Remember folks, theres a large percentage of folks in Germany that don't drive, don't have a license, or don't own a car. Theres plenty of alternatives to the car in Germany, cities like Hamburg, and Berlin have all kinds of mass transportation such as Buses, Trams, Passenger rail, & Subways. Every other large city in Germany offers all of the above except subways, and most mid sized cities offer everything except subways, and sometimes Trams. Bicycles is a practical alternative to the car as well, since you can basically ride it anywhere in the city, and even to other cities via trains... trains and trams in some cities offer special cars for bicycles and their riders. Even in the countryside theres bus lines that'll transport you back and forth to the larger towns and cities every hour or less.
So basically theres a large percentage of people that have been polled, that I bet don't even have a driver's license, or own a car... so of course a survey will show that most people favor speed limits. Most people that drive don't give much thought in regards to the environment, so I bet most won't favor a speed limit thats environmentally conscious, and the fact that diesel car sales are about to overtake gasoline cars is evidence enough to show drivers won't favor a speed limit. Also a speed limit will do nothing but hurt Germany's economy as mentioned earlier, most of the resources used to maintain the autobahn comes from fuel taxes, and if speed limits are introduced that cause even the slightest decrease in fuels sold, then it'll have a pretty big impact on Germany's budget for maintaining/building autobahns. The only way they'd be able to maintain their roads and autobahns is to start tolling the autobahn, and you better believe that it will never be supported in Germany, sure they got away with it with trucks... but it'll never happen with cars.
Theres already a recommended speed limit of 130kmh anyway, and the green party isn't in control in Berlin so it'll stay that way.