I have been told by traffic experts that speed itself is not nearly as great a danger as differences in speed between vehicles traveling the same road.
Speed differential
Each day in our bodies cells die and new cells
grow in their places. Skin and stomach turn over
every week or so; bone and muscle take years.
Heart cells, once thought irreplaceable, regenerate
so slowly we would live at least two
centuries before all had been renewed.
And so we exist in a state of wide and varied
flux, but our hearts refresh at such an incremental rate
they wear out before they can change completely.
The Daily Compost 