signalized intersections, which can improve intersection performance. Furthermore, the delay parameter derived from HCM2016 and simulation was evaluated for different scenarios. For this purpose, simulation was performed using Synchro software. In this study, a symmetrical, 90-degree, undersaturated (volume per capacity lower than 1, V/C < 1), and four-leg intersection were considered according to AASHTO Green Book 2018 suggestions. HCM2016 results indicated the delay parameter was less sensitive to the undersaturated condition than the right-turn volume variation for both single and dual-lane bays. However, the simulation results indicated that the delay parameter was not constant, depending on the number of right-turn lanes and volumes. On the other hand, there was a significant difference in delay parameters between the single and dual left-turn lanes for both simulation and HCM cases estimated 77.4% and 59.7%, respectively. Overall, this study can provide a vision for traffic engineers to modify the geometry of the four-leg signalized intersections, if the right-turn or left-turn demand volume of an undersaturated signalized intersection is larger than the through approach.


---

## Body

## 1. Introduction

The intersection capacity plays an important role in determining the traffic capacity of an urban road network. Regarding their heavy traffic, signalized intersections have been the focus of many studies because a large portion of a vehicle’s lost time is related to signalized intersections. The main methods to reduce delays and improve the intersection capacity are to optimize the traffic light timing and modify the geometric characteristics of the intersections [1].Safety and capacity play the most important role in the geometric design of an isolated signalized intersection [2, 3]. Traffic signal timing and phasing optimization without changing the geometric characteristics of the intersection are possible to limited values. Therefore, geometric modification of the intersection is considered when it is not feasible to reduce the delay time of a heavy traffic intersection through traffic light timing and phasing. Right-turn splitter islands and left-turn storage bays can be alternative geometric characteristics to improve the signalized intersections. These techniques can significantly reduce collisions and contribute to smooth traffic flow [4, 5].Traffic flow has been less considered as a statistical criterion to change left- and right-turn volumes [6]. If accurate predictions can be made to estimate the effect of geometric modifications on left and right turns, a better vision can be outlined for the design of signalized intersections before saturation. The geometric parameters for the design of the left and right turns in a signalized intersection include bay width, channelized right-turn radius, right-turn lane length, left-turn storage length, taper length, and the number of lanes [7]. Most studies have considered right-turn lane length and left-turn storage length, while the increased number of lanes and considering the storage or deceleration length can prevent lane blockage for incremental turning volume of the intersections [8, 9].The majority of studies have been dedicated to oversaturated (i.e., V/C ≥ 1) or saturated signalized intersections [10]. Signalized intersection phasing and timing can be a complex task in some cases due to the saturated incoming intersection traffic [11]. It is necessary to avoid saturation as much as possible while designing and selecting the geometric alternatives of an intersection.According to the Highway Capacity Manual (HCM) 2016 regulation, the delay is the main criterion (measure of effectiveness (MOE)) for at-grade intersection efficiency evaluation [12]. Based on HCM2016, the delay calculation methodology for undersaturated signalized intersections is less sensitive to changing the number of channelized right-turn lanes. In other words, the delay remained constant for right-turn overdemand and fixed through approach volume. Also, the extent to which the changing number of bays affects the intersection delay is unknown.Therefore, this study seeks to evaluate the effect of the right-turn lane and left-turn storage bay quantity on the capacity of an undersaturated signalized intersection. To this end, a symmetrical 90-degree four-leg signalized intersection without longitudinal grade was designed based on the AASHTO guideline [13]. The effect of the number of lanes on the delay parameter was compared to the HCM2016 model. The traffic and geometric characteristics of the undersaturated isolated signalized intersection were modeled and analyzed by Synchro11 software. The analyses were based on the delay and volume per capacity (V/C) ratio.


