They might not look like a problem, but grass clippings left in the road can create dangerous conditions for motorcyclists and bicyclists, plus they can be washed into the creek causing environmental problems.
Grass clippings can cause dangerous loss of traction (spinning and slipping) for motorcycles even at moderate speeds.
Additionally, grass clippings can make their way to stormwater drains leading to the creek adding unwanted nutrients to the surface water prompting algae growth and depleting oxygen in the waters. This can cause stream life to suffocate from lack of oxygen.
The best thing to do is blow the clippings back into your lawn, allowing them to compost and provide a valuable source of nutrients for your lawn.
For more information, visit the Illinois River Watershed Partnership website here: http://irwp.org/conservation-and-restoration/homeownertips/