Many renewable energy sources are intermittent. what does this mean, and why does it pose a challenge to the use of these sources for electrical power generation

Explanation:

Many intermittent

Intermittent renewable energy sources which are intermittent are difficult and challenging which means that they have an disrupting effect on the conventional procedures used to plan the daily maintenance and operations      of the electric grid. The power fluctuates over different and many time horizons which will force the operators to adjust it before time  i.e an hour ahead to day ahead to adjust the operating procedures manually.  

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.