So I got one of those OBD (automotive On-Board-Diagnostic) plug-in dealies so the insurance company could see what a great driver I am. About 25-30 years ago I learned how to actually drive, as opposed to merely "operating a car." Haven't had a ticket or an accident that was my fault since then, and only one, at that. (Knock wood.)
It monitors a couple of things, among which are braking events and night-time driving events. They say driving between the hours of midnight and 4 AM is "bad." OK, but since I retired, I do a lot of erranding in those hours since I'm a natural-born nightowl. OK, so I'll have to talk to them about that.
But my second week's report showed three "evil" braking events with my comments as to what I was doing while I was out erranding as follows:
Week Jun 8 - Jun 14, 2015
6 Trips - 18.3 Miles - 1.1 Hour -
3 Braking Events
Date & Time / Duration / Description
06/14/15, 09:31 1.0 Seconds 24 - 16 Mph
^ Traffic light at Wadsworth & 38th, sudden slowdown by everybody in front of me.
8 mph/sec
06/14/15, 09:56 1.0 Seconds 37 - 30 Mph
^Downhill on 44th from radar sensor flashing at me (my bad)
7 mph/sec
06/14/15, 09:56 3.0 Seconds 25 - 2 Mph
^Traffic light turned yellow, nobody behind me
7.66 mph/sec
The bolding shows my calculations of deceleration.
I've got a lot of questions about this whole sensing affair, like how often do they transmit to their receivers, how do they sense "speeding" on different roads, and a whole bunch of stuff I've been prowling around the net on for answers.
But a couple of things bother me right now.
I have smaller-than-standard tires on my car for unavoidable circumstances --one standard tire got destroyed and I had to replace "it" with all four tires and all they had was smaller ones.
As a consequence, my speedometer reads about 31-32 mph when, according to the radar stations along various streets, I'm actually going only 30.
Now I figure they can probably sense a "braking event" in a couple of ways...
(A) ...by an accelerometer built in to the nanny box.
(B) ...by brake pedal position (which I found out some OBD II systems can do) or various brake pressure readings like power brake diaphragm and brake line pressures.
(C) ...by other engine/transmission parameters such as calculations from RPM, gear ratios, manifold vacuum, etc.
(I doubt they could do it from GPS data because I don't think GPS responds that fast or that accurately.)
If it's an accelerometer in the nanny box itself, assuming proper calibration (which is sometimes questioned, according to what I picked in my amateurish search), I have no quarrel, except for the low threshold to create a braking event. It seems too low, probably around 7mph per second. I also wonder how they're figuring in downhill or uphill braking accelerations, where gee would have a cosine effect.
If it's by brake pedal position, the first thing I noticed when I bought the car was that the brake pedal felt low to me compared to my previous car. However, it has not changed in the years I've been driving that car, so maybe that's the "standard" for that car.
If it's by engine/transmission parameters, then it's clearly unfair by some small amount due to the smaller tire size. (Braking from 31-32 mph (on the speedometer) to zero will give a higher deceleration reading than from the "real" 30 mph to zero.
So, while I'm still <ahem> "researching" this whole nanny box thing, I wondered if youse* guys and gals would like to kick it around and see if youse had any hints or other comments on this stuff.
Terry, 230RN
That's Brooklynese for "you-all."