Jennie can cover the 200-mile distance in 5 hours when there is heavy traffic, so her average speed is 200/5 = <<200/5=40>>40mph
She can cover the same distance in 4 hours when there is no traffic for an average speed of 200/4 = <<200/4=50>>50mph
The difference in average speed between the two scenarios is (50-40)mph = <<(50-40)=10>>10mph
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