Is Tai Chi a martial art that can be applied in a street fight or against an experienced fighter?
Taiji (Tai Chi) can be very effective for fighting. Historically speaking, taiji masters like Yang Luchan were among the most skilled fighters in China. In his time, taiji was known for fighting, and not for health benefits.
However, it is necessary to understand how taiji has changed in the years since this time. In the 20th century, China experienced a shortage of trained doctors. Knowing that martial arts like taiji were beneficial for health, the government had martial arts masters simplify the martial training for the masses so they could reap the health benefits. The result is large groups of people in China practicing taiji and qi gong in the parks, with perhaps little connection to the martial roots.
The result of this public health program is an inversion of the source of taiji's fame. Formerly known for martial skill with little known health benefits, taiji is now known for health benefits and little known for martial skill.
From The Harvard Medical School Guide to Tai Chi by Peter Wayne, with Mark Fuerst p. 21:
A noteworthy development in Tai Chi's widespread promotion for health was the development of a 24-posture simplified form, or the Beijing Form. This form was developed in 1956 by the National Physical Culture and Sports Commission of the People's Republic of China as a part of the drive to standardize Tai Chi training for social reform and sport. This form was part of a national fitness program. Today you can go to parks across China and see millions practicing Tai Chi. This practice is clearly part of the country's health maintenance system, and many Chinese hospitals integrate Tai Chi into rehabilitation.
In the present day, it is common to have taiji classes that do not actually teach fighting, or may teach only push hands. As you may expect, the martial results of this training are not so good.
I think that this question can not be answered easily as it is depending on each person how he can transfer the knowledge and practice he learned in classes to the street scenarios under pressure and adrenaline pump.
However, you have to realize that a punch is a punch and a kick is a kick, and that if the moment comes, all flashy movements disappear and what will be left, will be just a punch and just a kick ... sometimes a little hugging on the ground too ;-). (took - a punch is a punch and a kick is a kick from Bruce Lee)
My personal believe is, if you want to have a direct or at least close to street training, you should choose a martial art with a lots of sparring and competition aspect in it - and again, even then there is no guarantee that you will use it on the street.
Also, don´t forget that in real life there are also. let´s call it 3rd party objects, that you may have to defend against that you will not see in training ex. glas, baseball bat, knife, ashtray, billiard ball or bat etc.
Yes I think Tai Chi could be used in real fights. For example Nick Osipczak was a Pro MMA fighter that practices Tai Chi.
However I think having knowledge of many different martial arts is advantageous in a "real fight". Many Tai Chi techniques are used in other forms of fighting like Sanda Kickboxing. Krav maga, Gracie jiu jitsu, kickboxing, and wrestling are also valuable skills to have in a street fight. But honestly training to running fast in the opposite direction is going to be way more effective than trying to fight someone in the street with Tai Chi.