I'm a U.S. native speaker. My phone has speech-to-text conversion technology in it. I opened up the "memo" pad, pressed the microphone, and said--slowly and carefully--"He is riding." and "He is writing." I felt that I pronounced "riding" and "writing" slightly differently, but my phone transcribed both of them as "He is writing." I then tried adding context, and this is what the phone transcribed. In each case, the first quotation is what I (thought I) said, the second is what appeared on the screen. "He is writing a bike." -> "He is writing a bike." (Unlikely, yet transcribed correctly). "He is riding a bike." -> "He is riding a bike." "He is writing a book." -> "He is writing a book." "He is riding a book." -> "He is writing a book." Make what you will of this. I don't think I pronounced "writing" and "riding" identically, but obviously the difference must be too small for a computer to "perceive" accurately. Also, the computer apparently has a "mindset" for hearing the word "writing" rather than "riding."

It's one area where British accents are clearer than American ones. There are other areas where the reverse is true. The /t/ of writing might become a glottal stop but not a /d/ which to me sounds distinctively N American.

Yes, in American English 'writing' and 'riding' will normally sound the same; 't' between vowels is pronounced as a 'flap' or 'tap' (as is 'd' between vowels, too!). There are more examples in this video: https://youtu.be/Y7FUneS1mBs

It seems weird, because usually the evolution of pronunciation appears to be in the direction of sloppiness, but I've noticed one change in the U.S. that seems to be going the opposite way. I pronounce the word "student" with a very light "d" that is probably that sloppy t-like "flap" sound. I pronounce the "e" as a schwa or omit it altogether: "stood'nt" with a very light "d" sound. Heavy accent on the "stoo," very light stress on "-d'nt". More and more often, though, I am hearing it pronounced as "stoo-dent" with a very clear "e," a very distinct "d," and practically equal stress on the two syllables.