Nexus 5 has no settings to change runtime I'm using Nexus 5. It was running on KitKat and I updated to Android L.

I wanted to try and check out if it's running on Dalvik or ART, but the settings to change runtime in the Developer Option is not there. 

Does my phone have to be rooted?


Screenshot (click image for larger variant)
From the official Android Lollipop website,

Android 5.0 provides a faster, smoother and more powerful computing experience.
Android now runs exclusively on the new ART runtime, built from the ground up to support a mix of ahead-of-time (AOT), just-in-time (JIT), and interpreted code. It’s supported on ARM, x86, and MIPS architectures and is fully 64-bit compatible.

(Emphasis mine)
So, starting from Android 5.0 Lollipop, it will run on Android Runtime (ART). AFAIK, there is no way to change back to Dalvik runtime.