user11066: I would like to design a system in which I am able to make a light flash when a motion sensor detects a movement of a non human object such as a car. 
(The goal here being that when a car is moving behind me in my blind spot around the corner of a wall in my parking spot I don't back into it.)
I have looked into a few tutorials using arduinos to detect this motion but so far I have only read about the PIR (which only detects humans and animals).
http://www.instructables.com/id/DIY-Arduino-Motion-Sensor-Lighting-Control/?ALLSTEPS
What sensor would be best for this situation or do I need to rethink my system?
(I have no intention of harming anyone or anything :) )

user313: I hate to be the wet blanket here but maybe the job is too simple for this much technology. Wouldn't a suitably placed curved mirror give you the information you need?
Or put a PIR sensor around the corner where it can see the car, and put the flashing light where you can see it while backing.

user15519: I am not sure why no one has suggested a Microwave Doppler sensor. I am not sure how it would work in an outdoor environment but it is designed to detect motion regardless of whether it is a human. This link has some information about using a cheap Microwave sensor with an Arduino.

user4143: If your car is moving at the time, then relative to your car everything is moving.
Automotive companies have spent billions of dollars working on this problem.  There is no "sensor" that can differentiate one moving object from a background that is also moving.
All existing automotive systems that perform this task use cameras and computers, with software such as OpenCV.  This is not something that any Arduino could even come close to doing.
The closest you could get is "is there something in this area that is within a certain distance of me" which you may be able to do with an ultrasound "Ping" sensor.  It won't differentiate between a moving car or a bollard or a child or just the wall you are reversing towards.

user8611: I would try the PIR sensor before writing it off.  It detects changes in infrared radiation from an initial baseline that it reads, so whenever the ambient heat that it sees changes by some threshold, it notifies you.  They might be sensitive enough to see a car coming through, as suggested by others.
A more robust solution could be the ultrasonic proximity sensor, like Majenko mentioned.  Here's one as a starting point: ultrasonic distance sensor.  This will be best if you know that the car will be passing in front of where you point the sensor.  If it's hard to point the sensor to a point where you're sure the car will be coming through, then that's where a PIR would have advantages, as it "sees" a wider area.

11066: <p>I would like to design a system in which I am able to make a light flash when a motion sensor detects a movement of a non human object such as a car. </p>

<p>(The goal here being that when a car is moving behind me in my blind spot around the corner of a wall in my parking spot I don't back into it.)</p>

<p>I have looked into a few tutorials using arduinos to detect this motion but so far I have only read about the PIR (which only detects humans and animals).</p>

<p><a href="http://www.instructables.com/id/DIY-Arduino-Motion-Sensor-Lighting-Control/?ALLSTEPS" rel="nofollow">http://www.instructables.com/id/DIY-Arduino-Motion-Sensor-Lighting-Control/?ALLSTEPS</a></p>

<p>What sensor would be best for this situation or do I need to rethink my system?</p>

<p>(I have no intention of harming anyone or anything :) )</p>
 313: <p>I hate to be the wet blanket here but maybe the job is too simple for this much technology. Wouldn't a suitably placed curved mirror give you the information you need?</p>

<p>Or put a PIR sensor around the corner where it can see the car, and put the flashing light where you can see it while backing.</p>
 15519: <p>I am not sure why no one has suggested a Microwave Doppler sensor. I am not sure how it would work in an outdoor environment but it is designed to detect motion regardless of whether it is a human. This <a href="http://jpnelson.com/post/37623468093/microwave-motion-sensors-for-fun-and-profit-hb100" rel="nofollow">link</a> has some information about using a cheap Microwave sensor with an Arduino.</p>
 4143: <p>If your car is moving at the time, then relative to your car <em>everything</em> is moving.</p>

<p>Automotive companies have spent billions of dollars working on this problem.  There is no "sensor" that can differentiate one moving object from a background that is also moving.</p>

<p>All existing automotive systems that perform this task use cameras and computers, with software such as OpenCV.  This is not something that any Arduino could even come close to doing.</p>

<p>The closest you could get is "is there something in this area that is within a certain distance of me" which you may be able to do with an ultrasound "Ping" sensor.  It won't differentiate between a moving car or a bollard or a child or just the wall you are reversing towards.</p>
 8611: <p>I would try the PIR sensor before writing it off.  It detects changes in infrared radiation from an initial baseline that it reads, so whenever the ambient heat that it sees changes by some threshold, it notifies you.  They might be sensitive enough to see a car coming through, as suggested by others.</p>

<p>A more robust solution could be the ultrasonic proximity sensor, like Majenko mentioned.  Here's one as a starting point: <a href="http://rads.stackoverflow.com/amzn/click/B00E0NXTJW" rel="nofollow">ultrasonic distance sensor</a>.  This will be best if you know that the car will be passing in front of where you point the sensor.  If it's hard to point the sensor to a point where you're sure the car will be coming through, then that's where a PIR would have advantages, as it "sees" a wider area.</p>