## 2. Literature Review

Previous studies have focused on evaluating the performance of signalized intersections with left- and right-turn storage bays and channelized right-turn lanes in terms of delay, V/C, and queue length. In general, the research regarding the capacity of signalized intersections with storage bays can be divided into two categories: (i) the phasing and timing of signalized intersections and (ii) the effect of geometric characteristics on the capacity of signalized intersections.For the first category, it was assumed that the capacity of an isolated signalized intersection was improved by phasing and timing optimization [11, 14–16]. In the second category, the intersection throughput was improved by changing the geometric characteristics of the left and right turns, such as right-turn channelization and increased left- or right-turn storage length [17–20]. The current study investigates an undersaturated isolated signalized intersection considering the geometric characteristics (i.e., number of lanes) of channelized right-turn approaches and the storage length of the left-turn approach.A summary of some previous studies related to signalized intersections is shown in Table1. Based on the movement in different signalized intersection approaches, the intersection assessment method (probability, simulation, or both), traffic flow parameters (queue length, delay, and V/C), geometric parameters, and also channelized right-turn movement were reviewed. The HCM-based studies are also presented. These studies propose a model or process to complement the HCM methodology or compensate for its weaknesses in signalized intersections. The literature shown in Table 1 is categorized based on the movement, including (1) left-turn lane, (2) right-turn lane, (3) right turn, left turn, and through, and (4) right turn, left turn, through, and U-turn.Table 1
Studies related to signalized intersections.
Study (year)Movement typeHCM checkedEvaluation methodsTraffic parametersProposed modelGeometric parametersRight-turn channelizedSimulationProbabilityQueueDelayv/cKikuchi et al. (2004) [2]Left-turn lane——✓✓———Length of left-turn lane—Qi et al. (2007) [21]——✓✓——✓Length of left-turn lane—Yin et al. (2010) [22]✓CORSIM✓✓——✓Length of left-turn lane—Kikuchi and Kronprasert (2010) [23]—Monte Carlo method and VISSIM✓✓——✓Length of left-turn lane—Reynolds et al. (2010) [24]✓VISSIM—✓—✓✓Length of left-turn lane—DYNASMARTGuo et al. (2011) [8]——✓——✓✓Length of left-turn lane—Yin et al. (2011) [25]—VISSIM✓✓✓—✓Length of left-turn lane—Qi et al. (2012) [26]✓HSC+—✓———Length of left-turn lane—SynchroVISSIMSimTrafficYao and Michael Zhang (2013) [11]✓VISSIM—✓✓—✓Length of left-turn lane—Yao (2016) [27]✓—✓—✓✓✓Length of left-turn lane—Ma et al. (2017) [28]✓VISSIM✓——✓✓Number of left-turn lane in waiting area—Liu et al. (2018) [3]✓VISSIM✓——✓✓Length of left-turn lane—Zheng et al. (2020) [18]✓VISSIM—✓✓—✓Length of left-turn lane—Guo et al. (2021) [15]—VISSIM—✓———Length of left-turn lane—Kikuchi & Kronprasert (2008) [29]Right-turn lane——✓✓———Length of right-turn lane—Macfarlane et al. (2011) [30]✓Synchro (SimTraffic)——✓—✓——Chen et al. (2012) [31]✓VISSIM✓—✓✓✓——Chen et al. (2013) [32]✓—✓———✓—✓Farivar et al. (2016) [9]✓VISSIM✓—✓—✓Length of right-turn lane✓Farivar et al. (2017) [9]✓VISSIM✓——✓✓Length of right-turn lane✓Li et al. (2019) [33]✓VISSIM———✓✓—✓Kikuchi et al. (2007) [34]Right turn, left turn, and through✓VISSIM✓✓———Length of right-turn and left-turn lane✓Li and Elefteriadou (2013) [17]✓CORSIM—✓✓————Shatnawi et al. (2018) [19]✓VISSIM—✓✓————Table1 indicates that the geometric parameter evaluations aimed to improve the capacity of signalized intersections were focused on left- and right-turn storage bays. The simultaneous effect of geometric and traffic parameter variations, including the number of right- and left-turn lanes and their traffic demand volume, was not included. These studies were carried out for near-saturated or oversaturated signalized intersections. If the left- and right-turn volume of a signalized intersection increases, it is important to recognize its limitations and maintain the intersection in an undersaturated state.The first novelty of this study is to evaluate the effect of the number of left- and right-turn lanes with storage length on the undersaturated signalized intersection capacity. The assessment was performed for traffic volume scenarios of variant left- and right-turn approaches and constant through approach volume. Thus, the combined effect of the number of lanes and traffic flow (left- and right-turn volume) was investigated. HCM assumes a constant delay for the unsaturated state up to a certain right-turn volume. Therefore, another novelty of the study is to evaluate the delay and V/C variations as the main criteria to determine the signalized intersection capacity relative to HCM.


## 3. Methodology

Synchro traffic simulator was employed to define the geometric characteristics, traffic volumes, traffic light timing, and delay. Many studies regarding traffic in at-grade signalized intersections used Synchro [35–40]. By considering different HCM criteria [41, 42], Synchro was selected because it contains the sixth edition of HCM (HCM2016). The average control delay (d) for each lane is based on the following formula:(1)d=3600c+900Tx−1+x−12+3600/cx450T+5×minx,1,where x = volume-to-capacity ratio of each lane, c = capacity of the subject lane (veh/h), and T = time period (h) = 0.25 (h).The control delay for an approach is calculated using the weighted average of the delay for each lane on the approach, weighted by the volume in each lane. The calculation is demonstrated in(2)da=∑divi∑vi,where da = control delay for the approach (s/veh), di=control delay for lane i (s/veh), and vi = flow rate for lane i (veh/h).The control delay for the intersection as a whole is similarly calculated by computing a weighted average of the delay for each approach and weighted by the volume on each approach which is shown in the following equation:(3)dintersection=∑dava∑va,where dintersection = control delay for the entire intersection (s/veh), da  = control delay for approach a (s/veh), and va=flow rate for approach a (veh/h).Figure1 summarizes the defined inputs and outputs for simulation and the methodology details. The drivers’ behavior in terms of gaps was selected based on AASHTO 2018 selections. To this end, critical gaps and follow-up gaps were accounted for in the simulations. The results were used to compare the delay obtained by the simulation and HCM. The volume per capacity (V/C) obtained by the simulation was evaluated for each scenario to control whether the incoming right- and left-turn volumes were in an unsaturated state (V/C < 1). A single and dual-lane method was adopted to compare the right- and left-turn movements.Figure 1
Research method in detail.


