New York ( CNN ) -- A National Transportation Safety Board investigation has revealed new facts about the Bronx crash that killed 15 people in early March , with the head of the NTSB issuing renewed calls for changes in bus safety and regulation of bus companies . One of the primary findings of the investigation so far is that contrary to bus driver Ophadell Williams statement to authorities that a tractor-trailer may have clipped the bus causing the accident , the NTSB engineer who examined the bus found no evidence to indicate that a truck had come into contact with it , NTSB chairman Deborah Hersman told CNN . The investigation found that the bus , with the 40-year-old Williams at the wheel , drifted to the right before striking a roadside barrier , Hersman said Wednesday in testimony to a Senate hearing on bus safety . After hitting the barrier , the bus rolled and skidded for nearly 500 feet before colliding with a highway signpost that penetrated the windshield and speared the entire length of the bus , killing 15 passengers , according to Hersman . Investigators determined that the bus was traveling at 78 miles per hour at some point between leaving the Connecticut casino and the scene of the accident , while the speed limit at that stretch of roadway was 50 miles per hour for commercial vehicles , Hersman told CNN . A camera was also mounted on the bus windshield when the bus crashed , but it did not record the accident , the NTSB reported in a press release . The crash happened in the early hours of March 12 on the New England Thruway near the border of Westchester and the Bronx . En route from the Mohegan Sun Resort and Casino , the `` World Wide Tours '' bus was bound for Chinatown in Manhattan . `` Unlike when travelers get in their own automobiles , passengers boarding a motor coach place their lives in the hands of the motor coach operator and its driver , '' Hersman said Wednesday of the industry that transports 750 million passengers per year . This accident , along with other recent , serious motor coach crashes in New Jersey and New Hampshire , has led the NTSB to renew its emphasis bus safety recommendations . The first NTSB recommendation is to improve passenger protection with stronger roofs , a redesign of window emergency exits and improved standards for passenger seating compartments . The NTSB also recommended better government oversight of both drivers and vehicles , as well as more advanced vehicle technologies , such as forward-collision warning systems . The Department of Transportation currently is working on rules covering the recommendations , Hersman said in her testimony . The full report of what happened is expected to be completed within 12 months , a spokesman for the NTSB , Peter Knudson , told CNN .
Fifteen people died in a Bronx bus crash in early March . The driver was charged with criminally negligent homicide and other offenses related to the accident that killed 15 people. An NTSB investigation does not support the bus driver 's claim that a truck clipped the bus .