Synthesis of Spatial Network Level Metrics with Local Quantitative Metrics for Prioritizing Transportation Projects Abstract Prioritizing and selecting few critical projects from several competing transportation projects is a multiobjective combinatorial optimization problem (MOCO). In such problems, transportation planners and managers are interested in visualizing and analyzing the trade-offs involved. This paper develops methodology for synthesis of spatial network level metrics with local quantitative metrics for planning and prioritizing of a large and diverse portfolio of transportation investment projects. The methodology serves as an aid to planners, managers and engineers to visualize and compare costbenefit trade-offs. The methodology is based on incorporating network level metrics along with local metrics in formulating generic MOCO algorithm and visualizing multiobjective trade-offs on a spatial network. A district level case study demonstrates the use of methodology in trade-offs analysis for short- and long-range transportation plans. The methodology is adaptable to other areas like water supply management, manufacturing and service industry where spatial consideration is important. Key Words: MOCO problem; network visualization; Entropy; transportation planning; risk analysis. 1. Introduction Transportation professionals at district and state transportation agencies in the United States are increasingly pressed with the task of allocating limited public funds among potential highway improvement projects and the subsequent determination of the order in which selected projects should be undertaken. There are several projects competing for these limited funds. Prioritizing highway projects is basically a Multiobjective combinatorial optimization (MOCO) problem. Over the last decade, many algorithms have been developed to solve such types of problems. But these mathematical models have been rarely used by managers and professionals in actual decision making. The major problem is lack of visualizing and understanding these models and trade-offs involved. We believe that models should be represented in a clear and informative way. This paper aims at integrating network level metrics with local quantitative metrics to better understand the impact of selecting certain projects on the specified geographical region. The scope of the paper includes: developing a generic MOCO model for prioritizing and planning transportation projects, developing a spatial network and network level metrics, identifying useful quantitative metrics for the comparison of diverse projects, developing an interface for visualizing project portfolio information and relative trade-offs on the spatial network and presenting a case study with short and longrange plans from the Culpeper district in the state of Virginia. The organization of this paper is as follows. Section 2 provides background for generic MOCO formulation of projects planning and prioritizations problem. Section 3 describes the methodology for integrating network level metrics with quantitative metrics. Section 4 presents the case study and discusses the scalability of the developed methodology. The last section provides a summary. 2. Background The purpose of this section is to formulate the projects planning and prioritization problem as a multiple objective combinatorial optimization problem. Projects planning and prioritization problem is combinatorial in nature and it involves multiple and conflicting objectives. In other words, we are interested in studying finite set of competing projects and combinations and arrangements of these projects that satisfy specified criteria. Our MOCO formulation is based on the general model as per Ehrgott and Gandibleaux (2002). The finite action set A = {a1,…, an} represents n competing transportation projects, where ai represents a unique project. The power set of A is the set of all subsets of A and is denoted by P(A). The feasible set of the MOCO problem can be defined as a subset X such that, X P(A) The problem can be formulated in terms of binary decision variables. A binary decision variable xi is introduced for every element ai A. Then, a basic feasible solution S X can be represented by a binary vector {x1,…, xn} where xi = 1 0 ai S otherwise In other words xi =1 indicates that ith project is selected in the final solution and xi=0 suggests that ith project has not been selected in the final solution. A typical multiobjective problem can have Q objectives or criteria, zj