The following information comes from my copy of the January 1993 California Vehicle Code (CVC), which I purchased at my local DMV for $3.00.
These sections of the CVC relate directly to the use of "speedtraps" within California:
40801 Speed Trap Prohibition
40802 Definition of a Speed Trap
40803 Speed Trap Evidence
40804 Testimony Based on Speed Trap
40805 Admission of Speed Trap Evidence
40808 Speed Trap Evidense
Here are the most important paragraphs:
40801:
No peace officer or other person shall use a speed trap in arresting,
or participating or assisting in the arrest of, any person for any
alleged violation of this code nor shall any speed trap be used in
securing evidence as to the speed of any vehicle for the purpose of
an arrest or prosecution under this code.
40802 A "speed trap" is either of the following:
(a) A particular section of a highway measured as to distance and
with boundaries marked, designated, or otherwise determined in order
that the speed of a vehicle may be calculated by securing the time it
takes the vehicle to travel the known distance.
This means that "airplane surveillance" tickets are illegal in California, unless the arresting officer paces your vehicle to determine your speed. I received two such tickets (in 1990 and 1992) and contested them. In both cases it was not necessary for me to say anything -- the judge asked the arresting officer if this case was related to airplane surveillance, and the charges were dismissed "in the interest of justice."
Why does the California Highway Patrol continue with airplane surveillance? Because most people don't know the law and just send in their fine, and because many drivers *are* paced, not realizing that a CHP cruiser has pulled in behind them.
CHP officers do know the law. One officer laughed when he told me "The airplane gotcha." His demeanor changed when I asked if he had read section 40801, and if he had used something other than a speed trap to determine my speed. He said "I'm just the gofer. I have to give you the ticket anyway."
Section 40802(b):
defines where "radar or other electronic devices" can be used to determine
speed -- in essence, either on "local streets and roads" or on highways
where a traffic survey has been conducted within the last five years. I
find that California police are very familiar with this law, and with the
local traffic surveys. I contested a radar ticket several years ago, and
found that I was on a stretch that had recently been surveyed. If I had
been two miles further I would have beaten the ticket.
I have some additional information on the use of radar in California. To use radar, the enforced facility (road, highway, etc) must have a valid Speed Zone Survey, as your other correspondent noted. What he did not say, though, was the posted speed limit must be in accordance with the speed limit indicated by the survey. In other words, if the survey recommends a speed limit of 40 MPH, and the local jurisdiction posts "30 MPH" signs, any ticket written should be thrown out of court, irrespective of the transgressor's speed, since the police would be enforcing a speed limit not legally set.
Additionally, the appropriate standard for determining the proper speed limit is based on the 85th percentile of the surveyed speeds. the survey must set forth ANY justification for reduction of the recommended speed from the 85th percentile of the survey data. To put it another way, the law assumes arbitrarily that 85% of drivers are driving at or below the maximum safe speed, and 15% exceed it. Therefore, after the survey data are collected, a basic statistical analysis determines the 85th percentile, and--voila!--the recommended speed limit. (This figure is rounded to the nearest 5 MPH increment.)
Mitigating factors, which must be cited on the survey, include a high accident rate due to speed, hidden hazards (defined as those not readily apparent to the driver--if you can SEE it, it's not a hidden hazard, no matter how unusual), and others. Local agencies have gotten fairly creative in these justifications, but there must be something to justify a posted speed limit at less than the 85th percentile, or no valid ticket.
There are a couple of other things I failed to mention about Speed Zone Surveys in California. They are public documents, and cannot legally be withheld. Some jurisdictions don't want to give them out, but be persistent, and you will be rewarded. Also, Speed Zone Surveys don't apply to highway speeds--technically, a SZS is used to lower a speed limit from 65 MPH (55 where applicable). The law recognizes 65/55 as the absolute speed limit, with a SZS justifying a reduction. Bear in mind that not every speed limit lower than 55 is justified by a SZS. The important point to remember here is that those speed limits not enforced by RADAR need not be justified by a survey.
For interested parties here is the CA Vehicle code