Yes, as Rushen mentioned, this is indeed a male Pope's Pit Viper.
One of the most stunning green pit viper species, though quite common. Did you find it inside the trail at km17 or near the Pranburi/ road?
Was it eating? Or did it strike at you? Because it has its mouth a bit open, and the throat seems 'swolen'...
The female of this species also has red eyes, but lacks the red ventral line and has no lines on cheeks. Also the body color is a bit more yellowish-green than the male.
Below is an image of a big female near the 3rd river crossing.
So far I have only seen White-lipped Pit Viper (Trimeresurus albolabris) before km16, and after km17 the range of these Pope's Pit Vipers seems to start, which can be found all the way up to Panoen Thung. I have the feeling it is more common higher up in the park. In most literature you will find that this species is a high elevation species, but in KK it occurs quite low, km17 is below 400m asl.
Even though few bites of the green pit viper species result in death in humans, it's wise to watch out because it can cause serious tissue damage, and a bite is said to be extremely painful.
In March we saw a female that was surprisingly aggressive, she kept striking while she was being photographed, but generally these snakes are relatively calm as long as they are approached calmly. the chance to get bitten while simply walking by, is very small, unless you accidentily grab it. Most of the time these snakes stay a little (or high) off the ground, but I have seen one hide between dead leaflitter on the forest floor on a little slope in daytime.
I will post the other photos when they are done. It was inside the trail where the stream crossing is with the side trail that bring you out just further down the road at the big rooted fig tree. There were lots of snakes this weekend - I never see them but we saw 4 this weekend, all little ones laid in the various trails (its been wet). I was not set up for macro and this was taken with about 700 mm (500 plus 1.4 tx) of lens!! It was on a log about 12 inches from the floor - very docile but the nearest I got was when I had to walk over it to continue on the trail.
4 pit vipers, or different kind of snakes?
Wet weather is great because the frogs get active. Many pit viper species eat frogs, so as soon as it rains they will stay close to the forest floor and wait till a frog hops by.