Although stress does not create cancer cells, it may affect their growth by suppressing the activity of:

Corticosteroid hormones are produced during stress. The hormone is useful especially in enabling the body to cope with short periods of stressful situations.  However, when the hormone is produced for long periods, it tends to suppress the immune system.  The activity of Natural Killer cells involved in cancer regulation is repressed by stress hence allowing progression of cancer.

the answer is; comparing the frequency of collisions with and without the device

this makes the anti-collision system an independent variable and the collisions a dependent variable. if there are fewer collisions with an anti-collision system that without, then it can be stated that the system is a success. essentially, collisions under an anti-collision system are compared with the frequency of collisions by cars with the human drivers.