If you want an 'idiot proof' system with negligible installation
requirements, use the same technology as that found in
  'radar' door openers (X-band RF to get narrow beams).

Using doppler off the road surface (oblique angle measuring),
you can measure vehicle velocity directly, and thereby the
rate of change of velocity to detect acceleration and braking.

By measuring the rate of closing of a following vehicle
(again with X band radio waves), you have part two of your
product (follow too close warning).
The typical alarm X-band motion sensor works out to
about 40 feet for human sized objects, which should be far
enough for what you want to detect (big metal car).
A dielectric lens can increase this range substantially.

By mounting both waveguides orthogonally in a waterproof package
that mounts to the rear bumper, and perhaps using wireless
communications (if not hard wired) with your flashing lights
indicator, you have what you describe.

The problem is your target price. I don't see how to make it
as cheap as you want except in huge quantities.

RF motion sensors as used in alarm systems are in the $40
range. (x2) You need an indicator bank ($10), and a some processor
(presumably a PIC) to tie it all together. And then there is
packaging and profit.

In North America there is a requirement for all new vehicles to
have a eye level mounted brake light. This has worked well to
reduce the rear end accident rate. Inexpensive retrofit kits
were available in the years following the regulation.
Since you are in Denver, CO, I'm not sure how your device would
improve things.

There IS a way to use just ONE RF sensor/waveguide for both functions.
Contact me offlist for the technical details.

Please send any royalties my way.

Robert

P.S. The use of a variable rate blink to show braking rate
is the subject of a 1950's patent, which will count as 'prior art'
should you try to patent this anywhere.

Rich Mulvey wrote:

> Gus Salavatore Calabrese wrote:
> 
>> I am really happy to read all the useful and inciteful ideas put forth.
>>
>>  I realize I did not supply enough information in my original post.

Seems to be a common problem on the PICLIST.

>> ---->  I have been designing a vehicle tail light which would start  
>> flashing at a slow rate
>> as the brakes were applied and at a faster rate as the brakes were  
>> applied harder.
>> Ideally this unit would be standalone except for power ( which might  
>> be solar provided ).
>>
>> I see no way to differentiate between braking and going downhill. Any  
>> ideas ?   <---
>>
>> My desire is to build a product that will sell for $30-$50 max.  Less  
>> would be great.
>> The product requires NO skill to install.  If it requires power that  
>> could come from the
>> cigarette lighter socket.  The user can put it in the back window or  
>> mount it anywhere
>> they want.
>> The desired result is to have the light FLASH to alert other drivers  
>> to wake up.
>> False positives are not that bad.  False negatives are to be avoided.
>> If it works correctly 90% of the time it is certainly better than  
>> "nothing".
>>
>> Your comments made me realize that another product could measure the
>> speed at which a car was approaching the rear and flash the brake lights
>> if the closing vector was excessive.  In addition, if an Italian  
>> driver ( note
>> my last name ) is following on the bumper, the light would flash.
>>
> 
>   While I'm aware that The World Is A Very Big Place, I have to wonder 
> how many places, like my state, restrict the use of blinking lights in 
> civilian vehicles, other than turn signals.
> 
> - Rich

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