Improving Intersection Design Practices Adam Kirk, Chris Jones, and Nikiforos Stamatiadis engineers from considering one or more of the alternatives, even though such designs may be appropriate. Unwarranted operational or safety problems or unwarranted costs may be incurred when suboptimal designs are constructed. The goal of this project is to improve intersection design practices by (a) expanding the scope of intersection design alternatives considered and (b) providing a structured and objective evaluation process to compare alternative design concepts. This goal is achieved through the development of a process capable of evaluating 13 alternative traffic control designs through the use of more than 12,000 unique lane configurations for a given location. To facilitate this evaluation, the methodologies described in this paper were developed into a software-based tool identified as the Intersection Design Alternative Tool (IDAT). The tool provides designers with a list of potential solutions that are based on the minimum number of lanes required to achieve a targeted level of operation.
This study aims to use operational characteristics to help determine the size and the design of intersections on the basis of a targeted level of operation. This approach will allow for a preliminary evaluation of a broader range of possible designs, by screening out those designs considered less desirable or inappropriate on the basis of operational performance. This approach will also allow for a more objective comparison of all alternatives because all options target the same operational service level. The use of the critical lane analysis method was considered an appropriate approach for developing such size estimates for intersections. Similar methods for stop-controlled and yield-controlled intersections were also identified because it was necessary to expand these methods to include unsignalized designs as well. The Intersection Design Alternative Tool developed through this effort is capable of evaluating 13 intersection alternatives and identifying preferred lane configurations from more than 12,000 available configurations.
Intersections are a critical component of the roadway system and frequently act as choke points on the transportation system. Intersection design is a balancing act of various elements and constraints aiming to produce a solution that addresses mobility, safety, environmental, and financial aspects of the project. To achieve this balance, alternative strategies and options must be identified, developed, and evaluated in a systematic manner. Significant advances in transportation engineering have identified new traffic control measures, practices, and design concepts capable of further increasing the operational efficiency and safety of intersections. Moreover, no significant efforts have been undertaken to provide an effective comparison between various types of intersection designs or traffic control measures that could demonstrate their effectiveness. For example, the Highway Capacity Manual uses two different delay metrics for estimating level of service (LOS) for unsignalized and signalized intersections, thus rendering comparisons between these two types of intersection control infeasible. Current practice, while achieving great strides in improving the efficiency, lacks a systematic, objective, and well-defined approach to evaluating individual design alternatives. A review of current literature identified 13 alternative intersection designs. No systematic process can be identified that compares these alternative designs. Most guidelines identify the need for comparative studies but do not identify the factors or methods that one should apply in determining the most beneficial design. The lack of specific guidance at both the national and state levels is likely to discourage
Alternative Intersection Designs A comprehensive list of alternative intersection designs was obtained from the Maryland State Highway Administration’s Unconventional Arterial Intersection Design Tool, which provides conceptual information and considerations for a wide range of alternative intersection designs (1). A list of the at-grade intersections in this tool is provided below: • Unsignalized inside left turn; • Median U-turn, signalized; • Median U-turn, unsignalized; • Superstreet, unsignalized; • Superstreet, signalized; • Continuous flow; • Continuous green T; • Jughandle; • Bowtie; • Modern roundabout; and • Paired. Several of these alternative intersection designs have been used throughout the country and aim to improve intersection operation and safety. The median U-turn design has been used in Michigan extensively for years, the jughandle design in New Jersey, and the continuous flow in New York and Maryland. The use of modern roundabouts is perhaps the most adopted alternative, and its use is increasing rapidly throughout the United States. Despite their use, limited guidance is available on the evaluation, design, and implementation of these designs. AASHTO’s A Policy on Geometric Design of Highways and Streets contains guidelines on the design of standard intersections and some guidance on the median U-turn, jughandle, and roundabout alternatives (2). However, this
A. Kirk, Kentucky Transportation Center, and N. Stamatiadis, Department of Civil Engineering, University of Kentucky, 176 Raymond Building, Lexington KY 40506. C. Jones, Stantec, 1409 North Forbes Road, Lexington KY 40511-2050. Corresponding author: N. Stamatiadis,
[email protected]. Transportation Research Record: Journal of the Transportation Research Board, No. 2223, Transportation Research Board of the National Academies, Washington, D.C., 2011, pp. 1–8. DOI: 10.3141/2223-01 1
2 Transportation Research Record 2223
