Yes, there are single rider lines for some attractions in DL and DCA. Disneyland.com has a list of attractions that use the SRL. The length of the lines can vary depending on crowds, time of the day, etc. In my experience, Matterhorn has the most variable SRL in terms of how long it will take to be seated. It can take anywhere from less than 5 minutes to over 30 minutes to seat three people depending on the configuration of the groups in the regular line.
The SRL vary a ton throughout the day, depending on a ton of factors with seemingly little rhyme or reason. They are usually a big time saver though, except Soarin in DCA, which seems to break even in most cases.
I usually go in the afternoon to Indiana Jones and wait about 20-30 minutes in the SRL. Haven't done Splash in awhile. I did the SRL for Matterhorn around 10:30 pm and waited about 10 minutes. The lengths do vary though depending on the crowds.