When should my employer pay me for travel

The rule of thumb is that you do not have to be paid for your morning commute to work or evening commute from work. If you have a office (or warehouse, workshop, etc. which serves as your "office"--i.e. it's the place you'd go to for work if not going directly to a job or construction site), the general rule is, you are not paid for the first and last trips of the day, when you go directly to a job site, to the extent they are more or less the same length trips as it would take you to go to your "office." You would be paid for trips in excess in of that.
Example: say that your employer either has an official office or uses his home address as his business's office. Say that it is 1/2 hour from where you live. If you go directly to a job site, you would not have to be paid for the first 1/2 hour of travel, give or take (i.e. 35 minutes would still effectivley be unpaid; the law allows a little leeway in this regard). Beyond 1/2 hour, you should be paid for your time, at your hourly rate. That is, under the law, if you have to be paid for travel, you are paid as if it is work time, not a per-mile rate.
(Mid-day travel is ALL paid; so if you go from jobsite 1 to jobsite 2 at, say, 1pm, you should be paid your hourly rate for the entire trip from site 1 to site 2.)
HOWEVER, bear in mind that your employer can set a lower hourly rate for travel than for time on the job site; it is legal to have different rates for different locations or part of a job. So as long as it is at least equal to minimum wage, your employer could pay you less than your normal $18/hour for travel time--but your pay would still be based on time, not mileage, and for the morning (1st trip from your home) and afternoon/evening (last trip back to home), you only count the time once it exceeds what it would take to go to the employer's "office."