Keywords

1 Introduction

The intersection can be consider as one of the significant part of the network due to the conflict that will be happened at this point and may be leads to many problems such as traffic congestion, traffic accidents. According to Eastern Asia Society for Transportation Studies, there are two types of intersections grade and grade separated. At the same time the intersections can be classified as signalized and unsignalized due to the type of control [1]. Traffic capacity, the key items that are used to determine traffic activity in the intersection are the volume to capability ratio, deviation, and quality of service [2]. The capacity of intersection can be defined as the maximum number vehicles that can move through a given section during one hour under the dominant conditions, the capacity for the intersection is measured for the lane group (for all lanes in the approach) [3]. The volume to capacity ratio refer to the degree of saturation of the intersection which refer to the ability of the intersection to operate the traffic demand under best condition. Any intersection with volume to capacity ratio less than 0.85 is consider as an ideal condition for the intersection and the traffic volume for this intersection will not expected any congestion and delay. On the other hand, any intersection with volume to capacity ratio more than 1.0, the intersection will operate with unstable condition and the traffic volume in the intersection will expected more delay and there will be queuing in all approaches [4]. Delay can be consider as an adequate indicator to assess the traffic operation for the intersection [5]. Based on the HCM 2010 the delay can be define as the “the additional travel time experienced by a driver, passenger, or pedestrian”. The delay of the intersection can be classified into: uniform delay, incremental delay and initial queue delay [2].

The average control delay can be calculated according to the Eq. 1:

$${{\rm d}} = {{\rm d}}_{1} + {{\rm d}}_{2} + {{\rm d}}_{3}$$
(1)

Where:

d1: is uniform control delay,

d2: is incremental delay, and.

d3: is initial queue delay.

The uniform control delay is:

$${d}_{1}=\frac{0.5C(1-g/C{)}^{2}}{1-[min(1,X)g/C]} (2)$$

Where

C: is cycle length in seconds and g is the lane group's efficient green period (second).

X: represents the lane group's v/c ratio.

The incremental latency is as follows:

$${d}_{2}=900T\left[\left(X-1\right)+\sqrt{(X-1{)}^{2}+\frac{8KIX}{cT}}\right]$$
(3)

Where:

T: analysis period duration (hour),

K: signal controller mode-dependent delay adjustment factor,

I: upstream filtering/metering adjustment factor.

c: lane group potential (veh/hr),

X: lane group v/c ratio.

The initial queue delay is:

$$\begin{array}{c}{d}_{3}=\frac{3600}{vT}\left({t}_{A}\frac{{Q}_{b}+{Q}_{e}-{Q}_{\mathrm{eo}}}{2}+\frac{{Q}_{e}^{2}+{Q}_{\mathrm{eo}}^{2}}{2{\mathrm{c}}_{A}}-\frac{{Q}_{b}^{2}}{2{c}_{A}}\right)\\ {Q}_{e}={Q}_{b}+{t}_{A}\left(v-{c}_{A}\right)\\ \, {\text{I}}{{\rm f}} \, v\ge {c}_{A} \, {{\rm t}}{\text{h}}{{\rm e}}{\text{n}}{:} \, {Q}_{\mathrm{eo}}=T\left(v-{c}_{A}\right)\\ {t}_{A}=T\\ \, {\text{I}}{{\rm f}} \, v<{c}_{A} \, {\text{t}}{{\rm h}}{\text{e}}{{\rm n}}{:} \, \begin{array}{c}{Q}_{e0}=0.0\cdot veh\\ {t}_{A}={Q}_{b}/\left({c}_{A}-v\right)\le T\end{array}\end{array} (4)$$

Where:

T: is the analysis period's time in hours,

\(v\): the request flow rate in vehicles per hour, and.

tA: is the adjustment period for unmet demand during the analysis period (hour),

cA: average potential of lane category (veh/h),

Qb: denotes the initial queue at the start of the analysis period (veh),

Qe: denotes the initial queue at the end of the analysis period (veh),

Qeo: denotes the initial queue at the end of the analysis period when cA > cA and Qb = 0.0 (veh).

To evaluate the quality of the any part in the transportation network the traffic engineers used the term level of service (LOS) which is represented the delay that occur in the traffic stream that used this part of the network [6]. According to HCM 2010 the LOS can be defined as “a quantitative stratification of a performance measure or measures that represent the quality of service”. The LOS of intersection can be classified into six level centered on the normal intersection delay from A to F The LOS for a signalized intersection is shown in Table 1.

