The Use of GPS and GIS to Analyze Access Near Intersections

Transcription

1 Huffman 1 The Use of GPS and GIS to Analyze Access Near Intersections Presented to the Urban Street Symposium July 28 th 30 th, 2003 Anaheim, CA Chris Huffman, P.E. Corridor Management Administrator Kansas Department of Transportation 217 SE 4 th Street, 4 th Floor Topeka, KS (P) (785) (F) (785) Huffman@ksdot.org Key Terms: Access Management, Intersection Operation, Property Rights

2 Huffman 2 ABSTRACT The land adjacent to busy at-grade intersections on arterial streets is often the most attractive to commercial development. The goals of a commercial developer are generally to obtain the best land at the best price in order to build the greatest possible traffic generator, and to have ready access to the facility. These goals often come into conflict with the goals of a transportation agency to preserve capacity, flow and safety on the adjacent arterial streets. In determining reasonable access to a site, conventional wisdom holds that intersections (including access points) must not exist within one another s influence area. The influence area of an intersection, which is generally defined as perception-reaction distance plus deceleration distance plus storage length, may be greater than the available property frontage. To what lengths may transportation agencies go in requiring zero overlap in intersection influence areas? The first step in answering this, which is as much a legal question as an engineering question, is to determine the actual intersection influence area. Reliance upon generally accepted theoretical models may not be sufficient to prove reasonableness if a developer is able to show significant economic impacts from the regulation of access. This paper will focus upon a methodology for determining actual intersection influence area. This includes GPS technology for velocity profiling to establish back of queue and influence area and GIS technology for analyzing and displaying these data including geo-referenced imagery to demonstrate alternate access possibilities. The goals of the methodology: to provide the information necessary to develop an objective spacing requirement that is appropriate to the unique circumstances and to provide a means to display and explain the conclusions reached. INTRODUCTION There are various tools available to engineering and planning professionals that assist in analyzing traffic flow and behavior of traffic at and near signalized and stop-controlled intersections. The Highway Capacity Manual, for instance, discusses a wide range of intersection related topics including queuing of traffic at intersections. It is generally held that access to a street or highway from a development should not fall within the anticipated traffic queue of a nearby intersection. There are also software tools that allow the user to analyze a wide range of possible values of different variables to attempt to predict how an intersection may behave under a given set of circumstances. Consider a newly developing or redeveloping area in which the land in the quadrants of arterial intersections may be quite valuable and will generally develop to the most intense use possible in order to achieve maximum return on the substantial real estate investment. A developer/owner will most generally hire a consultant to analyze the development and prepare a traffic impact study or traffic management plan using available traffic analysis tools. A basic limitation of any nationally accepted analysis tool is that it will function according to a set of assumptions. These assumptions may be inherent in the tool itself, such as driver behavior parameters, or they may be made by the user, such as headway or start-up lost time. By manipulating these assumptions, sometimes in very subtle and difficult to detect ways, a good analyst can virtually always come up with a scenario that should work.

3 Huffman 3 Regulators often find themselves in a very difficult scenario in which a developer s consultant is able to show that, according to the analysis, the proposal produces acceptable levels of service across the board, yet a common sense review reveals significant flaws. A regulator then faces three challenges: 1. To challenge the assumptions made by the analyst as being inappropriate or inaccurate. 2. To convince political or other decision making bodies that the developer s proposal is unsound. 3. To, perhaps, convince the court that actions taken by the regulator were based upon reasonable consideration and were not arbitrary and capricious. This last point may be especially critical as a standard defense to inverse condemnation. If a regulator follows the standard of the practice as presented in nationally accepted manuals, then it is unlikely that he has acted arbitrarily. However, as discussed above, a good analyst can make virtually any piece of software say virtually anything that is desired. So, if a regulator is to challenge a study prepared by an expert, it will often be helpful to have something other than arguments over assumptions to work with. TRAVEL TIME STUDIES The Institute of Transportation Engineers (ITE) Manual of Transportation Engineering Studies contains a chapter on travel time and delay studies. In a nutshell, travel time varies inversely with speed. Thus, by measuring the amount of time needed to traverse a corridor is a function of the operating speed of the arterial. This is an important concept because arterial level of service is a function of speed and is often the most difficult level of service to determine. While an analyst may be able to show that an intersection will or will not work by way of intersection level of service (average vehicle delay), measuring the impact of that intersection on the arterial corridor is much more difficult. Travel time studies provide a baseline or existing condition that is not dependent upon the assumptions found in analytical software. This baseline can then be used to calibrate simulation models or other analytical software to analyze newly developing areas (urban fringes) or can be used to measure intersection influence areas directly for redevelopment of existing areas. The challenge with travel time studies is not in performing them, this is relatively straightforward, the challenge is in finding a way to display data in a meaningful way. While mountains of spreadsheets are often useful and revealing to a trained professional, they are cryptic and frustrating to a non-technical audience. A political decision making body, judge or jury need a display of travel time data that explains complex theories, concepts and methods in a way that can be grasped intuitively and, preferably, visually. USE OF GPS AND GIS IN TRAVEL TIME STUDIES Geographical Information Systems (GIS) are ideal in meeting this challenge. A GIS has the capability to connect to any number of data tables and display that data visually. The catch to that is that the data must have a spatial component, and this is where Global Positioning System (GPS) comes in. Collecting position, velocity and time-stamp information via a GPS receiver