where j = 1,…, Q. Then the overall problem takes the form of maximizing a set of objective functions: Maximize {z 1 (S ),..., z Q ( S )} Examples can be easily constructed to clarify this MOCO formulation. For example, assume that there are 3 competing projects in a specified region and the decision makers are considering two criteria for projects prioritization: first, to select a portfolio of projects that will address higher average daily traffic (ADT) problem is high and second, to select a portfolio of projects that will address higher crash rates concerns. ADT and crash rate are two important measures in transportation planning. These two objectives are not necessarily in conflict, but definitely they will not yield the same portfolio of projects. The action set, A = {a1, a2, a3} for this particular problem is of order n = 3. Then, the power set of A will be of order 23= 8 as follows: P(A) = {, {a1}, {a2}, {a3}, {a1, al2}, {a1, a3}, {a2, a3}, {a1, a2, a3}} The basic feasible set for this problem will be any subset X of the power set P(A). There are three binary decision variables x1, x2 and x3 and a basic solution S can be represented by a binary vector {x1, x2, x3}. One particular solution can be {1, 0, 1}, i.e., in final portfolio, project a1 and project a3 have been selected and project a2 has not been included. The two objective functions will be z1(S) and z2(S) representing two different criteria as mentioned above. Thus, the overall combinatorial model is: Maximize {z 1 ( S ), z 2 ( S )} SX X P(A) A = {a1, a2, a3} xi = 1 0 ai S otherwise 3. Methodology The purpose of this section is to develop the spatial network level metrics, to identify useful local quantitative metrics for the comparison of diverse projects and to implement generic MOCO algorithm and represent multiobjective trade-offs on the spatial network. The synthesis of spatial network level metrics with quantitative metrics will greatly enhance understanding and analysis of complex models for transportation planners and managers. The first step in proposed methodology is to develop a spatial network, G (N, E) of cities and highways for a geographical region under study where N is the collection of nodes and subset E of N X N is the collection of arcs. The network is undirected. The nodes in this specific network represent major cities in the specified region and arcs represent the highways connecting these major cities. The network can be a short one consisting of only a few cities or it can be a very detailed one consisting of large number of cities depending on the nature of analysis a decision maker is interested in. The second step is to locate the competing projects on the network, so that planners can have a comprehensive look at the distribution of projects throughout the specified region. The next step is to gather quantitative evidences for each candidate project from a given set of projects being considered for funding. There can be many local quantitative metrics based on the criteria to be satisfied while selecting a portfolio. Few good quantitative metrics are level of service, ADT, crash rate, volume to capacity ratio and flow rate on each arc in the network. Unemployment rates and environmental issues in different portions of the network are also important measures. Cost effectiveness of individual project is another important metric. Then there are some specialized measures developed by transportation agencies like bridge sufficiency rating, etc. The significance of each quantitative metric varies according to the criteria selected by planners and decision makers. For example, if safety is one of the main criteria, then crash rate is the most important metric. If economic development is a major criterion, then unemployment rate is a highly significant measure. Among several measures, we believe that ADT and crash rates on each individual arc and cost of the project are three meaningful and typical measures for transportation projects planning and prioritization. The paper mainly uses these three measures for the case study. But, it is not binding. Similar analysis can be performed using any of the other local quantitative metrics. We can apply generic MOCO formulation to the developed spatial network in which different arcs have different values of quantitative measures. For one criterion, individual arc crash rates might be more important and for another criterion, arc ADT might be more significant. Consideration of arc ADTs will give one portfolio of projects and