Rust-0leum has entered the spray chalk market. It seems they have been marketing it to police departments to draw outlines of dead bodies. Hopefully your city has more races than murders. I had a devil of a job finding it on their web site. http://www.rustoleum.com/product.asp?frm_product_id=153&SBL=2
Maybe, now that a major brand makes the stuff, you can get your hardware store to stock it.
I sometimes use the spray chalk for start lines or marking the road to show turns on course. I put the mark a little before runner or bike gets there.
Most of our courses are in parks so putting down a ductape start line is easy. One or two races start on roads that have real traffic. In theory it is easy to go out on the day of the race, after the road has closed and stick down a temp start line. Unfortunately I am often out on the course, setting up the course and have no extra time. I like to personally do the marking of the start and finish to make sure there is no mistake. Sometimes marking the start a few days in advance is better. For that I use spray chalk. To get the line straight I have some one run across the road with the other end of a string. We put in nails each end to hold it tight. When we are happy that its correct I walk across the road spraying the string. You end up with a chalky string and something approaching a straight line.
It lasts for a few weeks, depending on rain and traffic.
If you think that it may rain on race day and the road may be wet, then a chalk start line done in the week before the race is better than trying to stick something down on a wet road.
I have used tape, spray chalk and regular spray paint in dark colors to put down temp marks during measurement. What ever I use I try to make the temp marks discreet. I often adjust the course, or turn around, and do it all again until I am happy. This may mean there are several marks close to one place. I try to make sure that temp marks can't be confused with any thing else so I use removable marks or dark paint. (I don't measure in the dark)