Picking up Speed

It took all of two days for Jasper to decide to take his bike on "jumps" and balanced along thin concrete curbs.  Now he is picking up his legs and coasting down hills, then crashing at the bottom because his bike, alas, has no brakes.  I'm not sure where all this leads once you have a 2 year old who can ride a bike.  The only thing I can think to do is to relegate him to the bike trail and keep him off the roads and sidewalks mainly because he is too fast for me on foot and doesn't possess the forethought to assess risks.