Safety and Reliability Requirements

Please note that D1 and D2 type bulbs operate at high temperature with great pressure probably near or over 30 atmospheres. The internal quartz arc tube temperature is probably typically around 800 degrees C (1400-1500 degrees F or so). The outer bulb is not this hot, but it is definitely burning hot. The arc tube always has at least some miniscule risk of exploding and should only be operated in a headlight housing or other suitable container. Improper operation increases the risk of bulb explosion.
The bulb must be clean and free of dirt, grease, organic matter, ash, salt, or alkali. Salts, ash, and alkalis have a tendency to slowly leach into red-hot and nearly red hot quartz which will result in strains, weak spots, and maybe cracks.
A metal halide lamp does not like frequent starting. D1 and D2 types can be blinked, but this should only be done for a limited amount of time. Starting causes wear on the electrodes. Excessive evaporation of electrode material will deposit it onto the inner surface of the arc tube which results in darkening and overheating of the arc tube. In D1 and D2 and some other metal halide lamps, there is a halogen cycle which cleans deposited tungsten electrode material from the inner surface of the arc tube. Prolonged continuous operation at proper internal temperatures is required for the halogen cycle to work.