The areas that vehicles can be detected in are outlined below.

To give the driver a more consistent time to react, the buzzer can alert for faster vehicles from farther away.
Example: 
The rear cross traffic alert function is operational when
Conditions under which the rear cross traffic alert function will not detect a vehicle
The rear cross traffic alert function is not designed to detect the following types of vehicles and/or objects.
*: Depending on conditions, detection of a vehicle and/or object may occur.
Conditions under which the rear cross traffic alert function may not function correctly
The rear cross traffic alert function may not detect vehicles correctly in the following conditions:

The rear cross traffic alert function
Driving tipsParking Support Brake
function (rear-crossing
vehicles)
If a rear radar sensor detects a vehicle approaching from the
right or left at the rear of the vehicle and the system determines
that the possibility of a collision is high, this function
will perform brake control to reduce the likelihood of an
impact with the approaching vehicle.
Examples of funct ...
If you think something is
wrong
If you notice any of the following
symptoms, your
vehicle probably needs
adjustment or repair. Contact
your Toyota dealer as
soon as possible.
Visible symptoms
Fluid leaks under the vehicle
(Water dripping from the air
conditioning after use is normal.)
Flat-looking tires or uneven
tire wear
...
General maintenance (2005/11-2006/01)
General notes
Maintenance requirements vary depending on the
country.
Check the maintenance schedule in the owner's
manual supplement.
Following the maintenance schedule is mandatory.
Determine the appropriate time to service the vehicle
using either miles driven or time elapsed ...