guidance is limited and does not address when and how to use these alternatives. A recent report by FHWA addressed the restricted crossing U-turn intersections (superstreet) and median U-turn at-grade intersections (3). Most documented is the use of modern roundabouts. Twelve states have developed roundabout guides that address the planning, design, and operations of roundabouts, primarily based on the FHWA Roundabouts: An Informational Guide (4), although several states have much more comprehensive guides. In addition to published research, a review of design guides used by each state was undertaken to determine the factors considered in intersection design and how decisions on control type and size are reached. Of the 41 state transportation agencies reviewed, only Florida, Missouri, New Jersey, New York, Texas, and Washington have developed their own intersection design guidelines contained within a separate intersection design manual or included within their roadway design manuals. All states reviewed have intersection design guidance that adheres to or follows the AASHTO guidance and the Manual on Uniform Traffic Control Devices for determining traffic control (mainly for signalization). Of the states with independent guides, the most frequently considered design factors are operational analysis and construction cost (five of six states with specific guidance). These two factors are considered controlling for designing and evaluating intersection options because the factors define the operational and construction efficiency of the intersection. A few manuals mention alternative intersection designs but do not provide any guidance for when they could be considered as viable alternatives. Moreover, no manual provides specific guidance for selecting appropriate intersection design or control types; most manuals simply note that comparisons among alternatives should be performed. It is apparent that there is a lack of tools that provide designers or planners with an estimate of appropriateness for different intersection designs. The review of state design practices revealed that there is not a significant amount of research on alternative intersection designs or design procedures. The limited guidance available is provided by state agencies that have developed their own intersection design guidelines. No state has developed a systematic process that compares these alternative designs. Most manuals identify the need for comparative studies but none identify the factors that one should consider in weighing alternatives and determining the optimal design. Maryland is the only state that is in the process of developing such an approach, but not much progress has been made since 2005 when the concept was initiated. The development of separate manuals for roundabouts by a few states is a step in the right direction for identifying and considering alternative intersection designs; however, these manuals do not provide a means for comparison and may further segregate alternative designs from traditional or other alternative designs.
Research Findings From the literature review, the following 13 different intersection alternatives were identified for consideration in our research: • Signalized; • Roundabout; • All-way stop; • Two-way stop; • Unsignalized inside left turn; • Median U-turn, signalized; • Median U-turn, unsignalized;
• Superstreet, unsignalized; • Superstreet, signalized; • Continuous flow; • Continuous green T; • Jughandle; and • Bowtie. These intersections may be broadly grouped in two categories: signalized and unsignalized control.
Intersection Design Procedures A basic problem in comparative analysis of intersection designs is ensuring that the alternatives examined all deliver a similar level of operational performance. For instance, depending on the volume distribution, a signalized intersection with two approach lanes may service the same volume as a single-lane roundabout. However, during the initial concept development, both alternatives may be compared with two-lane approaches. This method leads to comparison of alternatives with vastly different operational performance in addition to costs and right-of-way and environmental impacts. The approach taken by this project was to identify the minimum footprint for each intersection design for a given traffic demand, capable of meeting targeted operational parameters. A volume-to-capacity ratio (v/c) of 0.90 was chosen as the threshold in the development of IDAT. If an alternative meets this threshold, it is considered to be feasible and no further operational evaluation should be carried forward in consideration and development. This approach allows for full comparison of other design factors such as construction costs, right-of-way, and environmental impacts. An example of the benefits provided by this approach is provided in the following section. The critical aspect of this approach is to determine the optimum design scenario that can meet the desired operational threshold. The optimum design in this instance is that with the smallest footprint, and subsequently, smallest construction costs and impacts. Capacity analysis software may be used for design and sizing intersections; however, an iterative process is required for each alternative to achieve the desired level of capacity. More than 12,000 feasible lane configurations have been identified for signalized intersections alone, most of which can be developed for each of the 13 alternative designs considered. Using traditional capacity analysis software or microsimulation models to design the appropriate lane configuration for each alternative would be time-consuming and limit the range of alternatives to be considered. Therefore, the research team set out to identify methodologies that could be used to directly link the traffic demand (i.e., design hour turning movement volumes) to the optimum lane configuration for each alternative. It was understood that if achieved, the methodologies would have to apply to signal-controlled, stop-controlled, and yield-controlled (roundabout) alternatives. The most promising approach that could be used in designing and evaluating comparative alternatives was critical lane analysis (CLA). This method allows for the automation of the design process of signalized intersections by systematically linking traffic demand, geometric design, and operational LOS. CLA, as developed by Messer and Fambro, uses the geometry of the intersection and intersection traffic volumes as the basis for establishing a measure of potential performance and, by extent, of capacity (5). CLA uses the volumes of the approaches for an intersection to estimate their distribution among the available lanes. Once volumes are apportioned to each of the lanes, phasing plans are developed that allow for the appropriate
