Overview

Overview, Introduction, and Getting Started Printable Version

This case study is about a Traffic Impact Assessment for a proposed site development in Clifton Park, New York (see Exhibit 2-1). The large parcel of land in question is south of where Maxwell Drive intersects with Clifton Park Boulevard (State Route 146). Like most impact assessments, the main question is this: in the horizon year for the analysis, what geometric improvements will be required to mitigate the impacts of the site development in conjunction with normal traffic growth?

The Town of Clifton Park and New York State Department of Transportation (NYSDOT) both have roles to play in the decision-making. So do Saratoga County, the town’s citizens, the nearby land owners, and the developer. There are issues about how many intersections should be studied, what time periods should be examined, what goals and objectives should be used, and what performance measures should be employed.

To address these issues and others, the case study includes six problems. They range in scope from a single intersection to a sub-area network. Each one illustrates some aspect of the impact assessment. Each one also illustrates how various traffic analysis tools in the Highway Capacity Manual can be applied to assist traffic analysts, engineers, planners, and decision-makers in making sound investment decisions regarding changes to a transportation system.

The problems focus on the chapters of the HCM that deal with interrupted flow facilities, especially signalized intersections, arterials, freeway interchanges, and arterial weaving. After studying this material, you should be able to:

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Overview

There are many ways to classify traffic analysis problems that are appropriate for analysis with the HCM. The graphic below includes seven categories. The kinds of problems that are addressed in this chapter, within these seven categories, are highlighted in  BLUE.

Introduction

Overview, Introduction, and Getting Started Printable Version

Route 146 (Clifton Park Boulevard) is a major multi-lane arterial in Clifton Park, New York. It serves trips to and from three major shopping centers, a local school district campus, and many commercial and residential areas. Route 146 also connects these areas to Interstate 87 (the Northway) for trips toward Albany and Montreal.

Route 146 is the main street for the town. Shopping centers on either side are the downtown. Route 146 is also a major link between Schenectady on the west and Mechanicville on the east. Exhibit 2-1 shows the section of Route 146 that is the focus of this case study. 

Much vacant land south of intersection C (Maxell Road) is still available for development, as is evident in Exhibit 2-2. As development occurs, traffic impacts will result. Geometric and operational changes will be needed to maintain acceptable operations at roadway intersections. At a minimum, the northbound approach will have to be added at intersection C, but there will be other improvements as well.

We have been tasked with determining what those geometric and operational changes should be. We’re going to do a generic traffic impact analysis (TIA) for this corridor. As is common in TIA’s, we are going to focus on three conditions (the AM peak hour, the PM peak hour, and the peak hour of the generator, with the implicit assumption, at least initially, that the last is one of the former two) for existing and future conditions. Moreover, for the future conditions, we’re going to examine the network’s performance with and without the development.

We are not going to present the entire TIA, just portions of it. We’re going to use the TIA setting as a backdrop against which to illustrate use of the HCM analysis procedures. Consequently, we’re going to focus on specific intersections and conditions to illustrate specific types of analyses and issues.

Getting Started

Overview, Introduction, and Getting Started Printable Version

To begin this case study, issues relating to setting the scope, goals, objectives, the analysis methodology, the sequencing of the analysis, the tools to use, and the data to be employed will need to be identified. Each of these topics will be addressed in this section.

Getting Started

Scope
To begin the case study, as with the traffic impact assessment, decisions have to be made about a variety of study issues. One is the scope of the analysis: that is, the intersections to include, the components of the freeway interchange that might be necessary to consider, and the time periods for which the analysis is to be performed. Additionally, identification of the stakeholders, their issues, and how their issues translate into goals, objectives, and performance measures for the study will need to be considered, as will who the audience is. Each of these will affect other issues, like the analyses to perform, sequencing, tools to use, and data to collect.

Let’s think about the scope first. How should the system be defined? What facilities should be involved beyond the main site-access at intersection C? What about intersections further away? Often, you look at the amount of traffic contributed to the total intersecting traffic by the site-related traffic, and include the intersection if the resulting percentage increase is above a given threshold. You also might decide on which facilities to include based on whether a LOS change is likely. In the latter case, you might decide a performance assessment is necessary for a decision. For the purposes of this case study, we’ve decided that intersections A through F should be included, as shown in Exhibit 2-1. When we get to the end of the analysis, we’ll reflect on whether that decision was correct.

The time periods to consider are a second important aspect of the scope. We’ve indicated that the AM peak, PM peak, and peak hour of the generator should all be considered. Sometimes it is important to look at the Saturday midday and the Friday peak hours as well (e.g. for a shopping center) or a collection of heavy use days during the year (e.g., for a sports facility). In the case of this particular system, it’s also important to study peak shopping days in November and December because the traffic volumes are significantly above average then.

Getting Started

Next, let’s consider who the stakeholders are. The town is clearly one of them. The users are another. Other interested parties would be the county (Saratoga County), nearby property owners, the shopping center owners and tenants, and the school district. The school district is important because its main campus is just south of intersection A. When you’ve decided who all these groups are, consider the benefits and costs that accrue to each of these groups in every aspect of the case study.

Related to the stakeholders is the choice of audience. In this case, it is the town, with the caveat that the town would then forward your report to NYSDOT. Your audience could also be the system users or the adjacent land owners, if they request a presentation about your findings.

