Vehicle searches are conducted on a daily basis by police officers, deputy sheriffs, and state troopers on roadways throughout the country. And every day defense lawyers are successful in getting judges to suppress evidence obtained by showing that the vehicle operator's consent to search was not voluntary.
In order to keep the Exclusionary Rule at bay, law enforcers requesting consent need to be sure that the consent was granted knowingly and voluntarily without a hint of cooercion or duress present. Some agencies do this on a verbal basis, while others have a policy and/or practice in place that call for both verbal and written formats of consent to search authorization.
Whatever the case, consent seeking officers should make it clear to the driver that the traffic stop portion of the encounter has concluded and that they are free to leave. One way to sabotage your efforts is to keep possession of the driver's license even while obtaining consent.
Be sure to return the driver's license to the vehicle operator prior to obtaining consent. If your vehicle search does turn into an arrest, document the driver's license physical return to the operator in your report's narrative. That will stop the defense attorney from successfully raising the issue of the consent not being given voluntarily and thus any evidence arising from the search being excluded from the legal proceedings.