Kirk, Jones, and Stamatiadis
3
intersection movements. Critical volumes for each phase are determined on the basis of certain rules, and these volumes are summed to determine the total critical lane volume for the intersection. This sum can then be directly related to the LOS definition for signalized intersections. Similar techniques (i.e., estimates of capacity) have been developed for unsignalized intersection designs as well. The Highway Capacity Manual provided intersection capacity estimates based solely on conflicting movements and reserve capacity while considering intersection geometry (6). A recent report offered another consideration for estimating capacity for roundabouts (7). This report develops control delay models for single-lane and multilane roundabouts by using the critical lane methodology shown in Equations 1 and 2, respectively. Single-lane roundabouts: ccrit = 1130 ext ( −0.0010 × vc )
(1)
Multilane roundabouts: ccrit = 1130 ext ( −0.0007 × vc )
(2)
where ccrit is entry capacity of critical lane [passenger car unit (pcu)/h] and vc is conflicting flow (pcu/h). The methods discussed for estimating intersection capacity present simple estimates based on intersection geometry and turning volumes. These methods, although not as refined as current microsimulation models or more complex macro models, allow for direct linkage between intersection design and operation. Such simple models allow for manipulation through computational models, which permit the automation of preliminary designs for establishing the basic geometry needed to achieve a desired intersection capacity. CLA and unsignalized intersection LOS methods have served as the foundation for the calculating procedures used in the current version of the Highway Capacity Manual. These approaches are viewed as a basic, fundamental process for evaluating intersection design alternatives. The focal point of all these approaches is that they provide the potential for a common basis of comparison (i.e., v/c or unused capacity), which can be used in targeting design options and provide a common basis for comparisons.
Evaluation Approach The concepts presented above have not been proven, and the first step in this effort was to verify the relationship between delays and intersection design on the basis of CLA. Verification of these relationships was achieved through a simulation effort of various intersection designs and traffic control strategies. Different scenarios of volumes were identified to be simulated and obtain estimates of control delays. These delays were then used in determining the relationship between traffic volumes and delays for each intersection control option evaluated. This study considered only four-leg intersections, and the control types examined are two-way stop, all-way stop, signal, and roundabout. The corridor simulation (CORSIM) software was chosen because of its microscopic nature and ability to simulate traffic conditions in various traffic control environments. The first step of this work was to determine the different traffic volume scenarios to use in the simulation models. The east–west cardinal directions were considered the major street approaches, and
TABLE 1 Intersection Approach Volumes, Major Streets Total Major Street Volume (vph) 1,800 1,400 1,000 600
Eastbound
Westbound
1,080 840 600 360
720 560 400 240
TABLE 2 Intersection Approach Volumes, Minor Streets Total Minor Street Volume (vph) 1,200 800 400 860 570 285
Northbound 600 400 200 600 400 200
Southbound 600 400 200 260 170 85
the north–south directions were the minor street. A combination of different volumes and turning percentages was determined for the east–west direction and the north–south direction. The volumes were selected to be greater than the minimum volumes to satisfy the 4-h signalization warrant (8). The volume combinations used are shown in Tables 1 and 2. The east–west street used two different turning percentages. The first was 10% left turns and 10% right turns, and the second was 15% left turns and 15% right turns for each of the four different volumes. The turn percentages used for the north–south street were not uniform and were based on the total northbound approach volume (Table 3). Ninety-six scenarios were created on the basis of these approach volumes and turn percentages. For each scenario, different calculations were needed to determine the total critical volume for the allway stop-controlled and signalized intersections, the approach and conflicting volumes for roundabouts, or a critical approach volume for two-way stop-controlled intersections. The process followed for each of the traffic control options is described next. CLA was used to determine the lane configuration for signalcontrolled intersections, and approach and conflicting volume totals were used to determine the lane configuration for roundabouts. The lane configuration used for signalized intersections was also used for two-way and all-way stop-controlled intersections. Auxiliary TABLE 3 Minor Street Turn Percentages Northbound Approach Left Turn Volume (vph) (%) 600 400 200
10 20 10 20 30 30
Right Turn (%) 10 30 10 30 60 60
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70
Intersection Control Delay
60 50 40 30 20 10 0 0
200
400
600 800 1000 Critical Volume
1200
1400
1600
FIGURE 1 Signalized intersection delay and critical volume.
left- and right-turn lanes were provided when turning volumes exceeded 100 vehicles per hour (vph).
