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Jason @ Hackszine writes -

The greatest feature of the navigation system used in Lexus and Toyota autos is undoubtedly the lockout mode which occurs automatically whenever the car is in motion. Somewhere deep inside the navigation system, the fascism bit is toggled for your protection and certain features, namely destination input, are completely disabled.

Thus, use of the navigation system requires one to tap away at a touchscreen keyboard while parked in an intersection or along the shoulder of a busy freeway, reducing the risk of potential typos over ten percent when compared to touchscreening in a moving vehicle. If you happen to Griswold your way into the wrong area and need navigation assistance, the lockout feature will even cause your parked Lexus to broadcast a visual distress signal, increasing the chance that you will be approached by a friendly drug dealer or prostitute who can help point you in the right direction.

Kids, are you noticing all this plight?

I've found that the best way to appreciate the navigation lockout feature is to disable it and see what driving without it is like. The navigation system contains a hidden service menu with a big Override button that lets you do just that.

There's a secret combination to get to the service menu. The code you use depends on which version of the navigation system you have. They are all roughly the same, so just try them out and find the one that works with your vehicle.

Note: From what I understand, you can't get to this service menu on models from 2007 or later. There's more information on this at lexusoverride.com.

Getting to the Service Menu
Press the following combination to get to the service menu of your Lexus or Prius navigation system.

Newer vehicles: Menu, Volume, top left, bottom left, top left, bottom left

Older vehicles (I can confirm this one): Menu, System, top left, bottom left, top left, bottom left, bottom right

Older vehicles (alternate version): Menu, DVD, top left, bottom left, top left, bottom left, bottom right

Disable Navigation Lockout
On the service menu screen there are a number of diagnostic buttons hiding untold wonders. One of the more useful is the GPS button which gives you detailed lat/lon and status information from the GPS module. To disable the lockout feature, look for the button that says Override. Press and hold it until it beeps, then press the back button to exit the service menu.

You and your copilot will now have the ability to enter destination information while the vehicle is in motion. This is best done in heavy traffic while talking on your cell phone, eating a hamburger, cradling a hot cup of coffee between your legs and yelling at the kids in the back seat.

References:
Overriding Navigation Lockout - Link, Video
How to Override Lexus Navigation Motion Lock - Link