consideration of arc crash rates will give another. The problem is essentially of which combination of projects will be satisfy these two criteria. It is generally observed that the decision makers will try to choose a portfolio that maximizes these local quantitative measures. With many different local criteria under consideration, there is one more important criterion, i.e. topology of the network, which is generally neglected by the decision makers. Fair distribution of the resources among all arcs (highways) is as important as maximizing the local criteria. The advantage of the methodology is that decision makers can easily visualize the distribution of the projects throughout the network. They get good understanding of whether the resources are all concentrated in a specific portion of the network or whether they are equally distributed. But mere visualization of the distribution is not sufficient. It can be subjective. One decision maker might interpret it differently than the other. To solve this problem, we propose other network level quantitative metrics which can be added as separate criteria along with local level criteria to be satisfied. We use a special network complexity index to see how uniformly the resources are distributed throughout the network. Different indices can be developed for different network level quantitative metrics. One of such network complexity index can analyze the distribution of the projects throughout the network. This index is denoted by, H 1 Pi (ln( Pi ) i where, Pi Ni N Here N denotes the total number of projects in the selected portfolio and Ni denotes the number of projects from the portfolio that are on the ith arc. Higher value of the indicates more uniform distribution of projects throughout the spatial network and lower value of index indicates that the distribution of projects is not uniform throughout the network or the projects are concentrated among few cities or only a few highways in the network. In general, heavy concentration in any particular area of the network is not advisable unless there is a good justification for it. Similarly, another network complexity index can be developed to analyze the distribution of the funds throughout the network by modifying Pi in the previous equation as: Pi Ci C Where C denotes the cumulative cost for all the projects in the selected portfolio and Ci denotes the total cost of the projects from the portfolio that are on the ith arc. Thus our goal is to maximize these network level indices along with other local objectives in the MOCO formulation. Many solution methods have been developed to solve MOCO problems. These methods are mainly classified into three categories based on the role of the decision maker in the portfolio selection process: priori mode, posteriori mode and interactive mode (Ehrgott and Gandibleaux, 2002). We prefer interactive mode because in this mode, managers and planners are actively involved in the solution process. The last step in the methodology is to show the trade-offs graphically. A key aspect of the methodology is the interface used to present project information on the network. Graphing and visualizing two objectives trade-off problem in the decision space or functional space is relatively easier. But when a typical multiobjective problem consists of three or more objectives, suddenly visualizing and understanding the tradeoffs involved becomes difficult. The problem essentially turns into visualizing multivariate data involving several variables simultaneously. Many techniques have been discusseed to represent trivariate or multivariate data effectively (Cleveland, 1994, Gower & Digby, 1981, Tufte, 1997). Trivariate data can be represented by bubble plots, in which values of two variables are indicated by the location of the bubble on a two- dimensional scatter plot and value of third variable is represented by the size on the bubble. One important technique to represent multivariate data is scatter plot matrix. A scatterplot matrix is defined as a square, symmetric table or “matrix” of bivariate scatterplots (Cleveland, 1994). We propose a matrix of trivariate bubble plots to visualize and understand the trade-offs involved in projects planning and prioritization. Our symmetric matrix has m rows and m columns. Number of rows and columns are equal to the number of nodes (cities) in the spatial network developed earlier. The intersection of row i and column j contains a bubble plot showing the total number of projects on a typical arc between two specific cities. Three criteria can be shown simultaneously on this bubble plot. For example, a typical matrix will have bubble plots showing average daily traffic on x-axis, crash rate on y-axis and cost of the project will be indicated by the size of the bubble. One can develop similar matrix different quantitative metrics and criteria. This matrix presents a large amount of information to the decision maker in a very efficient manner. 