private float CalculateBrakeAccel(float targetPos)
{
    // targetPos has the position in the z axis the object should stop
    // i.e.: velocity is 0.0f
    float remainingDistance = (targetPos - rigidbody.position.z);
    // formula: a = 0.5 * v0^2 / (e0 - e)
    float brake = 0.5f * rigidbody.velocity.sqrMagnitude / remainingDistance;
    return brake;
}

public float Accel = 8f;
private float maxVelocity = 19.44f; // in m/s, approx. 70km/h
[...]
void FixedUpdate ()
{
    Vector3 forwardForce;
    Vector3 projected;
    if (playerController.PowerInput < 0.0f)  // if user braking, PowerInput = -1
    {
        float brakeAccel = CalculateBrakeAccel(playerController.BrakeTarget); // BrakeTarget is the transform.position.z where the user pushed the brakes + 10 meters
        forwardForce = new Vector3(0.0f, 0.0f, brakeAccel * playerController.PowerInput);
        Vector3 drag = rigidbody.velocity * Mathf.Clamp01(1f - rigidbody.drag * Time.fixedDeltaTime);
        if (drag.z > 0.0f)
            forwardForce += drag;
        else
        {
            forwardForce -= drag;
        }
        rigidbody.AddForce(myTransform.TransformDirection(forwardForce));
    }
    if (!CapAtMaxSpeed())    // if max. vel not reached, accel force can be applied:
    {
        forwardForce = new Vector3(0.0f, 0.0f, playerController.PowerInput * Accel);
        rigidbody.AddForce(myTransform.TransformDirection(forwardForce));
    }
}

bool CapAtMaxSpeed()
{
    // velocityMagnitude == rigidbody.velocity.magnitude but without sqrt() operation.
    velocityMagnitude = Vector3.Dot(rigidbody.velocity, myTransform.forward);
    if (velocityMagnitude > maxVelocity)
    {
        rigidbody.velocity = rigidbody.velocity.normalized * maxVelocity;
        return true;
    }
    return false;
}