You have to be careful at all the parking lots. Dont have anything visable of value. I have been warned by a cop that the parking areas at White Rock are also a haven for car break-ins.  
At far as the trail is concerned, there are a lot of blind corners and people riding two abreast or overtaking  on a corner resulting in many accidents. We used to go on it every week but now just drive to the lake as we feel more comfortable there.

Ken W had his car broken into at the small parking lot off Valley View Ln. Both John S (Clown #1) and Ken W had their vehicles broken into at the large Anderson Bonner parking lot, just South of IH-635. John and Ken are now parking at the small Anderson Bonner lot at Banner Dr. As John mentioned, no problems so far. I've not seen any evidence of a break-in (broken glass on pavement), since they've been parking there. The safest place to park at WRL is Sunset Bay. We've never experienced, nor have we seen any evidence of, break-ins in that lot in the several years we've been parking there. The Service Center is at that location, and there is a lot of vehicle and people traffic at that location, so not the right conditions for thieves to do their dirty work unobserved.
 
The White Rock Trail is crowded on Saturday mornings. It's less crowded on Sundays. The Clowns ride the Trail on Tuesdays and Thursdays, somewhere between 11:00 AM and 1:00 PM, and it's not crowded at all. There are two blind corners that need to be negotiated with caution. One is located where the trail goes under I-45 and the other at the golf course, near Greenville Ave. I went down on my Optima Baron under I-45 when I approached the blind corner too fast and met a roadie riding the other direction. I slow down and ring my bell, now. I really like the trail. Taking the curves on a bike or trike is fun, and riding it is nice break from riding laps at WRL. We haven't done it in a while, but after putting in a bunch of miles, it's a treat to reward yourself with an ice-cream bar at the Fair Oaks Tennis Center, near Abrams Rd.
 
Haven't ridden the Cottonwood Trail in a while, but those times I did I didn't experience any problems.

I think you mean US 75.  There have been breakins at every parking lot in the parks at some time, but they seem to be less common at this time.  The trails are quite crowded on the weekends, unless the weather is bad, which makes becoming a foul weather rider attractive (don't ride the trail during heavy rain, however, as there are spots prone to flash flooding).  During the week normal caution is sufficient.  The Cottonwood trail has attracted a few denizens that some people find objectionable, but my beef is the poorly controlled erosion areas that periodically cover the trail with mud or gravel, and the broken glass after the weekends.  Parts of the trail are showing their age (and the heavy vehicle traffic from the Parks Department), but that is slowly being improved.

We parked at Valley View on Labor Day and rode for 4 hours, had two cars.  Break-ins have been mentioned about this area, but we haven't had any problems in the dozen or so times we've been there.  Time of day & parking along the road helps to deter burglars.
 
The trail is really nice now that its finished.  From NW to WRC the trail is undergoing repairs and should be complete in a few weeks.  Not a problem, just use the road starting in front of Dallas Bike Works to WRC.