4. Case Study Following district level case study shows the application of the methodology. Culpeper district in the state of Virginia is selected for the case study. Table I shows sample project information collected for this case. Project data include the project ID, route, starting and end points, ADT, crash rate and cost, of the project. << Table 1>> Specified region consists of 42 competing projects. Thus, the initial action set for the problem, A = {a1, …, a42} is of order n = 42. The developed spatial network is shown in Figure 1. The network consists of 14 nodes (cities) and 21 arcs. The number on each arc represents actual highway number. The network consists of mainly primary highways in the Culpeper district. Interstate highways are not included in the network since interstate highway projects come under a different state level interstate highway projects prioritization program. The methodology is scalable. A similar state-level interstate highways network can be developed for interstate highway projects prioritization plan. << Figure 1>> Projects are grouped together according to their location in the network and can be described with corresponding quantitative information such as number of projects, costs, average cost, ADT, ADT per dollar, etc. As an example, Figure 2 shows the total number of projects on each arc in the network. Such aggregate statistics can suggest the network complexity and the potential gross efficiencies of investments in projects on each individual arc. Another good example of quantitative evidence can be the ratio of cumulative accident rate and the total project cost. << Figure 2>> After projects are grouped based on their location in the network, analysis can be done to compare projects within a group. Figure 3 shows an example of such an analysis for projects planning and prioritization. Shown are the projects on a particular arc. Coordinate axes are used with average daily traffic and crash rates as measures of crash exposure and crash intensity. Each of the projects is then represented by a bubble icon, centered on coordinate values of the project and whose size represents the project cost. The variations among project costs and the contribution of each project in terms of crash exposure and crash intensity is shown in the figure. << Figure 3>> The power set of the initial action set is of the order 242. Implementing the methodology on every element of power set is both unnecessary and impossible. Some of the solutions in the power set are obviously inferior. It is important to remove such obvious inferior solutions at this stage to simplify the application methodology. For example, empty set, and sets of only one project can not be efficient. Similarly set of all the projects is also meaningless. As mentioned previously, using interactive mode with the decision maker, many such inferior solutions can be identified to develop a small set of meaningful solutions. It is also helpful to know the approximate number of projects the decision maker is interested in undertaking simultaneously. After initial analysis, the final set of efficient solutions consists of combinations of only 20 projects from initial action set of 42 competing projects. These 20 projects along with other relevant information are shown in Table II. 5. Summary The developed methodology for synthesis of network level metrics with quantitative metrics for projects prioritization can serve as a decision aid among the transportation planners. 