Table 1. LOS criteria for signalized intersection [2]
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2 Previous Studies

The HCS software has been used in several studies to evaluate and improve the LOS for signalized intersections in various Iraqi cities. HCS 2000 was used to assess the Al-Thawra signalized intersection in Al-Hilla district, Iraq. The intersection operates with a 263.7 s/veh F LOS delay. The analysis proposed building a flyover to boost the LOS; as a result, the LOS would be C with a latency of 22.8 s/veh [7]. Karim used HCS 2000 in 2011 to assess the Al-Quds signalized intersection in Baghdad, Iraq. With an average delay of 328.7 s per vehicle, the intersection was found to fit with LOS F. By inserting one lane for each approach, the intersection's LOS improves to C, with an average delay of 34.6 s per vehicle [8]. Another research used HCS 2000 to assess the AL-Mustansiriyah Intersection in Baghdad, Iraq. The best idea for improving the LOS in this intersection was to construct a flyover between Al-Mustansiriyah University Street and Al-Talibia Street, according to the study [9]. In addition, the LOS for the AL-Kafa'at signalized intersection in AL-kut district, Iraq, is evaluated using HCS 2000. The current LOS for this intersection is F, with an average delay of 102.8 s per vehicle; but, according to this report, adding more lanes for the right turn would increase the LOS to D, with an average delay of 38.1 s per vehicle [10]. HCS 2010 was used to test the LOS for the Al-Furqan intersection in Al-Fallujah district, Iraq. The operational review for this intersection indicates that the intersection operates at LOS F with an average delay of 105.2 s per vehicle. This study recommended that traffic from the west bound be avoided in order to increase the LOS from C, which has an average pause of 34.5 s per vehicle [11].

3 Objectives of the Study

The main objectives in this study are:

  • Establish the peak hour for Al-Amreia intersection, which is consider as the highest traffic volume in all approaches.

  • Evaluate the current LOS at the Al-Amreia intersection with both approaches.

  • Suggestion different proposals to improve the LOS at Al-Amreia intersection.

  • Evaluation the LOS for all suggested proposals for all approaches at Al-Amreia intersection.

  • Selections the best propsal to improve the LOS at Al-Amreia intersection for the base and target year

4 Study Area

Baghdad is the capital of Iraq; it considers as one of the congested cities in the world because the huge number of vehicles that using the network in this city especially after 2003. All intersections in Baghdad city are operated under breakdown condition with LOS F. For this reason, one of the congested intersection are selected in this study. Al-Amreia intersection is selected as a case study for these reasons:

  • Al-Amreia intersection connects the traffic volume that are coming from the West provinces to Baghdad city.

  • This intersection has high traffic volumes in all approaches

  • There are many attraction locations (residential, educational and commercial) close to the study area.

Figure 1 represented Al-Amreia intersection and the boundary area of the selected intersection study.

Fig. 1.
figure 1

Satellite image for Al-Amreia intersection in Bagdad city, Iraq [open street]

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5 Methodology

To obtain the LOS for Al-Amreia intersection this study will follow the methodology that describe in HCM 2010. Figure 2 shows the main steps that must be follow to obtain the LOS which is the primary output.

Fig. 2.
figure 2

Signalized intersection methodology

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6 Data Collection

To evaluate the traffic operation at Al-Amreia intersection in terms of LOS a field data survey is made by special teams these data including traffic and geometric data. The measurements of these data are made manually on working days (Monday to Thursday) at January 2021 to spot the peak hours.

6.1 Traffic Volumes

Traffic data survey is made for Al-Amreia intersection at the workdays from (6:00 am to 7:00 pm) during the 2nd week on January 2021, the traffic volume is counted in each approach for the three movement (left, through and right) and the highest number of traffic volume during the survey time was highlighted as peak hour. The traffic volume flow is classified into two categories:

  • Passenger vehicles: Any vehicle contains four tires only.

  • Heavy vehicles: Any vehicle contains more than four tires.

Table 2 shows the traffic volume at Al-Amreia intersection for each approach according to their movement form (6:00 am to 7:00 pm).

Table 2. One-hour traffic level at the Al-Amreia intersection for both approaches
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While Table 3 shows the Heavy vehicles percentage at Al-Amreia intersection.

Table 3. Heavy vehicles percentage at Al-Amreia intersection.
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6.2 Saturation Flow Rate

One of the main effective parameter on the capacity of intersection is the saturation flow rate. To calculate this parameter for Al-Amreia intersection. The software HCS 2010 is employed. The calculated saturation flow for each approach at Al-Amreia intersection is shown in Table 4.

