IEEE TRANSACTIONS ON INTELLIGENT TRANSPORTATION SYSTEMS

Among them, "under review" means that the manuscript is in the hands of the AE responsible for your submission.
"Awaiting reviewer assignment" means that the AE has sent your manuscript to a certain number of reviewers, waiting for their agreement to review. Generally, the number of reviewers who agree to review is greater than or equal to 3 (in a few cases, 2 is also acceptable), and the status will change to "awaiting reviewer scores". If less than three reviewers agree to review and all the reviewers who were sent the manuscript have responded (agree to review/decline to review), or if the "awaiting reviewer assignment" status lasts for more than a certain period of time (I guess one month), the status will change to "under review". This means that the number of reviewers has not reached 3, and the AE needs to find new reviewers for you. However, in this case, the reviewers who have already accepted the review will generally complete it within one month (a one-month deadline is required, and reminder emails will be sent five days before the deadline, on the day of the deadline, and ten days after the deadline).
Therefore, "under review" does not mean that it has not been sent for review, but it is possible that it has been sent for review but the number of reviewers is not sufficient.

Give everyone some reference. This article has been submitted to TITS many times. This journal is sometimes fast and sometimes slow, but usually very slow. There are often reverse reviews (reverse status). For example:
(1) Article 1 took seven months for the first review. In the sixth month, it was awaiting AE recommendation, and in the seventh month, it became awaiting reviewer scores.
(2) Article 2 took four months for the first review. After a month and a half, it was requested for revisions, and after three months, it is still under review (although it changed to awaiting reviewer assignment midway).
(3) Article 3 has been under review for four months since submission, and it changed to awaiting reviewer assignment once during the process.
In general, the speed of handling manuscripts by the AEs of this journal is slow. I speculate that many AEs may log in to the system once a month. The Editor-in-Chief's frequency of logging into the system is also not high, probably about once a week (sometimes longer). Some reviewers are not responsible enough, and there are cases where they do not want to review the manuscript even if it has received good evaluations in the first review.
That's all for now.