Illinois field sobriety challenges start with the NHTSA manual. The three standardized tests, horizontal gaze nystagmus, walk-and-turn, and one-leg stand, have strict administration and scoring rules. Deviations from the manual undermine reliability. Physical conditions such as knee injuries, inner-ear disorders, obesity, age over 65, and prescribed medication all affect scoring. Roadside conditions including sloped shoulders, gravel, poor lighting, weather, and vehicle traffic distort performance. Officers who fail to demonstrate, fail to ask about medical conditions, or fail to give complete instructions weaken the state's case. A Chicago DUI lawyer uses cross-examination against the same officer who scored the test to expose the gap between the manual and what actually happened.
