PIR Motionsensor, detecting cars, non parallel position

Hello you guys!

Ive just picked up this whole world of arduino once again and am in a bit of a pickle.

I have a thought of a project I would like to do but I need some help from you more experienced as to avoid to much trial and error.

Please take a look at the attached picture I drew with my amazing paint skills.
I have a two-door garage with a very short driveway, about 6-7 meters/20-23 feet before it hits the main road.

To prevent my wife (and sometimes me) and guests from parking to close to the garage door so I't wont block me from opening it I want to use an arduino mini pro 5v combined with the ultrasonic sensor module HC-SR04 for distance measuring to approaching car. How ever I do not want the sensor to be operating constantly so I was thinking of adding a PIR sensor that wakes my arduino from a sleep state. When the car gets within a predefined distance where the garage door would hit it when it opens I will fire off some red LEDs.

Here comes the questions:

  • Would a PIR sensor go high with the approach of a car, with a hot running engine, with a placement of the sensor as seen in the picture (position 1). It would be at an height of approx. 0.5 meters/~20 inches
  • If I would like to drive this on batteries, any suggestion as to what would do?
  • With regards to position 2 in the picture, could i do with just 1 module for detecting both sides with 1 ultrasonic sensor and 2 pir sensors that fire off depending on what side a car drives up on? Or can the distance sensor only reliably measure distance in a straight forward pulse to an object as opposed to at an angle?

Please excuse my english ^^.

Ultrasonic sensors are quite directional, but PIR is fairly omnidirectional - in fact you'll likely get false triggering
of the PIR by anything in its field of view, a tree moving in the wind, passing people, etc. PIR is normally
used indoors or just for triggering security lights. But still it will probably allow a lot of sleep-mode
time and reduce battery load. So PIR could be on the wall rather than door it, will see large hot objects
like car radiator grill very easily I suspect.