I see that Skip has chimed in on the point below as well. I gotta stop this work stuff and spend more time online I guess ...
To address the initial point of the thread:
OK first let's clear up a minor technical point. Like "Woodsy" said above police speed radar is a straight line tool. Targets not moving directly to or away from the radar will have their measured speed reduced as has been discussed previously (
http://www.winnipesaukee.com/forums/...ead.php?t=1217). While laser radar (aka lidar) works by measuring timing from transmitted pulse to received pulse (multiple times over a small interval) to get an average speed over the measurement interval, the radar guns in question use the Doppler principle. So as not to bore everyone I won't bother explaining it here, you can google it to find out if you're curious. The difference is w/o distinction though as radar be it military, civilian or speed control, has some problems over water that don't exist in as much magnitude as they do on land. Some of these (ie - discrimination) were discussed in the thread above. Let me turn to the topic of radar reflectivity (aka radar cross section, RCS) of fiberglass boats. As many have said (and it's true) fiberglass isn't a good reflector of RF energy. Look at the covering over the marine and weather radar antennas, they're plastic and fiberglass because they don't reflect back too much energy. That said it's not like a boat will be invisible, it's just that the range at which the gun will lock on and display a speed will be reduced (in many cases) when compared to ranges for autos. I've tried to find some unclassified literature on boat RCS that I can share with y'all and the following is the best I can come up with at present.
http://www.iee.org/oncomms/pn/radar/...4%20%20RCS.pdf
In this they used civilian marine radar (CMR) to measure the RCSs of various targets, among them "small" (when compared to ships) boats. Of particular interest is the measured RCS of a 25' fishing boat (another reference lists a 28' commercial fishing boat at same RCSs), which was something between 1 and 10 square meters (m^2). A dingy was 0.01 to .1 m^2. I'd estimate a fiberglass boat might be around 0.5 to 5 m^2 (? maybe smaller ?). Consider that average numbers for a car are something like 100 - 120 m^2 and you can begin to appreciate the issue. Neglecting any other interfering effects and the max range is reduced to something between a quarter to a half of what it is for cars. It might even be worse if the measured boat RCSs above were for a beam aspects (looking at side of boat) rather than at it's bow from head on. In the later case the boat's RCS is certainly smaller again. Add in time varying reflections off waves and pointing error/beam loss of the gun and you've reduced the effective range even more. The end point is that while speed guns may be useful in some scenarios the boater has even more of a chance than a car does. What I'd predict would happen is easy to extrapolate from what's already happened in the car world. People will have radar detectors and perhaps "bear detectors" (detectors of police radio emissions) and the VHF (akin to the CB of the '70s). The MP will have certain locations where a trap might work and these will become known. In other open water locations any radar gun will be detected (heck the MP boat will probably be seen) before it can register the boat. You'll get people to slow down where and when the radar is in use and not in other places. This may have the desired effect of slowing people down in those places but I don't see it chasing performance boats off the lake. It'll become a big game as it was back in the bad ole days of the NMSL/55.
Lidar doesn't suffer some of the same problems that RF radar does but it has some of it's own. The big one is that the beamwidth is very small (about 10' wide @ 1500') and so it has to be held very steady to stay on target, something that will be very hard to do in a rocking boat (think about using a telescope in a boat). This is why you don't see moving lidar units like you see moving radar units. Use off a boat might catch some people (more when the water is calm, less normally) but now lidar detectors, which are pretty much useless in cars, have a chance to operate. If the beam isn't held steady the detector will go off and give some advance warning before the aim is re-centered. Whether this will be enough I don't know for sure. I also suspect there's some logic in the lidar guns that rejects readings that are too disparate from pulse to pulse and so even a somewhat steady (less unsteady) beam may be painting the bow then windsheild then ?? resulting in a longer time to "lock" than would be normal in auto usage. The guys who are long range rifle shooters and can breathe, time their squeeze and maintain sight picture during triggering may do OK with marine lidar. Others will be less effective.
Oh yeah, it's going to be really, really hard
to aim existing lidar guns to enforce the 25 mph night time limit. Are we going to have lidar for day and radar for night ?
To address speed limits in general :
When I have some more time to type I'll add some more things to think about. Suffice it to say that putting aside whether a SL is needed or not, effective or not, and whether the resources would be better used otherwise; how would one go about figuring out what it should be ? What is reasonable and why is that number reasonable ? What are the factors involved ? There was a time before politics got involved that SLs were set by analysis. Has anyone (other than me) done any analysis ? Hint: the answer is somewhere between NWS and 1000 MPH