### 3.1. Geometric Design of Intersections

For all designed intersections, the intersections were 90 degrees without a longitudinal slope. The radius of the channelized right-turn horizontal arc edge was 15.3 m. The ideal lane width was 3.65 m. The length of the left-turn storage bay was based on NCHRP 780 [43], which is suggested by the latest version of the AASHTO Green Book (2018). To this end, the left-turn storage bay was considered 30.5 m long because the through movement volume of the opposite traffic flow was 400 Veh/h. However, since 2% of the total incoming traffic was associated with heavy vehicles, 7.6 m was added to the left-turn storage bay length, according to AASHTO 2018, leading to a 38.1 m long left-turn storage bay.Figures2(a) and 2(b) show the length of the right-turn bays and left-turn storage length after the geometric modifications. All the scenarios were considered in separate phasing. First, the traffic light optimization was carried out by optimizing the intersection cycle length, green light timing for each route, and approaches. The optimization was performed by the Synchro simulator. Then, the total delay of the intersection was evaluated. The undersaturated state (V/C < 1) was the criterion for selecting the defined light or heavy traffic volumes(e.g., Figures 3 and 4). In addition to one right- and left-turn lane, dual right- and left-turn lanes were also addressed in the current study.Figure 2
Geometric design elements for turning movement at an intersection: (a) single right-turn lane and (b) single left-turn lane.
(a)(b)Figure 3
Sample of simulated scenarios in Synchro: (a) dual left-turn lanes, (b) dual right-turn lanes, and (c) single right- and left-turn lanes.
(a)(b)(c)Figure 4
Simulation of intersection in Synchro (3D).


### 3.2. Traffic Demand Characteristics at the Intersection

Two general volumetric cases with V/C < 1 were considered to define the scenarios. In the first case, the through and left-turn approach volumes were fixed (400 and 150 Veh/h, respectively). The volume growth rate was defined as 50 Veh/h for each new scenario for right-turn movements. The single right-turn and dual right-turn lane volumes were in the range of 100–800 and 200–1, 450 Veh/h, respectively. In the second case, the through and right-turn approach volumes were fixed (400 and 150 Veh/h, respectively). The volume growth rate was defined as 50 Veh/h for each new scenario for left-turn movements. The single left-turn lane and dual left-turn lane volumes were in the range of 50–400 and 200–1, 450 Veh/h, respectively (Table2). After applying the passing volume, the traffic light cycle length was optimized by Synchro. Figure 3 shows the intersection plans for dual left-turn lanes (3a), dual right-turn lanes (3b), and single left- and right-turn lanes (3c). Figure 4 depicts the 3D view of the simulation of a scenario.Table 2
Summary of scenarios, traffic demand volume, and outputs.
Type of turnNumber of lanesThrough traffic volume (Veh/h)Right-turn traffic volume (Veh/h)Left-turn traffic volume (Veh/h)OutputsRight-turnSingle lane400100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800150Delay and volume to capacity (V/C)Dual lane400200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, 1000, 1050, 1100, 1150, 1200, 1250, 1300, 1350, 1400, 1450150Left-turnSingle lane40015050, 100, 150, 200, 250, 300, 350, 400Dual lane400150100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700Figure 5
The effect of volume variations on delay for a single right-turn lane.


## 3.1. Geometric Design of Intersections

For all designed intersections, the intersections were 90 degrees without a longitudinal slope. The radius of the channelized right-turn horizontal arc edge was 15.3 m. The ideal lane width was 3.65 m. The length of the left-turn storage bay was based on NCHRP 780 [43], which is suggested by the latest version of the AASHTO Green Book (2018). To this end, the left-turn storage bay was considered 30.5 m long because the through movement volume of the opposite traffic flow was 400 Veh/h. However, since 2% of the total incoming traffic was associated with heavy vehicles, 7.6 m was added to the left-turn storage bay length, according to AASHTO 2018, leading to a 38.1 m long left-turn storage bay.Figures2(a) and 2(b) show the length of the right-turn bays and left-turn storage length after the geometric modifications. All the scenarios were considered in separate phasing. First, the traffic light optimization was carried out by optimizing the intersection cycle length, green light timing for each route, and approaches. The optimization was performed by the Synchro simulator. Then, the total delay of the intersection was evaluated. The undersaturated state (V/C < 1) was the criterion for selecting the defined light or heavy traffic volumes(e.g., Figures 3 and 4). In addition to one right- and left-turn lane, dual right- and left-turn lanes were also addressed in the current study.Figure 2
Geometric design elements for turning movement at an intersection: (a) single right-turn lane and (b) single left-turn lane.
(a)(b)Figure 3
Sample of simulated scenarios in Synchro: (a) dual left-turn lanes, (b) dual right-turn lanes, and (c) single right- and left-turn lanes.
(a)(b)(c)Figure 4
Simulation of intersection in Synchro (3D).


