Exactly how does Left 4 Dead's adaptive pacing work (intensity estimate, build-up/peak/relax states, timings, exemptions), and what did a second shipped game keep or change?

Valve's designer documents a small, reproducible controller: estimate each player's 'emotional intensity' from damage and nearby deaths, take the team maximum, and cycle four states (Build Up with full threats until intensity passes a peak threshold, Sustain Peak for 3-5 s, Peak Fade with minimal threats until a natural lull, Relax for 30-45 s or until the team has travelled far enough). Wanderers, mobs and special infected are throttled; bosses are exempt. Warframe's lead AI programmer independently reports a similar damage/kill-based intensity score and a peak-then-hold-off rule at intensity 100. Valve's slides call the estimator crude and give no decay constants or thresholds, and no ablation, so the pattern is documented but its tuning is not. Curator qualifications: 11 of 11 excerpts present. Intensity inputs on the slide: 'When injured by the Infected, proportional to damage taken'; 'When the player becomes incapacitated'; 'When player is pulled/pushed off of a ledge by the Infected'; 'When nearby Infected dies, inversely proportional to distance'; 'Decay Survivor Intensity towards zero over time'; 'Do NOT decay ... if there are Infected actively engaging'. States: Build Up (full threats until peak threshold), Sustain Peak (3-5 s), Peak Fade (minimal threats until intensity decays out of peak range; Relax waits for a natural break), Relax (30-45 s or travel distance). Bosses not affected. No constants or data given. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

{
  "citations": [
    {
      "url": "https://steamcdn-a.akamaihd.net/apps/valve/2009/ai_systems_of_l4d_mike_booth.pdf",
      "note": "Booth, 'The AI Systems of Left 4 Dead' (AIIDE 2009) PDF (equals producer pin), slides 'Estimating the emotional intensity', 'Adaptive Dramatic Pacing', summary; GDC 2009 'Replayable Cooperative Game Design: Left 4 Dead' PDF (equals pin), state list and 'Robust AI Actors'; Game AI Pro Online Edition 2021 ch.7 (Warframe) PDF, sec. 4-5, Fig. 4"
    },
    {
      "url": "https://steamcdn-a.akamaihd.net/apps/valve/2009/GDC2009_ReplayableCooperativeGameDesign_Left4Dead.pdf",
      "note": "Booth, 'The AI Systems of Left 4 Dead' (AIIDE 2009) PDF (equals producer pin), slides 'Estimating the emotional intensity', 'Adaptive Dramatic Pacing', summary; GDC 2009 'Replayable Cooperative Game Design: Left 4 Dead' PDF (equals pin), state list and 'Robust AI Actors'; Game AI Pro Online Edition 2021 ch.7 (Warframe) PDF, sec. 4-5, Fig. 4"
    },
    {
      "url": "http://www.gameaipro.com/GameAIProOnlineEdition2021/GameAIProOnlineEdition2021_Chapter07_Managing_Pacing_in_Procedural_Levels_in_Warframe.pdf",
      "note": "Booth, 'The AI Systems of Left 4 Dead' (AIIDE 2009) PDF (equals producer pin), slides 'Estimating the emotional intensity', 'Adaptive Dramatic Pacing', summary; GDC 2009 'Replayable Cooperative Game Design: Left 4 Dead' PDF (equals pin), state list and 'Robust AI Actors'; Game AI Pro Online Edition 2021 ch.7 (Warframe) PDF, sec. 4-5, Fig. 4"
    }
  ]
}