I recently had the opportunity to try out a prototype 4-point seatbelt system intended for street vehicles. I was invited to try the system out on a short test drive, and take a survey afterwards. I won't divulge the company developing the system, I am sure they will come out when they are ready. The system was installed in a Ford Mustang GT, and was intended to be sold to the sports/enthusiast consumer.
The system consists of two shoulder belts, which you slip each arm through, and buckle together centered at your waist. The lower straps form a lap belt, with a slight arch upwards towards the buckle. The lower straps are attached to automatic tensioners, which prevent the buckle from riding up on your belly. This is key, because if the buckle is too high, you could slide under the belt ("submarine") in a severe accident. The upper straps have traditional springy tensioners, which allow you to twist left and right, or lean forward as needed.
The system was a little more work to get into, since you have to thread each arm through a loop, but once buckled in, it was quite comfortable. Having both shoulders covered gave a very secure feeling, and having the buckle centered at the waist made it very easy to find and release the belt. Driving did not require any significant amount of additional effort, even in a manual transmission car. I would buy a 4-point system if it was offered, without hesitation.
The engineer running the survey told me that NHTSA is very interested in 4-point belts, because they spread the deceleration loads more evenly and symmetrically over a person's torso. In order for any car maker to offer 4-point belts, the regulations of FMVSS 208 would have to be amended to allow the design.
A major concern is how consumers would perceive the 4-point belts. Some people would prefer them, because of the added safety and "racecar" image, but some people would be annoyed by the extra work in putting on the belt. Another question is how very large, or pregnant passengers would wear the belts.
Several automakers and suppliers are working on improved seat belt designs. Volvo has been experimenting with 4-point safety belt designs, including a crossover type which forms an X over an occupant's chest. Nissan has revealed a 4-point system in its Sport concept car. Ford recently announced an inflatable seatbelt concept, which would spread forces out over a wider area by inflating an airbag built into a 3-point belt.