Discussion:
Setting the scope is extremely important. Think about the highest use hours during the year. How would you decide what are the right time periods to consider? The right intersections? What criterion would you use to decide whether a given intersection or facility should be in the study or not?

Getting Started

Goals and Objectives
For purposes of the case study, we’re going to assume the goal is to mitigate negative impacts resulting from traffic related to the site and general background traffic growth. We have to separate the former from the latter to ensure that we know which impacts are due to general traffic growth and which are due to the site. The objectives are to identify the needed improvements in geometry and the operational changes required. The former is likely to be lane additions while the latter is likely to be changes in lane use designation and signal timing.

Related to the goals and objectives are performance measures by which the system’s performance is evaluated. In many cases, they’re easy to identify. Delay is one that’s common. Another is level of service (LOS). (Sometimes delay implies level of service.) Others are total vehicle hours of travel, total vehicle miles, air pollution outputs, and noise impacts. For this case study, we will focus on delays and queues.

Discussion:

What scenarios will we need to model to address these goals? What other potential issues may need to be addressed as part of this development?

Getting Started

What Analyses to Perform
The next issue we mentioned was what analyses to perform. In this case study, you need to examine each intersection in each time period, but you also might want to do some specialized analyses, say for heavier-than-typical traffic conditions that arise during special times during the year. You might also want to do a system-level analysis to ensure that you have accounted for all the impacts that arise. We’ll try to answer these questions as the case study proceeds.

For the entire TIA, at least 45 intersection-level analyses are involved. That’s based on five intersections (A, B, C, D, and G), three time periods (AM peak, PM peak, and Saturday midday), and three conditions (existing, future without, and future with). In addition, for the freeway interchange, you need to study at least seven locations: the terminus of the southbound-to-westbound off-ramp, the entrance to the southbound on-ramp, the terminus of the northbound off-ramp (signalized intersection F), the two ramp junctions on I-87 northbound, the ramp junction on I-87 southbound, and the eastbound weaving movement under the I-87 bridges between the southbound-to-eastbound off-ramp and the eastbound-to-northbound on-ramp. That means 63 more analyses: seven locations for three time periods and three conditions.

As indicated, we’re not going to present all 108 analyses. Instead, we’re going to focus on specific problems that let us illustrate how to use the HCM. The problems we are going to consider are listed in Exhibit 2-3.

For each, we’ve identified the situation that will be analyzed (e.g., the school complex entrance during the AM peak) and the learning objectives and analysis issues that will be highlighted by that problem. The issues can be divided into scope (e.g., geographic boundaries, time periods), demands (e.g., demand vs. volume), geometrics, and signal control (e.g., critical movements). After we've completed the problems, we will extend what we’ve observed to speak more generally about conclusions and insights that would relate to the overall TIA. These insights are important to effectively communicate important findings to your audience and the other stakeholders. They also help with sensitivity analysis and ensure that the solutions are feasible.

Discussion:

Take a careful look at the network diagram. What other analyses would you suggest? What ones would you want to include? What overall network do you think should be analyzed if you were doing a traffic impact assessment? Why?

Getting Started

Sequencing, Tools, and Data
Sequencing the analyses is another issue. We’re going to focus on the signalized intersections first, starting at Maxwell Drive (Intersection C), working west to Intersections B and A, then east to Intersections D and G (see Exhibit 2-1). This allows us to study the most affected intersection first, then ones that are less affected because they are further away. We’ll consider the freeway interchange after we’ve finished with the signalized intersections, then we will analyze the arterial segment consisting of intersections A to D.

Choosing the tools to use is another issue. You have to pick ones that strike a balance between the amount of effort they require and the amount of insight they provide. In this case study, we’re going to use two tools. The first is the Highway Capacity Manual. We’ll use it for all of the intersection analyses and the location-specific analyses at the freeway interchange. The second is Transyt-7F. We’ll use it to do an arterial analysis of intersections A through D. It will help us ascertain what coordinated signal timings would work best for that set of signals. It should be emphasized that we are using Transyt-7F here to illustrate the use of non-HCM procedures, and other programs could also be used with equal effectiveness.

Since the main purpose of this guidebook is to illustrate how the HCM can be used to study traffic issues, we’ll use this tool wherever possible. We’ll also show where it isn’t applicable, explain why, and illustrate how other non-HCM tools can be used.

Getting Started

In regards to data, we will need three types of inputs: 1) facility-related information (e.g., number of lanes, lane widths, lane configurations, saturation flow rates); 2) traffic-related information (e.g., vehicular and pedestrian volumes for all three conditions); and 3) operational information (e.g., signal timings). We’ll talk about the specific data required for each problem as it is presented. When we reach the system-wide level of analysis, since there are unsignalized intersections on either side of Route 146, we’ll also need behavioral information about the users such as critical gaps and move-up times. We also need to know how NYSDOT designs highway facilities and expectations for operational details (e.g., signal timings). There is some data we won’t need, even though they might be important in other settings. For example, we won’t need any information about on-street parking since none is allowed along any of the streets or highways. We also won’t need information about transit services since no bus routes pass through the study area.

Discussion:
We decided to start the analysis with the most heavily used intersection, and then work first west then east. What would you have done? Is there an advantage to the way we decided to proceed? If you were doing an analysis of the system all at once, might you miss some details if you didn't consider the intersections individually first? What are some of the important consistency issues you have to consider if you look at the intersections separately?