Modal vs. Separate Page: UX Decision Tree - Smashing Magazine

"While we often speak about a single modal UI component, we often ignore fine, intricate nuances between all the different types of modals. In fact, not every modal is the same. Modals, dialogs, overlays, and lightboxes - all sound similar, but they are actually quite different."

"As Anna Kaley highlights, most overlays appear at the wrong time, interrupt users during critical tasks, use poor language, and break users' flow. They are interruptive by nature, and typically with a high level of severity without a strong need for that."

"Surely users must be slowed down and interrupted if the consequences of their action have a high impact, but for most scenarios non-modals are much more subtle and a more friendly option to bring something to the user's attention. If anything, I always suggest it to be a default."

Choosing between modals and pages significantly impacts user flow, context retention, error rates, and task completion. Modals, dialogs, overlays, and lightboxes differ fundamentally: dialogs facilitate conversation, overlays display content panels, modals disable background interaction, non-modals allow background access, and lightboxes use dimmed backgrounds for focus. Most overlays appear at wrong times, interrupt critical tasks, use poor language, and disrupt user flow. Non-modal alternatives provide subtle, friendlier attention-getting for most scenarios. Modals serve specific purposes when action consequences carry high impact, but designers should default to non-modal solutions for better user experience.

Read at Smashing Magazine