increasing traffic volume gradually in gameplay

git-svn-id: http://svn.code.sf.net/p/sumo/code/trunk/sumo@15991 29f149b2-0c63-11e1-8f5c-a347f6a9acd0
Merge branch 'master' into planet
Merge branch 'planet' of https://github.com/planetsumo/sumo into planet