A segment of roadway that directs vehicular traffic from ordinary roads onto a freeway(noun)
Wiktionary.org : Text is available under the Creative Commons Attribution-ShareAlike License
Use "on-ramp" in a sentence
"Summer jobs provide young people a critical on-ramp to the road towards responsibility, community, work ethic, stability and economic self-sufficiency."
"Things improved coasting downhill, but when I accelerated up the on-ramp to merge with traffic, my real-time fuel economy dipped below 10 mpg."
"I made what I thought was a rather artful multiple-lane-change to access a very necessary and poorly signed (imagine ....) on-ramp."