Even as pedestrians we naturally take more care crossing a 60mph road than a 30mph one. The corollary is that we take less care on a lower speed road. We see more low speed accidents around town than we do on high speed motorways.
However at higher speeds the results of an accident can be more severe (unless the parties are travelling in the same direction at about the same speed). 
When assessing risk we need to look at both the chance of an accident occurring ( which can increase as speed reduces) as well as with the severity should one occur (which can increase with speed).

The DfT Guidance realises that speed limits can be too low as well as too high so bases much of its work not only on the road type and accident rates but also on mean traffic speeds as an indicator of the speed most people feel comfortable to travel.  

For further reading on the way we adjust our behaviour due to perceived risk see
http://en.wikipedia.org/wiki/Risk_compensation