## 3.2. Traffic Demand Characteristics at the Intersection

Two general volumetric cases with V/C < 1 were considered to define the scenarios. In the first case, the through and left-turn approach volumes were fixed (400 and 150 Veh/h, respectively). The volume growth rate was defined as 50 Veh/h for each new scenario for right-turn movements. The single right-turn and dual right-turn lane volumes were in the range of 100–800 and 200–1, 450 Veh/h, respectively. In the second case, the through and right-turn approach volumes were fixed (400 and 150 Veh/h, respectively). The volume growth rate was defined as 50 Veh/h for each new scenario for left-turn movements. The single left-turn lane and dual left-turn lane volumes were in the range of 50–400 and 200–1, 450 Veh/h, respectively (Table2). After applying the passing volume, the traffic light cycle length was optimized by Synchro. Figure 3 shows the intersection plans for dual left-turn lanes (3a), dual right-turn lanes (3b), and single left- and right-turn lanes (3c). Figure 4 depicts the 3D view of the simulation of a scenario.Table 2
Summary of scenarios, traffic demand volume, and outputs.
Type of turnNumber of lanesThrough traffic volume (Veh/h)Right-turn traffic volume (Veh/h)Left-turn traffic volume (Veh/h)OutputsRight-turnSingle lane400100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800150Delay and volume to capacity (V/C)Dual lane400200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, 1000, 1050, 1100, 1150, 1200, 1250, 1300, 1350, 1400, 1450150Left-turnSingle lane40015050, 100, 150, 200, 250, 300, 350, 400Dual lane400150100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700Figure 5
The effect of volume variations on delay for a single right-turn lane.


## 4. Results’ Analysis and Discussion

After the scenario simulation, the results were analyzed and compared. The results were estimated for a single right-turn lane, a single left-turn lane, a dual right-turn lane, and a dual left-turn lane. In addition, the results were presented in the form of volume-delay control and V/C curves to analyze the intersection performance and compare different scenarios. The scenarios were simulated for unsaturated traffic using Synchro, and the results were compared to HCM2016. Thus, the following results were obtained.


### 4.1. Single Right-Turn Lane

The through and left-turn volumes were assumed constant for all directions (i.e., 400 and 150 Veh/h). The right-turn volume varied in the range of 100–800 Veh/h with an incremental step of 50 Veh/h. The simulation results showed that the delay parameter initially decreased for the traffic volume up to 300 Veh/h (Figure5). For higher volumes, the delay parameter increased. HCM2016 result indicated that the delay was independent of the incoming volume below 500 Veh/h. As the volume increased from 500 to 600 Veh/h, the delay increased, and for higher volumes, the amount of delay increased significantly. For the traffic volume of 600 Veh/h, the delay reported by our simulations and HCM2016 was equal. For over 600 (Veh/h), the delay of both HCM and simulation increases because the delay of the single right-turn lane is sensitive to the incoming volume increase. In fact, the volume of 600 Veh/h was the threshold for a sudden change in delay. As the volume increased from 600 to 800 Veh/h, the growing trend of the delay in the simulation was more drastic than that of HCM2016, indicating that HCM2016 is a more conservative method for delay estimation. For instance, the delay parameter obtained by simulations was 11% higher at 650 Veh/h, while the difference was 26.7% at the volume of 800 Veh/h.In addition, the difference between the simulation and HCM2016 control delay results was verified statistically using a two-sampleF-test for variances method in a single right-turn lane. According to Table 3, the results demonstrate that there is a significant difference between simulation and HCM2016 in control delay results at a 95% confidence level (P value <0.05).Table 3
F-test performed between simulation and HCM2016.
MethodVarianceFP valueF criticalSimulation21.715.990.0012.48HCM20163.62


### 4.2. Single Left-Turn Lane

The through and right-turn volumes were considered constant for all directions (i.e., 400 and 150 Veh/h). The left-turn volume varied in the range of 100–400 Veh/h with an incremental step of 50 Veh/h. Figure6 shows that the intersection delay increased as the left-turn volume grew. This is true for both simulation and HCM2016 results. Comparing the results of simulation software and HCM2016 showed that the intersection delay difference was neglectable for the traffic volumes of 100–350 Veh/h. However, the difference became 10% and 34.6% for the volumes of 350 and 400 Veh/h, respectively. Thus, the volume of 350 Veh/h was the threshold for a sudden change in delay.Figure 6
The effect of volume variations on delay for single left-turn lane.