Simulation Results Each of the 96 volume scenarios for each of the four traffic control options was evaluated by using CORSIM. Default values were used for all parameters that were not modified among the various runs. Control delay per vehicle was measured for each scenario and control type. The multiple run feature was used to run each simulation four times using a different random number (i.e., representing a different traffic volume arrival pattern). The average control delay
value for each volume scenario and traffic control was recorded for each approach and calculated for the entire intersection. For each type of control evaluated, either the corresponding key volume or total volume was used to determine the relationship between control delay and this volume metric. Regression analysis was used to find a line of best fit for the data that could correlate delays to the corresponding volume metric. For the signal-controlled and all-way stop-controlled intersections, critical volume was used as the predicting variable. For the two-way stop-controlled intersections, the approach critical volume was used, and for the roundabout, the approach and conflicting volume was used. A definitive capacity threshold was identified in which delay is shown to increase exponentially for all control types examined. Figures 1 and 2 correlate the
Approach Control Delay Per Vehicle
60
50
40
30
20
10
0 0
200
400 600 800 Approach and Conflicting Volumes
1000
FIGURE 2 Roundabout approach delay and approach and conflicting volume.
1200
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effect of the predictor variables on intersection delay. This threshold was identified as the capacity of the intersection for the purposes of determining intersection operations and is summarized below: • Signalized intersections: 1,400 vph (based on critical volume), • All-way stop-controlled: 1,200 vph (based on critical volume), • Two-way stop-controlled: 900 vph (based on approach critical volume), and • Roundabouts: 1,000 vph (based on approach and conflicting volume). Regression models were developed for each of the four intersection controls to determine whether the relationship noted between the volume metrics used and the delay estimated was statistically significant. All tests indicated that the relationships were significant, and all coefficients and intercepts were significantly different than zero. Therefore, it can be concluded that the volume metrics used for each intersection control are capable of capturing the changes in the delays and can be used as indicators of the capacity and level of operation of the intersection as a result of the traffic control used. On the basis of this analysis, the derived critical volume procedures were validated. Capacity was identified by significant deflection identified in the delay curve. These critical volume capacities can be used to establish the ultimate threshold for the targeted performance values of each alternative design, thus ensuring that designs operate below capacity and at an acceptable LOS. Therefore, these volumes are 1,350 vph for signal-controlled, 1,200 vph for all-way stop, 1,000 vph for two-way stop, and 1,000 vph for roundabout. These values were used in determining the appropriate intersection design in IDAT. Intersection Design Alternative Tool Development
example, the median U-turn operates as a signalized intersection at its center, paired with two adjacent intersections to accommodate left-turning movements. Each alternative has advantages and dis advantages as well as differing turn movement arrangements that will optimize efficiency of each design. Furthermore, each alternative may also be manipulated to accommodate a wide range of alternative lane configurations to meet the unique demands of each project. To evaluate the full range of alternatives and lane configuration combinations available, the authors decided that evaluation with a software application was appropriate. The following sections discuss the specific requirements used to develop IDAT software. Intersection Lane Configuration Combinations The 12,000 different lane configurations evaluated for each alternative include eight different left- and right-turn auxiliary configurations for each of one, two, and three through-lane combinations for a total of 24 combinations for each approach. All combinations of each approach were evaluated with each approach, except that the major and minor streets were restricted to the same number of through lanes. Figure 3 shows the eight different approach combinations for a single through-lane alternative. These eight approach configurations developed for the signalized intersections served as the basis for the other intersection alternatives, which were modified to meet the unique demands of the differing designs. Each of the eight approach configurations was scored according to (a) the total number of lanes used in the design and (b) the desirability of the configurations from an operational, safety, and driver expectancy (i.e., commonality of design used) standpoint. Lane configurations were rated as follows:
Most of the designs considered in this study manipulate a traditional design, evaluated above, through redirected or channelized turn movements to address problematic or heavy turning movements. For
• 1: 8 (highest score), • 2: 6.5, • 3: 6.5, • 4: 5,
ONLY Ln Config 1
ONLY Ln Config 4
ONLY ONLY Ln Config 7
ONLY
Ln Config 2
Ln Config 3
ONLY
ONLY ONLY ONLY Ln Config 5
ONLY
Ln Config 6
ONLY
ONLY ONLY ONLY Ln Config 8
FIGURE 3 Approach lane configurations (ln config 5 lane configuration).