4 Huffman 4 allows a single analyst to travel the corridor and collect data automatically, which improves both the safety and efficiency of collection. The corridor selected for review is Gage Boulevard in Topeka, Kansas. Gage is a minor arterial street that serves commercial, residential, institutional and public land uses and the development adjacent to it has been established long enough that some redevelopment is beginning to occur. For the purposes of this paper, a two-mile stretch between 10 th and 21 st Streets was chosen. The ITE Manual of Transportation Engineering Studies provides guidance on the number of runs that should be made for purposes of statistical validity. When designing such a study, particularly using GPS and GIS equipment and software, the analyst should take care to consider several factors. 1. Information Needed What information is necessary to review the study that has been submitted? How might this information be used to review future studies, or to conduct studies of your own. These are fundamental questions, but serious consideration to them should be given to avoid having to make repeated data runs to collect something that was left out. Most GPS software gives the user the capability to design data dictionaries so that features may be specified as well as attributes of those features. For the travel time runs themselves a very simple data dictionary may suffice, see Figure 1 below for an example of such a dictionary.

5 Huffman 5 Figure 1. Sample Data Dictionary for Travel Time Runs. However, simply collecting progression speed data may not be sufficient to conduct a complete review. For instance, intersection control is an important element of arterial progression and such information can also be collected via a data dictionary. If an intersection is signal controlled, then a signal identification number could also be entered into the data dictionary and that information could then be used to cross reference information regarding that signal including phasing and timing, which of course is important to any intersection analysis.

6 Huffman 6 Figure 2. Sample Data Dictionary for Intersection Control. Another critical element of determining the data needed is deciding what time periods are needed. If pm peak times are usually the design scenario, then travel time data must be collected during pm peak hours. Be careful to distinguish between data sets taken at different times of the day. 2. Availability of Other Data Sources In what context does the travel time data need to be displayed? Providing the audience with a backdrop will help put the data in perspective. For instance, if geo-referenced aerial imagery is available, this will help the audience identify with the information presented. Geo-referenced information on other major topographical or cultural features will also assist the audience.

7 Huffman 7 Figure 3. Backdrops such as aerial photography (of the greater Topeka area) and major features such as State Highway System (in red) will help the audience identify with the information presented. 3. How Will the Travel Time Data be Displayed? Many GPS receivers will give the user several options on how to collect and display data. The datalogger used by the Kansas Department of Transportation is the Trimble Geo- Explorer III. This unit gives the user the option to collect point, line or area features and to collect data in time or distance increments. Consideration of how the data can be most usefully displayed will effect how the data is collected. The companion software to the Geo- Explorer III is Trimble s Pathfinder Office software product. Once data is collected it is downloaded to a computer and can be displayed in a map window. This allows the analyst to identify any data anomalies and identify and eliminate any outliers.

8 Huffman 8 Figure 4. Graphically displaying the raw data allows the analyst to identify any data anomalies and, if necessary, eliminate outliers. 4. Determine What Constitutes Delay This decision is more political than analytical in nature. In order to make use of the travel time data to map out the influence area of an intersection a determination must first be made as to what constitutes influence. Is only stop condition considered delay, or are low speeds to be included? If low speeds are to be considered as part of delay, then what is the velocity threshold? Regardless of the number chosen, the threshold is a subjective measure based upon a level of delay that is considered acceptable. 5. Map The Speed Profile Each GIS system works differently, but the ability to map a spatial dataset according to a theme should be possible in virtually any GIS software. It may be useful to map the segments that fall within the category of delay in the color red and then marginal segments as another color. The style and scheme of the display are up to the analyst and can be customized as needed.

9 Huffman 9 CONCLUSION Simple modifications to the method described above can make it useful for an established corridor (evaluation of proposed redevelopment) or in newly developing areas (urban fringe). The data from travel time runs, assuming that velocity can either be logged or calculated, can be directly applied to determination of arterial level of service and can be used to calibrate simulation models. The shear numbers of GPS and GIS packages available make a detailed discussion of software steps useless, but the method presented should be applicable regardless of the hardware and software platforms chosen.