The elements, and why the second one matters
Intoxication assault under Texas Penal Code Section 49.07 requires the State to prove you operated a vehicle in a public place while intoxicated, and that by reason of that intoxication you caused serious bodily injury to another person.
That causation link is the whole case. Intoxication alone is not enough. If the other driver ran the light, if the road was wet, if a mechanical failure contributed, or if the injury does not meet the legal definition of serious bodily injury, the felony may not hold even where the intoxication evidence is strong.
Serious bodily injury has a specific statutory meaning: substantial risk of death, serious permanent disfigurement, or protracted loss or impairment of a bodily member or organ. A broken bone that heals is not automatically in that category, and the medical records are evidence that cuts both ways.
Punishment
Third degree felony. Two to ten years in TDCJ and a fine up to $10,000. If the injury was to a peace officer, firefighter, or emergency medical responder in the line of duty, or if it resulted in a traumatic brain injury causing a persistent vegetative state, the offense is enhanced further. It is also a 3g offense, which affects parole eligibility, and a conviction carries an affirmative finding on the use of a deadly weapon.
What gets investigated
Accident reconstruction, independently. The State's reconstruction is done by people who already believe the intoxicated driver caused it. Skid analysis, event data recorder downloads from both vehicles, sight lines, signal timing, and witness sequencing frequently tell a more complicated story.
Then the blood. In a felony case with a serious injury there is almost always a blood draw, often at a hospital rather than a jail, which raises its own questions about whether the sample was collected for treatment or for evidence, how it was preserved, and whether serum was converted to whole blood correctly. That conversion error is common and it inflates results.