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• 5: 4, • 6: 2, • 7: 3, and • 8: 1 (lowest score). For feasible combinations (i.e., v/c is less than 0.90), a total intersection score was then developed as the sum of the individual approach scores. The combination with the highest score was chosen as the preferred configuration for that alternative. For each alternative design, a preferred configuration was developed for single-lane, two-lane, and three-lane approaches on the major street. Volume transformations were used for each of the innovative intersection designs, and they were decomposed to the appropriate signalized and unsignalized elements to estimate the lane requirements. For example, in the median U-turn (unsignalized), all left-turning volume was added to the through movement and used the left turn at the U-turn location. The required number of lanes to serve the reconfigured volumes at the intersection produced the new configuration for the intersection, and a similar approach for an unsignalized intersection produced the requirements for the U-turn.
Preferred Alternative Selection IDAT provides a multiple-level screening analysis to allow the user to select the preferred alternative or alternatives to be carried forward for further analysis. All design alternatives that have been identified as feasible through CLA are carried forward and presented in the final output. This analysis includes the identification of the highest scoring single-lane, two-lane, and three-lane configurations for each alternative. If multiple approach lane configurations are feasible for a given alternative, those with a greater number of through lanes are identified as “not recommended” to indicate that a configuration with a smaller footprint is feasible. The feasible alternatives are evaluated using a weighted scoring scheme to assist in the final evaluation. The scoring system used to evaluate the intersection alternatives examines the size of the intersection (right-of-way requirements); safety performance for vehicles, pedestrians, and bicycles; and access management capabilities. These criteria are also assigned weights to indicate their level of importance in the project; IDAT default values assume an equal weight (0.33) for each criterion. Each alternative is scored against these criteria, and a composite total score is developed. The composite score is then determined as the sum of the products of the individual scores and criterion weight. The alternative scoring was developed by an expert panel employed for this task. The panel consisted of traffic operations, highway safety, and design engineers who were asked to evaluate and score each intersection design on their a priori understanding of the level of safety, pedestrian and bicycle accommodation, and access management control provided by the intersection. The scoring method used a fivepoint scale, where 1.0 represented the lowest score and 5.0 the highest. For intersection designs with similar control but a greater number of approach lanes, relative scores were developed according to the expertise of the panel. The safety level was scored individually for vehicular, pedestrian, and bicycle traffic, creating a unique score for each category because each travel mode has different safety concerns. The size of intersection was used to develop the scores for the right-of-way. Because all alternatives are developed to operate at the same level of efficiency, the size of the intersection becomes a critical determinant of suitability. Although this metric cannot provide precise estimates at the preliminary design stage because topographic or other constraints on the site, it can provide a relative comparison
between alternatives. The scoring method provides five points for an approach with a single lane. Approaches with five or more lanes receive zero points. Turn lanes, such as a left or right auxiliary lane, are counted as one-half of a lane because they will likely be required for only a short length. The average score of all approaches for the design is used in the final scoring. Two points were deducted from the overall score for jughandle and bowtie designs because of their increased space requirements. Even though intersection size may be disaggregated into components, including number of approach lanes, intersection number of lanes (including auxiliary lanes), and physical intersection area, such a detailed approach was not appropriate for the level of anticipated use of the evaluation tool. Example Application To test the capabilities of IDAT, an intersection project being considered by the Kentucky Transportation Cabinet was evaluated to identify potential alternative solutions. This section focuses on the practical solution developed for the intersection of KY-17 at Old Madison Pike in Fort Wright, Kentucky, as part of the KY-17 intersection improvement study conducted by the Northern Kentucky Area Planning Commission and KYTC. The existing stop-controlled intersection on the westbound approach operated at LOS F during the morning and afternoon peak periods (9). Figure 4 shows the projected turning movement counts at the intersection. The project was initially designed with a traditional design approach, which analyzed two alternatives. The first was a traditional signal installation, and the second introduced a modified modern roundabout. The design for these alternatives yielded project estimates between $5 million and $5.8 million because both alternatives caused major impacts to the existing bridge structure and rock cuts near the intersection. These alternatives are shown in Figure 5. IDAT results confirmed the proposed alternatives, which identified a three-lane approach on the major street for signalized operations and identified the roundabout as not feasible, thus requiring additional modification, as shown in Figure 6. Other feasible alternatives
KY 17
KY 17 FIGURE 4 Design hour volumes, KY-17 at Old Madison Pike.
