This is something that was briefly discussed not too long ago and the
idea is that a road's speed or class can be modified by the
presence of a POI that defines new values for either/both of them. This,
hopefully, will make ways that have stuff like traffic signals and
crossings, etc. less attractive to route over.

The values are specified using mkgmap:road-speed and mkgmap:road-class
tags added to the POI.

These tags expect an integer value, optionally preceded by a + or -
which make the adjustment relative to the original value.

For example, mkgmap:road-speed=3 sets the speed to 3 and
mkgmap:road-speed=-1 reduces the speed by 1 from whatever value it had
before.

Here's some possible rules for the points file:

highway=traffic_signals { add mkgmap:road-speed = '-1' }
highway=crossing { add mkgmap:road-speed = '-1' }
highway=mini_roundabout { add mkgmap:road-speed = '1' }