Present perfect continuous explained on englishpage.com
We use the Present Perfect Continuous to show that something started
in the past and has continued up until now. "For five minutes," "for
two weeks," and "since Tuesday" are all durations which can be used
with the Present Perfect Continuous.
Even if the accident has taken place this day and is not continuing now, there is a rule of the present period of time. If you say 'today' you refer to the present period of time.
Imagine it is 3 pm and compare:
- There has been an accident today (today is still continuing)
- There was an accident this morning (this morning was in the past)
- There was an accident at 11 am (11 am was in the past)
The rule of the current period of time is the most versatile one and easiest to understand.