Except you shouldn't need flash to have enough options on Riven.
What you should be doing is splitpushing (this is under the assumption you take tp/aggro-lane-SS, double-aggro-SS is a very team specific pick). Riven is a dueler, not a diver, she isn't Trynd or Shyv. Does she have a mobile kit? Yes. Does that mean you should dive into the enemy team when you are full-AD build, no armor or MR? No, and if you do it means your spirit animal is a Vayne. You need to think of Riven as the top-lane equivalent of Zed (who, depending on matchups, can see some success in top-lane), not a Garen that can do flips.
As far as flash on ADC's... is it safer to have flash and your escape? Yes. Is it necessary? Well, if I'm a 10/0 Ez with a Leona supp, Morg mid, Sej jung, and Naut top... I probably won't be needing that second escape, will I? This situation is, obviously, an exaggeration, but the point remains - if you have almost any support, coupled with at least 1 other source of hard cc, that right there will be 1 flash-like ability, plus 2 sources of hard cc and one soft (exhaust). If with all that their assassin still goes in on you, he will be too far in to escape, and if it's a tank who managed to get through your front line, and he is that big of a dmg threat that you need to flash away, well the enemy team probably already has a quadra at that point, flash won't help you much there.
tl;dr: Riven isn't a diver like Trynd or Shyv, if you need flash to dive in deeper you are in too deep or playing her wrong.
If you as adc need more than 2 sources of hard cc, one soft, plus a pseudo flash and still can't outplay one of their champs diving in way too far into your team - you probably did something wrong. Not even to mention that the estimate for number of cc's is a pretty conservative one.