6. Acknowledgment The authors are grateful to the members of the steering committee from Virginia Transportation Research Council and Virginia Department of Transportation: Chad Tucker. References Cleveland W.S., (1994). The Elements of Graphing Data (Rev. ed.). Summit, NJ: Hobart Press. Ehrgott M., Gandibleux X., (2002). Multiobjetive Combinatorial Optimization – Theory, Methodology, and Applications. Multiple Criteria Optimization: State of the Art Annotated Bibliographic Surveys. Gower J.C., Digby P.G.N. (1981). Expressing complex relationships in two dimensions. In V. Barnett (Ed.), Interpreting Multivariate Data. Chichester, UK: John Wiley and Sons. Jones C.V., (1996). Visualization and Optimization. Shannon C.E.,(1948). A Mathematical Theory of Communication. Bell System Technical Journal, Vol. 27, PP. 379-423 and 623-656. Tufte E.R., (1997). Visual Explanations. Cheshire, CT: Graphics Press. Baker J.A., Lambert J.H., (2001). Information systems for risks, costs, and benefits of infrastructure improvement projects. Public Works Management and Policy, 5(3):198-208. Capital Improvement Program (2000). Delaware Department of Transportation, Dover. (http://www.state.de.us/deldot/cip00-05/execsumm/priorities/sample.html). Charlottesville-Albemarle Regional Transportation (CHART) 2021 Plan Update. (2001). Thomas Jefferson Planning District Commission. (http://www.tjpdc.org/trans/chart/chart_2021.pdf). Cheng W., Washington S.P. (2005). Experimental evaluation of hotspot identification methods. Accident Analysis and Prevention, 37(5). Gray G.M., Hammitt J.K. (2000). Risk/risk trade-offs in pesticide regulation: an exploratory analysis of the public health effects of a ban on organophosphate and carbamate pesticides. Risk Analysis, 20(5): 665-680. Elvik R. (2003). How would setting policy priorities according to cost-benefit analyses affect the provision of road safety? Accident Analysis and Prevention, 35(4):557-570. Frohwein H., Lambert J.H., Haimes Y.Y., Schiff, L.A. (1999). Multicriteria framework to aid comparison of roadway improvement projects. Journal of Transportation Engineering, ASCE, 125: 224-230. Kulkarni R.B., Burns R.L., Wright J., Apper B., Baily T.O., Noack S.T. (1993). Decision analysis of alternative highway alignments. Journal of Transportation Engineering, 119 (3):317-332. Lambert J.H., Peterson, K. (2003). Decision aid for allocation of transportation funds to guardrails. Accident Analysis and Prevention, 35(1):47-57. Mahalel D., Hakkert A.S., Prashker (1982). A system for the allocation of safety resources on a road network, Accident Analysis and. Prevention, 14(1): 45–56. McDaniels T.L., Gregory, R.S., Fields, D. (1999). Democratizing risk management: Successful public involvement in local water management decision. Risk Analysis, 19(3): 497-510. Meijnders, A.L., Midden, J.H., and Wilke, H.A. (2001). Role of negative emotion in communication about CO2 risks. Risk Analysis, 21(5):955-966. Miaou S.P. and Song J.J. (2005). Bayesian ranking of sites for engineering safety improvements: Decision parameter, treatability concept, statistical criterion, and spatial dependence. Accident Analysis and Prevention, 37(4), 699-720. O’Connor R.E., Bord R.J., Fisher A. (1999). Risk perceptions, general environmental beliefs, and willingness to address climate change. Risk Analysis, 19(3):461-471. Performance Programming Process (2000). Montana Department of Transportation, Helena. (http://www.mdt.state.mt.us/planning/tranplanp3.pdf). Pigman J.G., Agent K.R. (1991). Guidelines for installation of guardrail. Transportation Research Record 1302, TRB. National Research Council, Washington, DC. Rene L., Meertens R.M., Bot I. (2002). Priorities in information desire about unknown risks. Risk Analysis, 22(4):765-776. Saaty T.L. (1995). Transport planning with multiple criteria: the analytic hierarchy process applications and progress review. Journal of Advanced Transportation, 29:81-126. Sjoberg L., Fromm J. (2001). Information technology risks as seen by the public. Risk Analysis, 21(3): 427-441. State Highway System Project Evaluation Criteria (1998). Alaska Department of Transportation and Public Facilities, Juneau. Tabucanon, M.T. and Lee, H.M. (1995). Multiple criteria evaluation of transportation system improvement projects: the case of Korea. Journal of Advanced Transportation, 29: 127-143. TEA-21. (2004). The Transportation Equity Act for the 21st Century. U.S. Department of Transportation (http://www.istea.org/). Transportation Programming Guide. (2001). Department of Public Works City of Sacramento. (http://www.pw.sacto.org). Transportation Review Advisory Council Policies for Selecting Major New Capacity Projects. (2000). Ohio Department of Transportation, Columbus. (http://www.dot.state.oh.us/trac). Tsamboulas D.T., Yiotis G.S., Panou K.D. (1999). Use of multicriteria methods for assessment of transport projects. Journal of Transportation Engineering, 125(5):407414. Figure 1. Spatial Network for the Culpeper District The plains Culpeper District Network View 14 cities { 17} Washington { 211} Warrenton External { 29, 