With that said, I agree with you regarding the security prospect of collecting data from cell phones. However, I think that many of the negative aspects can be mitigated.

First of all, there does not need to be many cell phones tracked in order to get traffic data. Very few, in fact. It is not important the number of cell phones in a given area to see the traffic - you can simply track the speed of one cell phone. I think any large scale implementation of a traffic monitoring system could use a very limited number of tracked cell phones.

Like you said, there needs to be a opt-in policy. I think it would also be awesome if the cell phone user could change whether they wanted their data to be used on the spot. Perhaps by using a switch or a setting on their phones, they could opt out of being part of the traffic study at any time. The cell phone would still need to have the GPS enabled for emergency purposes, of course.

The traffic data could be sent anonymously also. Instead of the traffic study being able to follow a specific cell phone, they would just get a signal from some cell phone and not be able to map it to any previous signals. Although I am sure this could still be hacked, at least the data is not being centrally stored.

Even with security concerns, I like the idea. I’d be willing to pitch in my cell phone signal for the cause.