Are there grounds for an appeal if a judge rules before closing arguments and denies a mistrial?

There is no inherent legal right in the court rules to render closing arguments in traffic or municipal court (your question references traffic); in a bench (judge only, no jury) trial, the judge can rule at any time after the defendant had the chance to actually put on his or her case (and if the defendant is getting repetitive or going into irrelevancies or trying to present inadmissible evidence, the judge can cut him/her short--the judge does not need to allow a defendant to keep going on if the testimony or evidence has already been given or is not germane); and therefore no right to a mistrial for the judge doing these things, and so no inherent right to an appeal.
The grounds for an appeal would be the same as the are in *any* case: 
1) An error of law: the judge can be shown to have misapplied or misunderstood the statutes or case law--you need to cite specifically to the error and provide legal back-up and analysis as to why it was in error; or
2) A CLEAR error of fact, such as the judge ruling (for example) that you were speeding 25 miles over the limit and applying that penalty, when the state's (i.e. the police officer's) testimony was that you were only 12 mph over, in which case you would need to get a transcript of the hearing and cite to the specific factual error in it.