On June 24, 1996, at 0930 mountain daylight time, an experimental home built Williams Velocity 173RG, N525V, registered to a private owner, exited the end of runway 29 onto soft ground during the takeoff roll at the Vance Brand Airport, Longmont, Colorado. The private pilot and the one passenger were not injured and the aircraft sustained substantial damage. The flight was operating under Title 14 CFR Part 91 when the accident occurred and no flight plan was filed. Visual meteorological conditions prevailed for this personal flight which was departing to Melbourne Beach, Florida, with the first en route stop planned at Salina, Kansas. According to the pilot, he was aware of the density altitude (7,500 feet) but said he had experienced no problems on takeoff from Hattiesburg, Mississippi, or Wichita Falls, Texas, on the trip to Colorado, from Florida, so he expected none on takeoff from Longmont. (Hattiesburg is 151 feet above sea level and Wichita Falls is 1,005 feet above sea level.) According to the Koch chart on altitude performance, given the conditions present at takeoff, the pilot would experience a takeoff run 130 percent longer than at sea level and climb performance would be degraded by 63 percent. No empirical data was provided by the manufacture of the kit airplane to assist the pilot in calculating takeoff and climb performance under non-standard conditions. The FAA currently does not require kit manufacturers to develop and distribute such information. The pilot stated that he was aware of density altitude and purposely had less than full fuel tanks on takeoff due to the high elevation of the airport. He reported that in the future he intended to confine his flying to airports 3,000 feet above mean sea level or less.