Answer by taking a quote from the following article:

James Byron Dean was born on February 8, 1931, at the Seven Gables apartment on the corner of 4th Street and McClure Street in Marion, Indiana, the only child of Winton Dean and Mildred Marie Wilson. He was primarily of English descent, with smaller amounts of German, Irish, Scottish, and Welsh ancestry. Six years after his father had left farming to become a dental technician, Dean and his family moved to Santa Monica, California. He was enrolled at Brentwood Public School in the Brentwood neighborhood of Los Angeles, California, but transferred soon afterward to the McKinley Elementary School.

Dean's first television appearance was in a Pepsi Cola commercial. He quit college to act full-time and was cast in his first speaking part, as John the Beloved Disciple, in Hill Number One, an Easter television special dramatizing the Resurrection of Jesus. Dean worked at the widely filmed Iverson Movie Ranch in the Chatsworth area of Los Angeles during production of the program, for which a replica of the tomb of Jesus was built on location at the ranch. Dean subsequently obtained three walk-on roles in movies: as a soldier in Fixed Bayonets!, a boxing cornerman in Sailor Beware, and a youth in Has Anybody Seen My Gal?  While struggling to get jobs in Hollywood, Dean also worked as a parking lot attendant at CBS Studios, during which time he met Rogers Brackett, a radio director for an advertising agency, who offered him professional help and guidance in his chosen career, as well as a place to stay.  In July 1951, Dean appeared on Alias Jane Doe, which was produced by Brackett. In October 1951, following the encouragement of actor James Whitmore and the advice of his mentor Rogers Brackett, Dean moved to New York City. There, he worked as a stunt tester for the game show Beat the Clock, but was subsequently fired for allegedly performing the tasks too quickly. He also appeared in episodes of several CBS television series The Web, Studio One, and Lux Video Theatre, before gaining admission to the Actors Studio to study method acting under Lee Strasberg.  Proud of this accomplishment, Dean referred to the Actors Studio in a 1952 letter to his family as "the greatest school of the theater. It houses great people like Marlon Brando, Julie Harris, Arthur Kennedy, Mildred Dunnock, Eli Wallach... Very few get into it ... It is the best thing that can happen to an actor. I am one of the youngest to belong." There, he was classmates and close friends with Carroll Baker, alongside whom he would eventually star in Giant (1956).  Dean's career picked up and he performed in further episodes of such early 1950s television shows as Kraft Television Theatre, Robert Montgomery Presents, The United States Steel Hour, Danger, and General Electric Theater. One early role, for the CBS series Omnibus in the episode "Glory in the Flower", saw Dean portraying the type of disaffected youth he would later portray in Rebel Without a Cause. This summer 1953 program was also notable for featuring the song "Crazy Man, Crazy", one of the first dramatic TV programs to feature rock and roll. Positive reviews for Dean's 1954 theatrical role as Bachir, a pandering North African houseboy, in an adaptation of Andre Gide's book The Immoralist, led to calls from Hollywood.  In 1953, director Elia Kazan was looking for a substantive actor to play the emotionally complex role of 'Cal Trask', for screenwriter Paul Osborn's adaptation of John Steinbeck's 1952 novel East of Eden. The lengthy novel deals with the story of the Trask and Hamilton families over the course of three generations, focusing especially on the lives of the latter two generations in Salinas Valley, California, from the mid-19th century through the 1910s. In contrast to the book, the film script focused on the last portion of the story, predominantly with the character of Cal. Though he initially seems more aloof and emotionally troubled than his twin brother Aron, Cal is soon seen to be more worldly, business savvy, and even sagacious than their pious and constantly disapproving father (played by Raymond Massey) who seeks to invent a vegetable refrigeration process. Cal is bothered by the mystery of their supposedly dead mother, and discovers she is still alive and a brothel-keeping 'madam'; the part was played by actress Jo Van Fleet.  