Ingress/egress, also known as Sliding Entry, involves simulating an occupants movements while entering (ingress) and exiting (egress) a vehicle. Seat cushion height (ground to seat) is an important factor. Additionally, the greater the distance between the seat cushion and the roof line, the more comfortable the ingress/egress.
Ease of ingress/egress is becoming increasingly more important as the population grows taller (on average) and lives to drive to an older age. Back & neck problems such as degenerative-arthritis-of-the-spine make it difficult for many people to deal with repetitive ingress/egress discomfort caused by low slung vehicles. The ingress/egress test is an entirely different test than "headroom" or "legroom" which merely measures capacity once the occupant has already climbed inside the vehicle.
Fortunately, some inexpensive and efficient new cars, such as the Toyota Yaris hatchback, Honda Fit, & Nissan Versa offer 60" in height, which provides fairly comfortable ingress/egress. Legroom is adequate for those under 6'3".
Here are some new models with good ingress/egress per size class: Subcompact - Honda Fit. Compact Sedan - Nissan Versa. Midsize Wagons - Kia Rondo, Honda Element. Midsize Sedan - 2010 Subaru Legacy. Large Sedan - Ford Taurus. Compact Wagon/SUV - Nissan Cube. Midsize SUV - Saturn VUE.
Hopefully, auto designers and manufacturers will increasingly consider the importance of ingress/egress for their product lines, and some of us will be able to spend less time at the chiropractor and more time enjoying the open road.