Cubic and higher-degree polynomial models
If a quadratic model does not adequately describe the trend in a time series, it is tempting to try increasing the order of the polynomial,
trend = b0 + b1 time + b2 time2 + b3 time3 + ...
Polynomial models of degree 3 or 4 often provides a fairly smooth description of trend but they usually behave badly (with sudden increases or decreases) beyond the data points, so
Polynomial models of degree greater than 2 should not be used for forecasting.
Fatal traffic crashes in New Zealand