The maximum deceleration rate which can be achieved will depend upon the capability of the braking system, the traction available between the tyres and road, the mass distribution of the vehicle, and the position of the centre of mass relative to the front and rear axles. A good introduction to this subject is available in "Fundamentals of Vehicle Dynamics' by Tom Gillespie.

When you say 'safe' rate of acceleration, do you mean while maintaining control? If so then the threshold would occur when the brakes on one axle or the other lock. In that case rear lock-up is the more dangerous scenario as it will typically lead to oversteer whereas front lock-up would lead to understeer which is undesirable but more stable.

It varies from vehicle to vehicle. Besides it is also dependent on the braking system. The deceleration rate would be as follows ABS>Disc Brakes>Combination of Disc + Drum brakes>Drum Brakes. This is an arbitrary comparison. In the real case scenario, the road conditions, weather conditions, wear and tear of tyres and brakes, weight of the vehicle (with Passengers), etc. come into play. In order to achieve better deceleration rate, my suggestion would be to drive at the prescribed speed limit on road thereby making your vehicle to function better.

P.S- For the racing track deceleration rate is a crucial number. You may check guide notes in case of Ferrari or Mercedes for this.

For on-road dynamics, one need not worry as long as your car listens to your braking.

Whilst there are many parameters to estimate deceleration (retardation) via the brake torque generated on the disc rotor, the friction coupling between the tyre and the road surface will govern the maximum deceleration. Slippage will initiate ABS modulation of the hydraulic system thus governing the maximum.

-0.85 g is a good starting point for the average light road vehicle application assuming the tyres are within specification on a sealed road.

Race vehicles may achieve greater than -1.3 g