### 4.3. Dual Right-Turn Lanes

Figure7 shows the effect of the number of right-turn lanes on the dual-lane intersection delay. The right-turn volume varied between 200 and 1,450 Veh/h with an incremental step of 50 Veh/h. The simulation results showed that the delay parameter initially decreased for the traffic volume up to 550 Veh/h. For higher volumes, the delay parameter increased. There was a 39.8% difference between the simulation results and HCM2016 at the volume of 550 Veh/h. HCM2016 results indicated that the delay was independent of the incoming volume below 950 Veh/h. The delay increased as the volume increased from 950 to 1,450 Veh/h. For the traffic volume of 1,200 Veh/h, the delay reported by the simulation and HCM2016 was the same. As the volume increased from 1,200 to 1,450 Veh/h, the growing trend of the delay in the simulation was more than that of HCM2016. For instance, the delay parameter obtained by simulation was 7.1% higher at the volume of 1,250 Veh/h, while the difference was 23.2% at the volume of 1,450 Veh/h.Figure 7
The effect of volume variations on delay for dual right-turn lane.As mentioned in section4.1, the difference between the simulation and HCM2016 control delay results was verified statistically using a two-sample F-test for variances method in dual right-turn lane. Table 4 shows a significant difference between simulation and HCM2016 in control delay results at a 95% confidence level (P value <0.05).Table 4
F-test performed between simulation and HCM2016.
MethodsVarianceFP valueF criticalSimulation19.446.040.0011.95HCM20163.21


### 4.4. Dual Left-Turn Lanes

The through and right-turn volumes were considered constant for all directions (i.e., 400 and 150 Veh/h). The left-turn volume varied between 100 and 700 Veh/h with an incremental step of 50 Veh/h. Figure8 shows that the intersection delay increased as the left-turn volume grew. This is true for both simulation and HCM2016 results. Comparing simulation and HCM2016 results indicated that the intersection delay difference was 17% for the traffic volumes of 100–400 Veh/h. However, the difference decreased at the range of 450–650 Veh/h, and for the volumes of 550, 600, and 650 Veh/h, the difference became insignificant. For the traffic volume of 700 Veh/h, the difference between the simulation results and HCM2016 reached 11.4%.Figure 8
The effect of volume variations on delay for dual left-turn lane.


