I'm not sure its even that far. At 30mph every modern (this millennium) saloon car will stop in its own length when doing an emergency stop, provided tyres, brakes, and shocks are working. I could see 40 requiring an extra car length, maybe 2, but an extra 3 car lengths sounds too far to me.
I was basing it on simple math given the 40 mph and 2 seconds to stop while assuming constant deceleration. Simply put x = v0*t + 0.5*a*t^2 where v0 = 40 mph (58.67 ft/s), a = -20 mph/s (-29.33 ft/s^2), and t = 2 s which gives 58.67 and I rounded to 60 feet. Sure, I get the braking king is somewhere around 90 feet for a 60-0 stop (that's over 1.3 g deceleration) but those tests are typically done in near ideal conditions and also measure from the instant the brake is hit which takes the wetware out of the equation. I would expect a typical modern car to manage about 1 g deceleration which works out to about 120 feet from 60, 53 feet from 40, and 30 feet from 30 but as you say there are many other factors which impact that and I'm sure Tesla's recommended 45 psi tire inflation pressure is good for range but probably not ideal for stopping.