User`s manual

7-3
You can do either a one direction or two direction GAP study with the TDC
Ultra. It takes practice to do a two direction gap study, especially if the two
approaches are at right angles to one another, but it can be done. Like many
other things, practice makes perfect.
The TDC Ultra actually keeps track of three different gaps, even though you
only measure two. One for each of the GAP buttons that you press while
doing the study, and one that is a combination gap that is only valid when
both GAP buttons are simultaneously pressed.
The combination gap lets you measure the gaps in the N-S direction with
one key, the S-N direction with the other key, and still determine the distri-
bution of gaps across both lanes. Typically this is used at an unsignalized
intersection or a driveway access to determine the ease of making right
and left turns.
You also have the option of doing a volume count while you are doing the
gap study. A separate key (the COUNT key) is pressed for each vehicle.
The total number of vehicles is stored with the gap data at the end of each
interval. The analysis software prints the volume data next to the gap data
in the reports. Information about the volumes at a certain location helps put
the gap data into a proper perspective.
Imagine that at two different locations there were 100 gaps measured, and
the average gap was 10 seconds. Now imagine that at the first location
there were 125 cars counted, and at the second location there were 2000
counted. Aha! Location one is a quiet little street, with fairly steady traffic.
Location two is much heavier with platoons of cars. Knowing the volumes
helps a lot.
The volume count is optional. If you don’t need the data, or if you are doing
a two direction study and don’t have the time to keep track of the gaps and
count at the same time, then don’t worry about it.
Chapter 7 — Multi-direction Gap Studies