Many years ago (mid 70's) I wanted to put a motor on my bicycle. If I am recalling correctly there was one available from Sears at the time. Parents were NOT in favor of the idea and so I wound up buying a motorcycle instead (they thought it would be "safer" -- better braking and designed for a motor. Now around 30 plus years later (and many of those on a motorcycle) I finally have the motor on a bicycle that I originally wanted. AND I LOVE IT!!! Goes along at around 18 miles an hour -- fast enough to get me where I want to go and slow enough to enjoy the ride, allows me to enjoy bicycling with none of the drawbacks (hills, cargo and comfort) and extremely low fuel cost.
Following is what I have done to develop this idea. It is an ongoing process and I have absolutely no doubt that my ideas and opinions will change with time. These are things that have worked (or not) for me in my riding (Bangor is what you might describe as light urban) and legal (laws and regulations of the State of Maine) environment. They will almost certainly have to be modified for your own situation -- BE AWARE OF THIS.
Must be convent (we will use the transportation mode that is most so -- generally this means the cage umm I mean car....)
Comfortable (and the standard bike is NOT)
Can carry cargo (at least a standard grocery store run.
Fast enough to get your there in a timely manner
Can be used at night and during inclement weather if needed (keep in mind that I prefer to avoid traveling during both)
Enjoyable.
Note that I have not included safe as one of the initial objectives. After many years on motorcycles I have come to the conclusion that safety is a state of mind awareness that has little to do with the mode of transport (or safety gear used). Some folks manage to trip on cracks in the sidewalk and do themselves major injury -- others could take a skateboard on the freeway at rush hour and be untouched. You want to be safe -- then walk/ride/drive safe. Pay attention -- be aware of what is around you and comport yourself in a manner that is appropriate for conditions.
As many folks are finding out the giant (safe) SUV they cruise around in feeling invulnerable can be a death cage.
And it is not how fast you can accelerate -- it is how fast you can safely stop.