code: private function calculateSpeed ()
  	{
  		$startSpeed;
  		$endSpeed;
  		$acceleration = 0;
  
  		// Accelerated speed
  		if (abs ($this->acceleration) > 0)
  		{
  			$startSpeed = $this->startSpeed;
  			$acceleration = $this->acceleration;
  			$endSpeed = $this->startSpeed + ($acceleration * $this->getDuration ());
  		}
  
  		// Constant velocity
  		else
  		{
  			$startSpeed = $this->getDistance () / $this->getDuration ();
  			$endSpeed = $startSpeed;
  		}
  
  		return array 
  		(
  			'start' => $startSpeed,
  			'end' => $endSpeed,
  			'acceleration' => $acceleration
  		);
  	}
docs: Return an array of speed & acceleration
  Speed is defined in pixels / second