I can only speak for Georgia, but many states follow similar paths when it comes to DUI enforcement and the laws governing when and what tests you may take.
The machine that you blow into at the site of the traffic stop is a field test that will give you an approximation of the driver bac. The model that my department uses is a Alco Sensor and gives a read out that is very accurate and rarely will deviate from the court admissible test by more than .01 bac. In court I can only testify that it was one of the tests that I administered to determine that a driver was under the influence of alcohol. During a traffic stop I will not say the reading, nor will I show it to the driver, as the number that is revealed is not admissible.
It is not admissible due to the way that it is calibrated, and it does not use two samples to ensure that the results are consistent. To calibrate the device a can of compressed gas that equates to .08 bac is introduced to the machine and you adjust it up or down to get a display of .08 bac. Obviously this could be manipulated by a dishonest person.
In Georgia you would not have to participate in this test, or any other by the roadside. The failure to do so might actually hurt you if you weren't impaired, but that is up to the individual to decide.
The test that you must take in Georgia is the state administered test using an Intoxalizer 5000, these results are admissible in court. The machine takes two samples, and compares them against each other and there can only be a slight difference between the two in order for the results to be valid and admissible. The machine is tested by a representative of the state, and cannot be adjusted by the operator. ONLY after submitting to this test can you request a test of your blood, breath, or urine at your cost at a facility of your choice.
In cases where the person is injured, we would just request a test of the person's blood at the hospital, with the suspect having the same option to get a further test at their own expense. In Georgia we have " Implied Consent " which basically says that when you operate a motor vehicle on public roads you are giving your consent to have your blood, breath, urine, or other bodily substances test for intoxicants when probable cause exists to believe that you are under the influence.
If a driver refuses to submit to a requested test, their privilege to operate a motor vehicle in the State of Georgia would be suspended, and the DUI case would also continue. The two events arise out of the same incident, but the privilege to drive is a civil matter where the DUI case is criminal. In other words, beating one of the two has no bearing on the other.
It is important to note that you can also receive a DUI when you are less than than .08 bac. Again, I can only speak with certainty when it comes to the state of Georgia but I would assume that other states would have similar provisions. A person operating under a CDL can have no more than a .04 bac for example, while anything over a .02 bac for a driver under the age of 21 could be charged with dui.
The concept of a " less safe driver " also allows for successful dui prosecutions for other drivers at less than .08 bac for drivers not covered in the above examples. For example, if I were to stop a driver that was traveling the wrong way down a interstate highway and they were to blow a .06 bac it would be a fairly easy case to prove that at .06 bac they were a less safe driver and they would be charged with dui.
The problem with this concept is that it requires that the deputy employ good judgment and discretion in making cases. For the most part we do a good job, but if mistakes are made with military personnel, ATP holders, and similar professions the repercussions of the dui may be felt prior to the case going to court.
Most states will have similar set ups, but as others have said it would be best to figure out how things are handled where you are PRIOR to needing the information.