25, 17} { 28} { 522} Remington { 29, 15} Culpeper { 29} Madison { 15} { 522} { 29} Stanardsville Orange { 33} { 20} { 15} External { 29, 240, 250} Gordonsville { 33} { 22} Charlottesville {208} { 22, 208} { 15} { 20} Scottsville Columbia Louisa Mineral Figure 2. An example of comprehensive network-level aggregate information generated by the developed methodology for projects planning and prioritization showing total number of projects on each arc 1 Louisa Gordonsville Culpeper Columbia Charlottesville 2 Madison 2 Orange 5 Warrenton The plains 1 1 Mineral 3 Remington 2 Scottsville 1 Stanardsville 1 2 3 6 Local 1 1 Washington 10 Figure 3. Example of a simple analysis generated by the developed prototype for projects planning and prioritization showing project contributions in crash rates and ADT, and project cost represented by the size of icon 600 500 Crash Rate 400 300 200 100 0 0 20000 40000 ADT 60000 Table I. Sample project information for Culpeper District Case Study Project ID # 2070004 2070005 Route 00020 00020 From RTE 708 RTE 250 ADT 8805 10618 Crash rate 266.65 393.47 Cost 24826 20378 I-64 To RTE 53 BARN BRANCH RTE 250 UNDERPASS 2070010 00029 34938 126.86 10767 2070011 00029 RTE 643 GREENE CL 45407 93346 2070013 00240 RTE 250 W 6692 2070014 00250 14757 204.01 49630 2070015 00250 I-64 RTE 677 OLD BALLARD RTE 250 E RTE 677 OLD BALLARD 165.05 355.12 14757 320 22119 2070016 00250 22703 253.54 4724 2070017 2070018 2070019 00250 00250 00250 305.76 178.33 210.36 26970 19745 16125 00029 00015 00211 00229 00522 00015 00015 00015 00017 00028 00029 00211 00015 00029 I-64 RTE 22 FLUVANNA CL WCL CHARLOTTESVILLE RTE 686 FAUQUIER CL RTE 211 RTE 3 RTE 17 RTE 15/29 BYPASS WARRENTON SCL RTE 55 PR WILLIAM CL PR WILLIAM CL WARRENTON WCL RTE 250 RTE 33 35950 23526 10558 2070049 2070021 2070023 2070024 2070025 2070028 2070029 2070030 2070031 2070032 2070033 2070034 2070042 2070045 45097 7408 13704 8793 5778 34360 42597 9600 11918 12637 45537 15061 5314 30227 70.21 114.49 301.76 170.15 170.46 94.07 95.13 45.18 261.1 157.4 106.77 54.75 110.92 146.58 15478 26401 4085 14553 24314 21835 45193 4829 5535 45740 61913 24322 12701 34573 2070046 2070050 2070053 2070054 2070055 2070054 2070058 2070059 2070064 2070092 2070074 2070079 2070081 00033 00015 00022 00022 00022 00033 00033 00033 00208 00015 00015 00015 00020 RTE 29/250 BYPASS ECL CHARLOTTESVILLE I-64 RTE 22 RTE 250 UNDERPASS MADISON CL RTE 229 RTE 694 ORANGE CL RTE 15/29 BUS RTE 17 RTE 15/29 BYPASS RTE 17 SB RTE 15/29 RTE 15/29 BUS CULPEPER CL RTE 1001 ALBEMARLE CL SHENANDOAH PARK ENTRANCE RTE 617 RTE 33 E ECL LOUISA WCL MINERAL WCL LOUISA RTE 1005 RTE 22/208 RTE 522 I-64 ORANGE CL ORANGE NCL ALBEMARLE CL RTE 29/250 BYPASS WCL CHARLOTTESVILLE 4968 6095 11493 11493 11402 5242 5242 5846 4095 4101 8435 9377 2859 111.86 112.54 495.03 112.82 217.81 97.98 142.64 109.02 147.36 155 120.21 55.69 362.76 12648 25700 2376 14905 968 3091 1470 4329 3801 23985 14482 5797 804 2070088 00015 RTE 211 BUS RTE 33 BYP RTE 33 ECL LOUISA WCL MINERAL RTE 522 RTE T 669 RTE 22/208 ECL LOUISA SPOTSYLVANIA CL RTE 617 CULPEPER CL MADISON CL RTE 33W WINCHESTER STREET 33940 6.62 6708 28946 2070089 2070091 00015 00211 ALEXANDRIA PIKE WARRENTON WCL N1 N2 00029 RTE 29 RTE 33 W LEE HIGHWAY SHIRLEY AVENUE 0.5 MN NORTH RIVANNA RIVER RTE 15 S 12089 24513 28.34 8.52 3480 4208 57541 11660 438 45.19 151000 11930 Table II. Projects chosen for final analysis Project ID # 2070011 2070018 Route 00029 00250 2070017 2070033 00250 00029 From RTE 643 I-64 ECL CHARLOTTESVILLE RTE 15/29 BUS N1 2070029 2070054 2070053 2070055 00029 00015 00022 00022 00022 RTE 29 RTE 17 ECL LOUISA RTE 33 E WCL MINERAL 2070016 2070092 2070032 2070005 2070028 00250 00015 00028 00020 00015 2070049 2070023 2070054 2070024 2070079 2070042 00029 00211 00033 00229 00015 00015 RTE 29/250 BYPASS I-64 RTE 15/29 RTE 250 RTE 15/29 BUS RTE 250 UNDERPASS RTE 229 WCL LOUISA RTE 694 ORANGE NCL RTE 1001 Crash rate To GREENE CL RTE 22 ADT 45407 23526 165.05 178.33 Cost 93346 19745 I-64 PR WILLIAM CL 0.5 MN NORTH RIVANNA RIVER RTE 15/29 BYPASS WCL MINERAL ECL LOUISA RTE 522 WCL CHARLOTTESVILLE RTE 617 PR WILLIAM CL BARN BRANCH RTE 17 WCL CHARLOTTESVILLE FAUQUIER CL RTE T 669 RTE 211 MADISON CL RTE 250 35950 45537 305.76 106.77 26970 61913 57541 42597 11493 11493 11402 438 95.13 112.82 495.03 217.81 151000 45193 14905 2376 968 22703 4101 12637 10618 34360 253.54 155 157.4 393.47 94.07 4724 23985 45740 20378 21835 45097 13704 5242 8793 9377 5314 70.21 301.76 97.98 170.15 55.69 110.92 15478 4085 3091 14553 5797 12701