I apologize in advance. You're not going to like this answer much, or find it to be very helpful.
It's not clear to me from your question, or even from your blog, how confident you are that the police ever identified the man who hit you. Their assertion that there is "conflicting evidence" could mean that they have identified him, and that after interviewing him and perhaps other potential witnesses, they don't believe that the can obtain a conviction, or in any event that they think the chances of that are sufficiently low that they'd be better off using their limited resources on other cases. Or it may be that they haven't done all that, or any of it, and their assertion may just be an excuse, a "white lie" intended to make you feel better and used to conceal an inadequate investigation (as you would judge it).
However, police and, ultimately, prosecutors have ENORMOUS amounts of discretion as to what criminal charges they choose to bring. There's no practical way to appeal their decisions not to charge.
It's conceivable that you might still find someone within the police or the Harris County DA's office who you could persuade to take a second or third look. It's likely that at some point in that process, though, they're eventually going to mentally write you off as a "kook" and stop listening, regardless of the merits of your arguments.
In terms of making the criminal justice system focus its attention on this assault, then, I don't have much hope to give you.
The same conduct may also create civil liabilities: You could sue for money damages in civil court, IF you can find the defendant's name and then locate him to get him served. There's always a serious question in such cases as to whether the defendant, when and if he's located, is "judgment-worthy" or "judgment-proof." Most likely he has no assets that could be seized and sold to satisfy any civil judgment you might win against him. Most likely any insurance he had (e.g., car insurance) will have disclaimers and exclusions for "intentional torts" like deliberate assaults. Again, without knowing the identity of the guy, this is going to be hard for any lawyer to assess, and it's not very likely that a lawyer is going to be eager to invest his own time in your case by betting that he'll eventually find this guy, win the case, and then actually be able to collect an award out of which he can get paid a contingent-fee percentage. So if you want to get a civil lawyer to help you, for example, to try to compel the police or the DPS to turn over information that might help identify the defendant, you may have to pay that lawyer for his time on an hourly-rate basis, without much chance of ever getting that money back or anything substantial for it.
It's conceivable that you could use a civil lawsuit to find the guy and to obtain proof which might then be used to persuade the DA's office to reconsider the question of bringing criminal charges, but that's also a very long shot that's likely to require an out-of-pocket investment on your part which you'll likely never get back.
I sympathize with your situation: You feel twice victimized, once by the assaulter, and then again by the system. I wish I had more hope to hold out that a civil lawyer like myself might be able to help you, but it would be disrespectful and a further abuse to give you false hopes.
Mr. Dyer provided you with a very thorough answer. The only thing left to do is to try to talk to the investigator's supervisor. If that fails, put your request for information in writing to the investigator, with a copy to his supervisor.
In addition, if you believe that you can identify the person, have a private investigator run the license plate and find out the name of the owner and then have the private investigator interview the owner to see if you can find out who had the car that evening. Some times private investigators can come up with more because they are being paid for their time and are not overwhelmed on the job.
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