Before casting Cal, Elia Kazan said that he wanted "a Brando" for the role and Osborn suggested Dean, a relatively unknown young actor. Dean met with Steinbeck, who did not like the moody, complex young man personally, but thought him to be perfect for the part. Dean was cast in the role and on April 8, 1954, left New York City and headed for Los Angeles to begin shooting.  Much of Dean's performance in the film is unscripted, including his dance in the bean field and his fetal-like posturing while riding on top of a train boxcar (after searching out his mother in nearby Monterey).  The most famous improvisation of the film occurs when Cal's father rejects his gift of $5,000, money Cal earned by speculating in beans before the US became involved in World War I. Instead of running away from his father as the script called for, Dean instinctively turned to Massey and in a gesture of extreme emotion, lunged forward and grabbed him in a full embrace, crying. Kazan kept this and Massey's shocked reaction in the film. Dean's performance in the film foreshadowed his role as Jim Stark in Rebel Without A Cause. Both characters are angst-ridden protagonists and misunderstood outcasts, desperately craving approval from their fathers.  In recognition of his performance in East of Eden, Dean was nominated posthumously for the 1956 Academy Awards as Best Actor in a Leading Role of 1955, the first official posthumous acting nomination in Academy Awards history. (Jeanne Eagels was nominated for Best Actress in 1929, when the rules for selection of the winner were different.) East of Eden was the only film starring Dean that he would see released in his lifetime.  Longing to return to the "liberating prospects" of motor racing, Dean was scheduled to compete at a racing event in Salinas, California on September 30, 1955. Accompanying the actor to the occasion was stunt coordinator Bill Hickman, Collier's photographer Sanford Roth, and Rolf Wutherich, the German mechanic from the Porsche factory who maintained Dean's Porsche 550 Spyder "Little Bastard" car. Wutherich, who had encouraged Dean to drive the car from Los Angeles to Salinas to break it in, accompanied Dean in the Porsche. At 3:30 p.m. Dean was ticketed for speeding, as was Hickman who was following behind in another car.  As the group traveled to the event via U.S. Route 466, (currently SR 46) at approximately 5:15 p.m. a 1950 Ford Tudor was passing through an intersection while turning, ahead of the Porsche. Dean, unable to stop in time, slammed into the driver's side of the Ford resulting in Dean's car bouncing across the pavement onto the side of the highway. Dean's passenger, Wutherich, was thrown from the Porsche, while Dean was trapped in the car and sustained numerous fatal injuries, including a broken neck. The driver of the Ford, Donald Turnupseed, exited his damaged vehicle with minor injuries. The accident was witnessed by a number of passersby who stopped to help. A woman with nursing experience attended to Dean and detected a weak pulse, but "death appeared to have been instantaneous". Dean was pronounced dead on arrival shortly after he arrived by ambulance at the Paso Robles War Memorial Hospital at 6:20 p.m.  Though initially slow to reach newspapers in the Eastern United States, details of Dean's death rapidly spread via radio and television. By October 2, his death had received significant coverage from domestic and foreign media outlets. Dean's funeral was held on October 8, 1955 at the Fairmount Friends Church in Fairmount, Indiana. The coffin remained closed to conceal his severe injuries. An estimated 600 mourners were in attendance, while another 2400 fans gathered outside of the building during the procession. He is buried at Park Cemetery in Fairmount, second road to the right from the main entrance, and up the hill on the right, facing the drive.  An inquest into Dean's death occurred three days later at the Paso Robles City Hall, where a coroner's jury delivered a verdict that he was entirely at fault due to speeding, and that Turnupseed was innocent of any criminal act. However, according to an article in the Los Angeles Times of October 1, 2005, a former California Highway Patrol officer who had been called to the scene, Ron Nelson, said the "wreckage and the position of Dean's body indicated his speed at the time of the accident was more like 55 mph".

What accident did Dean have?
Dean, unable to stop in time, slammed into the driver's side of the Ford resulting in Dean's car bouncing across the pavement onto the side of the highway.