### 4.5. Comparison of Single and Dual Lanes

Figure9 shows the effect of the number of right-turn lanes on the intersection delay estimated by the simulation and HCM2016. As the number of right-turn lanes increased to two, the HCM2016 did not estimate any changes in the intersection delay for the single right-turn lane with a volume below 550 Veh/h and dual right-turn lanes with a volume below 1,000 Veh/h. On the other hand, the difference between the delays reported by HCM2016 was neglectable for single and dual right-turn lanes at traffic volumes below 500 Veh/h. However, the simulation results indicated that the delay was reduced for dual right-turn lanes with volumes in the range of 100–550 Veh/h. For higher volumes up to 1,450 Veh/h, the delay increased. This was the same for the single right-turn lane. The only difference was that the delay parameter decreased in the range of 100–300 Veh/h and then increased for higher volumes. The simulation results showed that in the range of 100–300 Veh/h, the delay associated with the single and dual lanes was equal. A significant difference was observed for volumes higher than 300 Veh/h. For a single right-turn lane, the delay was 23 s for the traffic volume of 800 Veh/h. At 1,450 Veh/h, the delay was 23 s for the dual right-turn lanes.Figure 9
Delay variations of the effect of changing traffic scenarios in single and dual right-turn lanes.The traffic scenarios of signalized intersections were analyzed for different V/C values with a maximum of 1. Figure10 shows the simulation results of the single and dual right-turn lanes. As the volume increased from 100 to 300 Veh/h, V/C remained fixed at 0.39 for the single right-turn lane. For traffic volumes in the range of 300–800 Veh/h, V/C suddenly increased to 0.89. For the dual right-turn lanes, V/C was constantly equal to 0.39 for the traffic volume of 100 to 600 Veh/h. For higher volumes, V/C gradually increased and reached 0.91 at 1,450 Veh/h. Comparing the single and dual right-turn lanes suggested that, similar to the delay parameter, V/C converged faster to 1 at low traffic volumes when using a single lane, but for large traffic volumes, the convergence speed was higher when using dual lanes. This is due to the effect of the number of right-turn lanes. For example, for V/C = 0.89, the volume was 800 Veh/h for a single lane and 1,400 Veh/h for dual lanes (i.e., 75% higher traffic volume).Figure 10
Comparison of changing a single and dual right-turn lane in volume to capacity (v/c) ratio.Figure11 illustrates that by increasing the number of left-turn lanes to two, the delay was 20 s for a single left-turn lane at 350 Veh/h, while the same delay was obtained by the dual left-turn lanes at 700 Veh/h, according to HCM2016. The simulation results indicated that increasing the number of left-turn lanes led to a delay reduction from 45.6 to 10.3 s for the left-turn volume of 400 Veh/h. This shows that even for smaller volumes (around 400 Veh/h) in which the signalized intersection was unsaturated, there was a significant difference in delay parameter between the single and dual left-turn lanes in both simulation results and HCM, respectively, 77.4% and 59.7%. Therefore, the difference can be attributed to the number of lanes.Figure 11
Delay variations of the effect of changing traffic scenarios in a single and dual left-turn lane.Figure12 depicts the simulation results of the single and dual left-turn lanes. As the left-turn volume increased from 50 to 100 Veh/h, V/C remained constant at 0.31 for the single right-turn lane. For traffic volumes in the range of 100–400 Veh/h, V/C increased to 0.98. For the dual right-turn lanes, V/C was constantly equal to 0.28 for the traffic volume of 50 to 200 Veh/h. For higher volumes, V/C gradually increased and reached 0.89 at 700 Veh/h. Comparing the single and dual left-turn lanes suggested that, similar to the delay parameter, V/C converged faster to 1 at low traffic volumes when using a single lane, but for large traffic volumes, the convergence speed was higher when using dual lanes. For example, for V/C = 0.86, the volume was 350 Veh/h for a single lane and 700 Veh/h for dual lanes (i.e., 100% higher traffic volume).Figure 12
Comparison of changing single to dual left-turn lane on volume to capacity (v/c) ratio.The difference between the single and dual lanes (both right and left turn) results was verified statistically using the two-samplet-test method. Table 5 shows a significant difference between single and dual lanes in volume results at a 95% confidence level (P value <0.05).Table 5
t-test performed between the single and dual lanes.
Turn typeMethodt statP valuet CriticalRight-turnSimulation and HCM2016−3.960.0012.02Left-turnSimulation and HCM2016−2.210.032.1AASHTO 2018 and NCHRP 780 provide recommendations based on driving behavior and traffic parameters for the length of right- and left-turn lanes in a given condition of geometric elements of at-grade signalized intersections. Also, previous studies conducted in this field have examined the length of the lane in different turning movements of an approach (see Table1). In contrast, the result analysis of this research shows that the number of traffic lanes can significantly impact increasing the throughput of intersections. In this study, we have focused on the effect of the number of turning lanes on the volume changes of right turn and left turn. Analyses in undersaturated conditions (V/C < 1) showed that the HCM2016 delay model is not sensitive to volume changes, especially in right-turning movements, up to a specific value. While much of the research reviewed in the background has been done for V/C = 1 or V/C > 1, this insensitivity of HCM is not seen in saturated and oversaturated conditions.


## 4.1. Single Right-Turn Lane

The through and left-turn volumes were assumed constant for all directions (i.e., 400 and 150 Veh/h). The right-turn volume varied in the range of 100–800 Veh/h with an incremental step of 50 Veh/h. The simulation results showed that the delay parameter initially decreased for the traffic volume up to 300 Veh/h (Figure5). For higher volumes, the delay parameter increased. HCM2016 result indicated that the delay was independent of the incoming volume below 500 Veh/h. As the volume increased from 500 to 600 Veh/h, the delay increased, and for higher volumes, the amount of delay increased significantly. For the traffic volume of 600 Veh/h, the delay reported by our simulations and HCM2016 was equal. For over 600 (Veh/h), the delay of both HCM and simulation increases because the delay of the single right-turn lane is sensitive to the incoming volume increase. In fact, the volume of 600 Veh/h was the threshold for a sudden change in delay. As the volume increased from 600 to 800 Veh/h, the growing trend of the delay in the simulation was more drastic than that of HCM2016, indicating that HCM2016 is a more conservative method for delay estimation. For instance, the delay parameter obtained by simulations was 11% higher at 650 Veh/h, while the difference was 26.7% at the volume of 800 Veh/h.In addition, the difference between the simulation and HCM2016 control delay results was verified statistically using a two-sampleF-test for variances method in a single right-turn lane. According to Table 3, the results demonstrate that there is a significant difference between simulation and HCM2016 in control delay results at a 95% confidence level (P value <0.05).Table 3
F-test performed between simulation and HCM2016.
MethodVarianceFP valueF criticalSimulation21.715.990.0012.48HCM20163.62


## 4.2. Single Left-Turn Lane

The through and right-turn volumes were considered constant for all directions (i.e., 400 and 150 Veh/h). The left-turn volume varied in the range of 100–400 Veh/h with an incremental step of 50 Veh/h. Figure6 shows that the intersection delay increased as the left-turn volume grew. This is true for both simulation and HCM2016 results. Comparing the results of simulation software and HCM2016 showed that the intersection delay difference was neglectable for the traffic volumes of 100–350 Veh/h. However, the difference became 10% and 34.6% for the volumes of 350 and 400 Veh/h, respectively. Thus, the volume of 350 Veh/h was the threshold for a sudden change in delay.Figure 6
The effect of volume variations on delay for single left-turn lane.


