On the G-III family you can change the setting for the turn distance. The Street Pilot uses 50 meters by default and this turns out to be a good setting for driving down the road. This, of course, will increase the length of data that can be collected at the expense of accuracy on turns. The etrex vista, legend, and venture have both time and distance. Automatic mode has a setting where you can adjust the sensitivity to distance from the projected straight line from less to more often.
Automatic mode is the only mode supported on basic etrex, summit, and emap units.
These three units have other methods of using the automatic mode. For example, they can use speed to help determine how often a trackpoint is laid down. When walking a departure of 10 feet from a projected straight line might be enough to trigger a new tracklog point but while driving this would not be enough. The users current zoom scale can also be used as an indicator of desired track resolution. Garmin has not documented how they are doing the tracklogs on the latest software releases of these models but empirical data from users has shown that some of these techniques are being used.