Transportation plays an essential role in the way societies function with significant impacts to the economic, social, political, and environmental sectors. It is undeniable that the need for robust, efficient, and sustainable transportation infrastructure remains a critical goal to support national and global advancements and is even more paramount with increasing population. Cities and economies must develop and evolve in innovative ways to meet the constraints of such opportunities and growth.
For developing countries to advance their transportation systems, they can look to adopt emerging global innovations and technologies. My engineering education and career began in Ghana, a developing country located in West Africa. Ghana has one of the fastest growing economies on the continent, and similar to other countries, has multiple opportunities for transportation infrastructure development. Having now lived in the United Kingdom and United States, I have gained more insight into how efficient transportation systems directly impact a nation’s development.
In a recent case study, we explored traffic-signal based strategies and innovative intersection designs for operational performance improvement. The case study focused on an existing intersection in Kumasi, the second largest metropolis in the country, and utilized capacity analysis, simulation, and modeling expertise for analyses. This case study was presented at the first Ghana Infrastructure Conference organized in August 2018 (GIC-2018) by the Ghana Transportation Professionals Forum of North America (GTPF) and its local engineering and academic partners. The goal of the conference was to explore best practices in infrastructure development as well as to engage leaders and key stakeholders in Ghana and to set forth a collaborative process to facilitate and effect change.
Study Location & Description
The case study focused on the Anloga intersection along the main divided arterial entering Kumasi from the capital city of Accra. It is a major signalized four-leg at-grade intersection of the Accra-Kumasi Road (East-West approach) and the Eastern Bypass (North-South approach). The existing intersection is characterized by recurring congestion, significant pedestrian volumes, long queues, low saturation rates, high side friction, and high left turn volumes predominantly during the morning and afternoon peak periods. The main east-west arterial has left turn lanes with protected left turn movements while the north-south Eastern bypass has left turn lanes and operates on a split phase. It operates on a fixed signal timing plan. Prior studies and plans have identified the location for a potential design modification to an interchange design. Various existing land use types and features as well as right-of-way costs could make an interchange a high cost alternative.
The study methodology included a four-step process comprising data collection, planning level analysis, preliminary operational analysis, and microsimulation operational analysis.
Geometric data for the site was collected via aerial images, site observations, and photos. Traffic data collected included volumes with classification, queues, travel times and speeds, and signal timings during the morning and afternoon peak periods.
Planning Level Analysis
The Planning Level Analysis involved the use of the Federal Highway Administration (FHWA) Capacity Analysis for Planning of Junctions (CAP-X) spreadsheet tool. The tool utilizes critical lane volume analysis to examine the suitability of various configurations based on existing traffic volumes. For the purposes of this case study, only the more congested morning peak period was analyzed. The results showed that the existing conventional at-grade intersection design was the lowest ranked option out of the 15 intersection types tested. The top five design options all had volume to capacity (v/c) ratios less than 0.7 and included three and two variants of the Displaced Left Turn (DLT) / Continuous Flow Intersection (CFI) and Quadrant Roadway Intersection designs, respectively. Continuous flow intersections (CFI) also called Displaced Left Turns (DLT) function primarily through the relocation of the left-turn movements on an approach to the opposing roadway at crossovers upstream of the main signalized intersection thereby eliminating the left-turn phase for the approach at the main intersection and subsequently providing more green time for the consecutive movement of left and through vehicles. A Quadrant Roadway Intersection (QRI) is typically a feasible option for the intersection of two busy suburban or urban roadways and works by adding two three-way intersections to the existing four-way intersection by rerouting turning movements from the main intersection onto a new roadway that connects the two intersecting roads.
Preliminary Operational Analysis
Based on the results of the planning level analysis, preliminary operational analyses were then completed using Synchro software. The existing base model was created using the collated field data and model parameters were adjusted as needed to replicate field conditions. The various design alternatives analyzed and initial projected results are summarized below:
- Existing with optimized signal timings
- Lane assignment change (N-S Shared through/left and left lanes
- Geometry/ Lane assignment changes (Double protected left for N-S)
- Full CFI/DLT
- Quadrant Roadway (South-West)
Intersection capacity analyses results are generally designated with the letters A to F for acceptable to failing levels of service (LOS). The existing (base) model operates at failing LOS F with a significant corresponding intersection delay of 205.0 seconds. Alternatives 1 to 3 were projected to provide decreases in delay but were not significant enough to improve the failing LOS F while all the approaches and intersections were projected to operate at LOS D or better in Alternatives 4 and 5.
Detailed Operational Analysis
The operational analysis in VISSIM microsimulation software was then completed to provide additional evaluation of the options. Generally, the microsimulation environment also takes into account individual vehicle interaction and provides the opportunity to adjust additional parameters for model calibration. This analysis was important because it allowed travel speed distribution, high side friction, driver aggressiveness, and additional vehicle classification data to be considered in the evaluation.
The quadrant roadway was eliminated from this analysis due to site constraints. The results are consistent with the preliminary analysis results and also project that the full CFI intersection provides the most significant improvement and would reduce intersection delay by 69 percent and improve the LOS to an acceptable C from the failing LOS F in existing conditions.
Considerations and Take-aways
The analyses supported the consideration of innovative intersection designs when conventional at-grade intersections experience failing operational performance. This is especially true for locations that may be viable candidates for grade-separated interchange designs but could benefit from the preservation of the land use and roadway corridor classification and accessibility to the roadway quadrants for all road users.
Short, medium- and long-term options were also outlined for the Anloga intersection. The short- and medium-term options focused on upgrades to the existing design including optimized signal timing, lane assignment changes, and improved pavement marking. It also highlighted the importance of undertaking access management and complete streets assessment of the existing layout for a thorough understanding of public transit and non-motorized access for inclusion in any viable design options. The long-term recommendation was to consider a CFI, full or partial, and to examine its potential in the context of driver expectancy, right-of-way constraints, pedestrian and transit accommodation, signing and markings, supplemental driver education and awareness requirements, overall safety concerns and its impact on and interaction with adjacent signalized intersections.
Based on feedback from the case study presentation at the GIC-2018 conference, the main take-aways were:
- There are alternative innovative interchange designs that can be considered for this location and others as applicable.
- Traffic engineering options to optimize use of existing capacity should be considered in addition to capacity increasing projects.
- Alternative analysis should be considered in the context of land use and all road users.
- Planning and Alternative analysis need to be considered on a site-specific basis as well as on a corridor or regional level.
Presently, GTPF and its partners are focused on a second installment of the conference series planned for August 2020 (GIC-2020 –www.gtpfconference.com). The goal is to build on the momentum from GIC-2018 and to continue to contribute to Ghana’s progress in infrastructure advancements.
Yolanda Oliver-Commey, PE, PTOE is a senior engineer and project manager at Pennoni. Learn more at www.pennoni.com.