## 4.3. Dual Right-Turn Lanes

Figure7 shows the effect of the number of right-turn lanes on the dual-lane intersection delay. The right-turn volume varied between 200 and 1,450 Veh/h with an incremental step of 50 Veh/h. The simulation results showed that the delay parameter initially decreased for the traffic volume up to 550 Veh/h. For higher volumes, the delay parameter increased. There was a 39.8% difference between the simulation results and HCM2016 at the volume of 550 Veh/h. HCM2016 results indicated that the delay was independent of the incoming volume below 950 Veh/h. The delay increased as the volume increased from 950 to 1,450 Veh/h. For the traffic volume of 1,200 Veh/h, the delay reported by the simulation and HCM2016 was the same. As the volume increased from 1,200 to 1,450 Veh/h, the growing trend of the delay in the simulation was more than that of HCM2016. For instance, the delay parameter obtained by simulation was 7.1% higher at the volume of 1,250 Veh/h, while the difference was 23.2% at the volume of 1,450 Veh/h.Figure 7
The effect of volume variations on delay for dual right-turn lane.As mentioned in section4.1, the difference between the simulation and HCM2016 control delay results was verified statistically using a two-sample F-test for variances method in dual right-turn lane. Table 4 shows a significant difference between simulation and HCM2016 in control delay results at a 95% confidence level (P value <0.05).Table 4
F-test performed between simulation and HCM2016.
MethodsVarianceFP valueF criticalSimulation19.446.040.0011.95HCM20163.21


## 4.4. Dual Left-Turn Lanes

The through and right-turn volumes were considered constant for all directions (i.e., 400 and 150 Veh/h). The left-turn volume varied between 100 and 700 Veh/h with an incremental step of 50 Veh/h. Figure8 shows that the intersection delay increased as the left-turn volume grew. This is true for both simulation and HCM2016 results. Comparing simulation and HCM2016 results indicated that the intersection delay difference was 17% for the traffic volumes of 100–400 Veh/h. However, the difference decreased at the range of 450–650 Veh/h, and for the volumes of 550, 600, and 650 Veh/h, the difference became insignificant. For the traffic volume of 700 Veh/h, the difference between the simulation results and HCM2016 reached 11.4%.Figure 8
The effect of volume variations on delay for dual left-turn lane.