Kirk, Jones, and Stamatiadis
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(a)
(b)
FIGURE 5 Initial design alternatives.
2-Way Stop Control*
Not Feasible
L2 U
2.5
A.M.
Bike
Ped.
Veh.
0
1.5
SCORE
L4 U
0.3
L2
0.1
L4
0.1
L1
0.1
L3
SAFETY
ROW
MINIMUM LANE CONFIGURATION
OPERATION EVALUATION
0.3
INTERSECTION ALTERNATIVE
2.5
1.0
0
4-Way Stop Control
Not Feasible
0
4.0
4.0
5.0
1.5
0
Signalized Intersection (1 lanes)
Not Feasible
0
4.0
4.0
3.5
2.5
0
Signalized Intersection (2 lanes)
Not Feasible
0
3.0
3.0
3.0
3.0
0
Signalized Intersection (3 lanes)
Feasible
4
2.5
2.5
2.0
3.0 3.08
Jughandle A EB (1 Lanes)
Not Feasible
-2
3.0
2.5
3.0
3.5
0
Jughandle A EB (2 Lanes)
Not Feasible
-2
2.5
1.5
2.5
4.0
0
Jughandle A EB (3 Lanes)
Feasible
2
2.0
1.5
1.5
4.0 2.53
Jughandle A WB (1 Lanes)
Not Feasible
-2
3.0
2.5
3.0
3.5
0
Jughandle A WB (2 Lanes)
Not Feasible
-2
2.5
1.5
2.5
4.0
0
Jughandle A WB (3 Lanes)
Feasible
2
2.0
1.5
1.5
4.0 2.53
Jughandle A EB-WB (1 Lanes)
Not Feasible
-2
3.0
2.5
3.0
3.5
0
Jughandle A EB-WB (2 Lanes)
Not Feasible
-2
2.5
1.5
2.5
4.0
0
Jughandle A EB-WB (3 Lanes)
Feasible
2
2.0
1.5
1.5
4.0 2.53
Roundabout
Not Feasible
0
5.0
4.0
4.0
5.0
0
Median U-Turn (Signalized) (1 Lanes)
Not Feasible
0
4.0
3.5
2.5
5.0
0
Median U-Turn (Signalized) (2 Lanes)
Not Feasible
0
3.0
2.0
2.0
5.0
0
Median U-Turn (Signalized) (3 Lanes)
Not Feasible
0
2.5
2.0
1.5
5.0
0
Median U-Turn (Unsignalized)*
Not Feasible
0
4.0
2.5
2.5
5.0
0
Superstreet (Signalized)
Feasible
1.5
3.5
2.5
1.5
4.0 2.64
Superstreet (Unsignalized)
Not Feasible
0
3.5
1.5
1.5
4.0
0
Inside Left Turn (Signalized) (NB 'T') (1 Lane)
Not Feasible
0
4.0
1.0
1.0
3.5
0
Inside Left Turn (Signalized) (NB 'T') (2 Lane)
Feasible
2.25
3.0
1.0
1.0
4.0 2.61
Inside Left Turn (Signalized) (NB 'T') (3 Lane)
Not Recommended
2
2.5
0.5
0.5
4.0 2.37
Inside Left Turn (Signalized) (SB 'T') (1 Lane)
Not Feasible
0
4.0
1.0
1.0
3.5
0
1
3.0
1.0
1.0
4.0 2.2
0.75
2.5
0.5
0.5
4.0 1.95
Not Feasible
0
3.5
1.0
1.0
3.0
0
Inside Left Turn (Unsignalized) (SB 'T')*
Not Feasible
0
3.5
1.0
1.0
3.0
0
Bowtie (1 Lane)
Not Feasible
-2
3.5
3.5
2.5
5.0
0
Bowtie (2 Lane)
Not Feasible
-2
2.5
2.5
2.0
5.0
0
Bowtie (3 Lane)
Not Feasible
-2
2.0
2.0
1.5
5.0
0
Inside Left Turn (Signalized) (SB 'T') (2 Lane)
Feasible
Inside Left Turn (Signalized) (SB 'T') (3 Lane)
Not Recommended
Inside Left Turn (Unsignalized) (NB 'T')*
FIGURE 6 IDAT output (EB 5 eastbound, WB 5 westbound, NB 5 northbound, SB 5 southbound).