Table 4. Saturation flow rate calculated at Al-Amreia intersection.
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6.3 Existing Geometric Design

It is important to determine the number of lanes and the direction of each movement when evaluating the quality of operation (LOS) at the Al-Amreia intersection. Figure 3 illustrates the intersection’s current geometric layout.

Fig. 3.
figure 3

Existing geometric design of Al-Amreia intersection

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7 Analysis and Results

7.1 Peak Hour Volumes

The following findings were drawn from the site inspection and traffic analysis:

  • The peak hour at the Al-Amreia intersection is between 2:00 and 3:00 p.m. At this hour, the overall traffic volume at the Al-Amreia intersection was 5038 pc/h (see Fig. 4).

Fig. 4.
figure 4

Distribution of traffic volume at Al-Amreia intersection from 6:00 a.m to 7:00 p.m.

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7.2 Peak Hour Factor (PHF)

According to HCM 2010, the PHF can be described as the ratio of total volume to the hour's maximum 15-min rate of flow. The following table summarizes the PHF values for both routes to the Al-Amreia intersection (Table 5).

Table 5. Peak hour factor at Al-Amreia intersection.
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7.3 Existing LOS

To assess the current LOS HSC 2010 software, it is implemented. The LOS at the base year was determined to be LOS (F), as seen in Table 6.

Table 6. Existing LOS at Al-Amreia intersection
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8 Proposals Design Alternative

8.1 First Proposal: Change the Cycle Length and Green Time for All Approaches

The first proposal that will be adopted it to change the cycle length form 90 s to 120 s. In addition, it will increase the green time for the congested direction approaches (EB (Al-Khadra) and SB (Al-Amreia)) in the intersection, it is found from the results shown in Table 7 that the change the cycle length and green time for all, the intersection became operational as a result of these methods (LOS F). As a result, this plan is not recommended for operational improvement, and another one must be adopted.

Table 7. LOS at Al-Amreia intersection within first proposal
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8.2 Second Proposal: Increase Number of Lanes

The second plan to enhance the intersection’s LOS proposed increasing the amount of lanes on both approaches by removing parking in the approach lanes. It is found from the results shown in Table 8 that the increase the number of operation lanes at all approaches caused the intersection to work (LOS F). As a result, this plan is not recommended for operational improvement, and another one must be adopted.

Table 8. LOS at Al-Amreia intersection within second proposal
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8.3 Third Proposal: Underground from East Bound Towards West Bound

The third proposal to improve the LOS for the intersection suggested to increase the number of lanes in all approaches by eliminating the parking in the execution of underground along EB (Al-Khadra) towards WB (Abu Ghareeb). It is found from the results shown in Table 8 that the increase the number of operation lanes at all approaches made the intersection operate on (LOS D). Therefore, this proposal is not recommended to improve the operation and it is necessary to adopt another proposal (Table 9).

Table 9. LOS at Al-Amreia intersection within third proposal
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8.4 Fourth Proposal: Fly Over from North Bound Towards South Bound

The fourth proposal is to execute a flyover that connects NB (Al- Ghazalia) towards SB (Al-Amreia), while the intersection is kept operating with four legs. It is clear from the results that were shown in Table 10 that the LOS was (C). Also the execution of this proposal will not make any improvement on the LOS, therefore; the fourth proposal was adopted.

Table 10. LOS at Al-Amreia intersection within third proposal
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9 Analysis of Forecasted Traffic Data

The HCS software is used to analyze forecasted data (after 20 years at a 2% annual growth rate) through power, pause, and LOS calculations for all approaches and the entire intersection. For the intended year. According to the data gathered, the LOS in the target year would be LOS (C), as seen in Table 11.

Table 11. LOS at Al-Amreia intersection within third proposal
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10 Conclusions

From the results that obtained from the analysis for Al-Amreia intersection it can be concluded that the existing LOS F with average delay 138.7 s/veh. The study suggested four proposals to improve the LOS for the intersection, it is concluded that the fourth proposal which is construct a flyover from NB (Al-Ghazalia) towards SB (Al-Amreia), the proposal reflects the best solution to improve the LOS for the intersection on base and target year. The intersection will operate at C LOS with average delay 25.9 s/veh for base year, while for target year the intersection will operate at C LOS with average delay 31.3 s/veh.