## 4.5. Comparison of Single and Dual Lanes

Figure9 shows the effect of the number of right-turn lanes on the intersection delay estimated by the simulation and HCM2016. As the number of right-turn lanes increased to two, the HCM2016 did not estimate any changes in the intersection delay for the single right-turn lane with a volume below 550 Veh/h and dual right-turn lanes with a volume below 1,000 Veh/h. On the other hand, the difference between the delays reported by HCM2016 was neglectable for single and dual right-turn lanes at traffic volumes below 500 Veh/h. However, the simulation results indicated that the delay was reduced for dual right-turn lanes with volumes in the range of 100–550 Veh/h. For higher volumes up to 1,450 Veh/h, the delay increased. This was the same for the single right-turn lane. The only difference was that the delay parameter decreased in the range of 100–300 Veh/h and then increased for higher volumes. The simulation results showed that in the range of 100–300 Veh/h, the delay associated with the single and dual lanes was equal. A significant difference was observed for volumes higher than 300 Veh/h. For a single right-turn lane, the delay was 23 s for the traffic volume of 800 Veh/h. At 1,450 Veh/h, the delay was 23 s for the dual right-turn lanes.Figure 9
Delay variations of the effect of changing traffic scenarios in single and dual right-turn lanes.The traffic scenarios of signalized intersections were analyzed for different V/C values with a maximum of 1. Figure10 shows the simulation results of the single and dual right-turn lanes. As the volume increased from 100 to 300 Veh/h, V/C remained fixed at 0.39 for the single right-turn lane. For traffic volumes in the range of 300–800 Veh/h, V/C suddenly increased to 0.89. For the dual right-turn lanes, V/C was constantly equal to 0.39 for the traffic volume of 100 to 600 Veh/h. For higher volumes, V/C gradually increased and reached 0.91 at 1,450 Veh/h. Comparing the single and dual right-turn lanes suggested that, similar to the delay parameter, V/C converged faster to 1 at low traffic volumes when using a single lane, but for large traffic volumes, the convergence speed was higher when using dual lanes. This is due to the effect of the number of right-turn lanes. For example, for V/C = 0.89, the volume was 800 Veh/h for a single lane and 1,400 Veh/h for dual lanes (i.e., 75% higher traffic volume).Figure 10
Comparison of changing a single and dual right-turn lane in volume to capacity (v/c) ratio.Figure11 illustrates that by increasing the number of left-turn lanes to two, the delay was 20 s for a single left-turn lane at 350 Veh/h, while the same delay was obtained by the dual left-turn lanes at 700 Veh/h, according to HCM2016. The simulation results indicated that increasing the number of left-turn lanes led to a delay reduction from 45.6 to 10.3 s for the left-turn volume of 400 Veh/h. This shows that even for smaller volumes (around 400 Veh/h) in which the signalized intersection was unsaturated, there was a significant difference in delay parameter between the single and dual left-turn lanes in both simulation results and HCM, respectively, 77.4% and 59.7%. Therefore, the difference can be attributed to the number of lanes.Figure 11
Delay variations of the effect of changing traffic scenarios in a single and dual left-turn lane.Figure12 depicts the simulation results of the single and dual left-turn lanes. As the left-turn volume increased from 50 to 100 Veh/h, V/C remained constant at 0.31 for the single right-turn lane. For traffic volumes in the range of 100–400 Veh/h, V/C increased to 0.98. For the dual right-turn lanes, V/C was constantly equal to 0.28 for the traffic volume of 50 to 200 Veh/h. For higher volumes, V/C gradually increased and reached 0.89 at 700 Veh/h. Comparing the single and dual left-turn lanes suggested that, similar to the delay parameter, V/C converged faster to 1 at low traffic volumes when using a single lane, but for large traffic volumes, the convergence speed was higher when using dual lanes. For example, for V/C = 0.86, the volume was 350 Veh/h for a single lane and 700 Veh/h for dual lanes (i.e., 100% higher traffic volume).Figure 12
Comparison of changing single to dual left-turn lane on volume to capacity (v/c) ratio.The difference between the single and dual lanes (both right and left turn) results was verified statistically using the two-samplet-test method. Table 5 shows a significant difference between single and dual lanes in volume results at a 95% confidence level (P value <0.05).Table 5
t-test performed between the single and dual lanes.
Turn typeMethodt statP valuet CriticalRight-turnSimulation and HCM2016−3.960.0012.02Left-turnSimulation and HCM2016−2.210.032.1AASHTO 2018 and NCHRP 780 provide recommendations based on driving behavior and traffic parameters for the length of right- and left-turn lanes in a given condition of geometric elements of at-grade signalized intersections. Also, previous studies conducted in this field have examined the length of the lane in different turning movements of an approach (see Table1). In contrast, the result analysis of this research shows that the number of traffic lanes can significantly impact increasing the throughput of intersections. In this study, we have focused on the effect of the number of turning lanes on the volume changes of right turn and left turn. Analyses in undersaturated conditions (V/C < 1) showed that the HCM2016 delay model is not sensitive to volume changes, especially in right-turning movements, up to a specific value. While much of the research reviewed in the background has been done for V/C = 1 or V/C > 1, this insensitivity of HCM is not seen in saturated and oversaturated conditions.


## 5. Conclusion

This study sought to assess the performance of signalized intersections by changing the number of right- and left-turn approaches and traffic scenarios for a symmetrical four-leg intersection using Synchro. For all traffic scenarios, the incoming volume of the through approach was considered constant, and the channelized single or dual right-turn lane approaches and the left-turn volume approach with single or dual lane storage length were increased under unsaturated traffic conditions. The following conclusions can be drawn:(i)
As the number of left-turn lanes increased, the results showed that for low traffic volumes (less than 400 Veh/h) in which the signalized intersection was unsaturated, the difference in delay parameter between the single and dual left-turn lanes was significant in simulation and HCM2016, respectively, 77.4% and 59.7%.(ii)
Generally, as the through approach volume remained constant, the delay parameter reported by HCM2016 was fixed up to a certain threshold (10.8 s) for increased channelized right-turn volume. The delay parameter of the undersaturated intersection was less sensitive to single and dual right-turn lane changes. However, the simulation results indicated that the delay parameter was not constant, depending on the number of right-turn lanes and volumes. Furthermore, at low volumes, the delay value decreased and then increased in both single and dual right-turn lanes.Overall, if the intersection-occupied area is enough, increasing the number of right- and left-turn lanes can be a suitable alternative for at-grade signalized intersections to improve the throughput of an intersection. Our results can offer a good vision for traffic engineers to modify the geometry of the four-leg undersaturated signalized intersections under conditions that the right or left-turn volumes are incremental, and the through volumes are constant.



---

*Source: 8764498-2023-02-16.xml*