In general, people can be recorded anywhere without consent so long as they do not have a reasonable expectation of privacy. For example, if you were having a telephone conversation inside your home or office, you would expect that conversation to be private, even if someone was eavesdropping from a window. On the other hand, if you were having a conversation on your front porch or office balcony, the courts would probably rule your conversation to be unprotected, since a reasonable person would know that their conversation outside could easily be overheard.
For taxicabs, the courts consider passengers to have a limited expectation of privacy since the passenger is paying a fare, has control over the destination, and can exclude other passengers. This means that if you are going to use a dashcam to record their ride, you first have to get consent to record the conversation. But consent laws can be tricky and they vary by state.
Fifteen states currently require that both parties to a conversation consent to the recording, while the other 35 states and the District of Columbia only require one person to consent to the recording. Federal law also only requires the consent of one party, but if you are in a state where the two-party rule applies, then in most cases, you will have to get consent from both parties.