On mountain highways, maintenance is typically carried out under "traffic-open-during-construction" conditions. How temporary signs are spaced in these work zones directly shapes what information reaches drivers and when. Existing design standards specify a minimum spacing but treat the entire work zone as a single segment, ignoring the different roles of warning, transition, and activity areas. We develop a spacing framework with two constraints: sight recognition distance sets the floor, and information load per unit segment sets the ceiling. From this we propose phased gradient spacing. The advance warning zone uses 1.2–1.5× sight recognition distance to give drivers enough time; the transition zone tightens to 0.8–1.0× to strengthen risk signals; and the activity zone keeps spacing at or above the sight distance so critical signs (stop, speed limit) are not missed. We calculated spacing for a two-lane mountain highway at 40 km/h under three common maintenance scenarios—curve-section pavement repair, straight-segment guardrail work, and bridge–tunnel transition construction. The recommended range is 20–30 m. For bridge–tunnel transitions, light–dark adaptation cuts effective sight distance, so we recommend tighter spacing and Class IV (or better) retroreflective sheeting. No new data collection is needed; practitioners can use the method with existing traffic management plans.