I'm very much into the DIY thing... and, in the name of being over-cautious for safety's sake, I'm going to play devils' advocate here a bit...
Not long ago I had installed some new 2"X6" spans in my attic to support an air conditioning unit drip try... not much weight at all. The wood lengths were installed carefully, holes were pre-drilled and screws used for zero cracking / stress on the wood. Well, I went up there the other day and one of the 2"x6" completely split all on it's own on one end to the point were if it had been supporting any more than 50 pounds it would have failed and collapsed. That's wood.
Also, not too long ago I had a few 2"X10"s that I used to lay flat on the ground and I'd drive my C6 Vette up on them to get an extra 1.5" of clearance so I could get one of my hydraulic jacks underneath. One of those 2"X10" completely split in half one day as I drove up on it. In this case it wasn't a problem, but just another reminder to me about how unpredictable wood can be.
If you are going to use wood to support a lot of weight, you should have redundancy... meaning, you should never rely on just one single piece of wood as a critical weight supporting member. Like, having single wood ramp planks (leading up to the rectangular sections)... even with the extra black ramps under the middle, one of those planks could split at the top end (length-wise along the grain)... once that would happen, it could move or shift or who knows, a wheel could drop and the frame of the car would slam down on another portion of this wood structure, could cause further failure and the whole car could go down.
I'm not a master carpenter, but if you study the way most modern homes are framed for instance, you will note a lot of redundancy. I've never seen an instance where one single "2-by" piece of wood was responsible for supporting something very heavy and important. Most headers or girders are made of two or three "2-bys" nailed together, presumably for redundancy, if one splits, there's another one or two pieces to keep things together.
Also, side note, drywall screws have very little strength and should not be used in instances where you're dealing with real load-bearing structures. If anything, use deck screws which are considerably better than drywall screws... and, depending on the situation, back them up with good common nails for greater shear strength.
Just don't want to see anyone get hurt. Sometimes with DIY wood ramps, unless an architect (someone who would know the properties / load limits of various wood members and how things should be constructed etc) was to review the design, it's really an unknown as to how safe they might be. They could last forever, or could fail after three uses. They may seem solid and stable, but a weak point (due to an inherent split in a key section of wood) could allow a sudden failure. One's life is at stake when under a suspended automobile, so it's not the type of thing you really want to roll the dice with.
When in doubt, utilize additional safety measures, such as adding good jack stands under the car before crawling under it, etc. Do not trust the wood solely for supporting a car in this manner. That's my opinion anyway after having done a lot of general carpentry / framing work.
I think a lot of folks place perhaps a bit more faith in wood than they should. What I know is that you could go to Home Depot right now, buy two "brand new" lengths of 2"X10", try to support the same amount of weight with each, and one might be fine forever, the other might fail... and it might fail immediately or fail a month from now... that's wood. No two pieces are the same and they are not "guaranteed" by any means to support a great deal of weight all alone.
Driving a typical musclecar over a pair of single planks means a concentrated moving load of maybe 1,000 pounds (front wheel) on each plank... that's a lot to ask of a single typical 2"X10" in my opinion, at least when used in a non-kosher manner (horizontally, etc). We're not talking about an evenly distributed typical load within a professionally designed frame.
Another possible idea for DIY ramps out of wood that would theoretically be pretty safe... use stacked layers of plywood or even OSB... 12" wide strips laying flat on the ground, screwed together... stagger the strips to create a gradual incline to drive up with a nice flat section on top with stops. Ramps would essentially be solid. Multiple layers of ply or OSB make for lots of "redundancy", even if one piece totally cracked and split everywhere (which would not happen anyway), the ramp would still not come apart or shift etc at all.
For the front wheels, make two ramps using ten 3/4" thick strips for 7.5" of raise height.... and for the rear wheels, make two ramps using however many strips you can fit under the rocker panels of the car... I'll guess maybe 3.5" or so... then place all four ramps in front of all four wheels and drive up... the front end will be up 7.5" and the rear about 3.5"... or vise-versa depending on whether you drive up or back up. This should be enough height for most common jobs (though maybe not enough for major jobs). The best part, such ramps could never fail.... would be pretty much 100% safe.
The downsides to such ramps... each ramp would be quite heavy.... and might cost a little more to build than other common designs.... though they would be WAY easier to construct as long as you could get someone to rip the ply or OSB into 12" strips for you (Home Depot might do this for free). You wouldn't have to be an engineering genius or master carpenter to put together such DIY ramps and have them be 100% safe, it's somewhat of an "idiot-proof" design in terms of the construction / assembly. The concept is, it's "over-built" / "over-designed" which I'd say is a good idea when using any type of WOOD to support a car.
Upon looking at the OP's ramps once again, they do seem quite neat, perhaps fine, but I did want to mention my general feelings about wood ramps in general and how wood really should not be trusted for critical applications unless a good design with adequate redundancy is used throughout.
Ok, devil's advocate hat removed now. Just a little safety awareness.