Rigidbody moves really fast

Physics
1

When both Horizontal and Vertical input is being pressed, the rigidbody moves twice as fast if only 1 being pressed.

rigidbody.AddRelativeForce(Input.GetAxis("Horizontal") * accel * Time.deltaTime, 0, Input.GetAxis("Vertical") * accel * Time.deltaTime);
2

What value does accel have? It may be worth rewriting it as it appears when both are pressed, whatever value accel has, it moves twice as fast because accel is being applied twice.

Try also to += accel instead of * accel!

3 
Vector3 temp = new Vector3(Input.GetAxis("Horizontal"),0,Input.GetAxis("Vertical")).normalized*accel*Time.deltaTime;
		rigidbody.AddRelativeForce(temp);

this way only direction with change and magnitude will be same.

4

Thanks Amon and Timer. It worked.