8 Transportation Research Record 2223
do not have to involve comparison of operations, but instead can focus on costs and impacts to the associated project area. The tool identifies the most efficient design (minimum number of lanes) that is capable of meeting a targeted level of operation. A designer is presented with several options that meet the minimum operational requirements, allowing examination of other trade-offs such as right-of-way impacts, safety considerations, and the like. This approach eliminates the need to compare different alternatives with varying performance levels across different types of traffic control measures. The proposed approach provides a greater efficiency in the evaluation and conceptual design of intersection alternatives, with the intent to achieve greater operational efficiency and improved safety performance. The approach allows for a more appropriate and properly customized design for each intersection, avoiding the use of standard or typical designs. FIGURE 7 IDAT recommended alternative.
References identified were jughandle and superstreet designs, which were not feasible because of sight constraints. However, an inside left-turn lane was also identified as feasible; it could operate with a reduced number of lanes (two) on KY-17 and have minimal impacts on Old Madison Pike. This alternative is shown in Figure 7. Implementation of the inside left-turn lane has the ability to maintain the existing cross section with minimal widening to accommodate a small channelizing median for the left turn. This unique design fit the roadway within its specific needs and constraints. The cost for the proposed improvements was estimated at $275,000, providing a savings of more than $4.5 million for the intersection. Detailed operational analysis of the intersection was estimated at LOS B, which exceeded the target value LOS D or E for the initial alternatives. Conclusion This paper documents the efforts and approach used to develop the operational component of IDAT. The tool and this approach provide a method to evaluate 13 different intersection design alternatives with more than 12,000 unique lane configurations each. The ultimate applicability of this approach is to identify a wider range of feasible intersection design alternatives with significantly less effort than is currently afforded through the independent evaluation of intersection designs through capacity software or microsimulation programs. Notable is the approach taken to sizing intersection alternatives to deliver a targeted performance level so that comparative evaluations
1. Applied Technology and Traffic Analysis Program. Unconventional Arterial Intersection Design. University of Maryland, College Park, Md., 2005. http://attap.umd.edu/bbs/zboard.php?id=projects&select_arrange= headnum&desc=asc&page_num=5&selected=&exec=&sn=off&ss= on&sc=off&category=10&ss=on&ss=on&keyword=. Accessed June 22, 2010. 2. A Policy on Geometric Design of Highways and Streets. AASHTO, Washington, D.C., 2004. 3. Hughes, W., and R. Jagannathan. Alternative Intersections/Interchanges: Information Report (AIIR). FHWA-HRT-09-060. FHWA, U.S. Department of Transportation, 2009. 4. Robinson, B., L. Rodegerdts, W. Scarborough, W. Kittelson, R. Troutbeck, W. Brilon, L. Bondzio, K. Courage, M. Kyte, J. Mason, A. Flannery, E. Myers, J. Bunker, and G. Jacque Mart. Roundabouts: An Informational Guide. FHWA-RD-00-067. FHWA, U.S. Department of Transportation, 2000. 5. Messer, C. J., and B. D. Fambro. Critical Lane Analysis for Intersection Design. In Transportation Research Record 644, TRB, National Research Council, Washington, D.C., 1977, pp. 26–35. 6. Special Report 209: Highway Capacity Manual. TRB, National Research Council, Washington, D.C., 1985. 7. Rodegerdts, L., M. Blogg, E. Wemple, E. Meyers, M. Kyte, M. P. Dixon, G. F. List, A. Flannery, R. Troutbeck, W. Brilon, N. Wu, B. N. Persaud, C. Lyon, D. L. Harkey, and D. Carter. NCHRP Report 572: Roundabouts in the United States. Transportation Research Board of the National Academies, Washington, D.C., 2007. 8. Manual on Uniform Traffic Control Devices. FHWA, U.S. Department of Transportation, 2009. 9. DLZ Kentucky Inc. Madison Pike (KY 17) Intersection Improvement Study. Northern Kentucky Area Planning Commission and Kentucky Transportation Cabinet, Fort Wright, Ky., 2006. The Operational Effects of Geometrics Committee peer-reviewed this paper.
