I dont think any road in Thailand is safe over Songkran, drinking and driving is very common in Thailand and is a big part of the issue, plus you are going to get water thrown over you.......................only last year I was riding through a small village on my X-Max and got a full bucket of water thrown at me, when doing 70kmh it hit me with some force.
If you cannot reschedule the ride I would still go and be very careful, not sure how much riding you have done in Thailand ?............................just be aware oncoming vehicles cutting the corners, vehicles pulling out from side roads, dogs and cows etc and obviously getting water thrown over you.
Saying that Songkran is a fun time of year with most people very happy and you will find it very hard not to participate, so just join in the fun