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Definitions

Definitions

Before any discussion of travel time data collection and analysis can begin, you must have a careful understanding of the terms we use and how we use them. This section defines the words used in the program that mean something more specific than the word itself might imply.

Run:    A single collection of travel time data.
For example, when data is collected along an arterial, the user drives to the beginning of the arterial under study, starts data collection, proceeds along the arterial to the end of the study area, and then stops data collection. He has just completed one run. If he turns around and collects data in the other direction, it is another run. All runs are stored as separate entities in the program.

Study:   A collection of runs.
When the user collects data, he is making data runs, and when he gets back to the office, he collects those runs into studies. The difference is important because runs can be collected into different studies. For example, a user may make a number of runs at an arterial during one or two days. Back in the office he may create a study with just the morning runs. He may also create a study with all of the runs, which of course use some of the morning runs.

There is one critical rule for studies:
All of the runs in a study must start at the same place, end at the same place, and follow the same route.
Only runs in the same direction can be part of the same study. Since you usually collect runs in two directions (up and back), you typically will create at least two studies for each data collection session.

Study Group: A folder where related runs and studies are stored.
This term is specific to the program. Since studies must be created from runs that start in the same place, end in the same place, and go in the same direction, it makes sense to store all runs that fit that criteria in one place on your computer, along with any studies that are created from those runs.

You typically create Study Groups when you first read the run data collected in the field. Since you usually collect at least two sets of runs, one in one direction and another in the opposite direction, you usually will create two Study Groups when you read the field data.

Fixed-Route: Data collection along a pre-determined route.
PC-Travel only supports Fixed-Route studies. Another type of study, called Chase Car studies, may be supported in the future, based on user interest.

When you do Fixed-route studies, you collect run data along the same route several times. One run is never sufficient to find the travel time characteristics of a route. You may be lucky and never hit a red light during your run, or you may be unlucky and hit several. If you collect several runs, the averages of the individual run data will be a better representative of the true traffic characteristics of the route.

Fixed-route studies usually have segments defined at the time the runs are made. The route is divided into geographic segments, using easily determined landmarks to separate the segments. For arterials, the segment boundaries may be signalized intersections. For freeways, the boundaries may be interchanges. You are free to define the segments any way you want.

Node:   The boundary between two segments of a run.
Every run has a starting node, which is where you start collecting data on a fixed-route study, an ending node, which is where you stop collecting data, and several segment nodes in between. The user records the location of the nodes by pressing a buttons in the field as the user passes the nodes during a run.

Primary Run: A run that has node data.
Secondary Run:  A run that does not have node data.
You do not have to collect segment node data while doing a run, especially if you use GPS receivers to collect your data since you can add node information to a run by editing a map in PC-Travel. If you use Jamar TDC boards or OBD Interfaces to collect your data then you will need to mark the nodes while you collect the data in the field. Runs where you collect node information in the field or add node information back in the office are called Primary runs; all others are called Secondary runs.

Before and After: A way to categorize a group of runs so that two different groups of runs can be compared.
The terms Before and After are used liberally in the program and these mean only that the data is summarized into two separate groups so the statistics of each group can be compared. If all of one set of runs are made under the same conditions, they may all be defined as before runs. Later, identical runs made under different conditions (after an arterial has been re-timed, for example) may be defined as after runs. The program lets you define runs as either before or after and then automatically calculates statistics for the before runs as a group, the after runs as a group, and changes in the various statistics from before to after.

Normal Speed: Ideal speed at which the traffic should travel on an arterial or freeway.
The Normal Speed is used in two places in the program. It is used to find Total Delay statistics for runs and studies (see Total Delay, below). It also is plotted on the Time/Space Diagrams to show perfect progression. You set the Normal Speed on the Study Summary screen. As with most ideals, real traffic rarely measures up to the ideal, but it is useful as a guide.

Travel Time:  The elapsed time to travel between two points, in seconds.
This is probably the most fundamental of the reported statistics. All run travel times are measured and reported to the nearest second. Study travel times, which are averages of the run travel times, are reported in tenths of second (technically speaking, the tenths of second are not significant in studies with less than about 5 runs, but that is rarely of concern in the vast majority of practical traffic evaluation situations).

Number of Stops: A stop is defined as a one-second interval where the speed is less than X MPH for one second when the speed was greater than X MPH in the previous second.
X is normally 5 MPH but can be set to any speed you want. This speed is called the Stop Speed and is set on the Study Summary screen. Each time the vehicle slows down and crosses the Stop Speed boundary, a stop is counted. The vehicle must speed up faster than the threshold before another stop can be counted.

Average Speed: The total distance covered divided by the elapsed time.
The program calculates the average speed for each section (node to node distance) and also separately calculates a total average speed for the entire route.

Total Delay:  Difference between actual travel time and ideal travel time.
Actual travel time is calculated from the data. The ideal travel time is based on the Normal Speed setting on the Study Summary screen.

Time <= X MPH: Total time the vehicle spent at or below the given speed.
The program gives you three speed categories, which you can set for different purposes. You can measure stopped delay (time vehicle is stopped) by setting Category 1 to 0 MPH. You can measure queue delay by setting Category 2 to 7 MPH. The third category might be set to 30 MPH to show how much time vehicles spent in car following mode rather than free flow (assuming free flow speed is 40 or 45 MPH). Many other uses for these three categories are possible, limited only by your imagination.