http://www.youtube.com/watch?v=RfFVzYub6wE
This is a video of one of the Philips Triangel urban lanterns at the road in which my hostel located, with HPS cycling.
The ignitor pulse rate is ~5 pulses per second, and the pulses are visible as soon as the lamp wents out. The reason because several flashes aren't visible in this movies is because my camera didn't capture them.
As most of the videos of cycling HPS and pulse start MH lamps in youtube you and another youtubers uploaded, exhibited lamps that were ignited at high frequency pulses (More then 1000 pulses per sec and so the discharge looks continuous and not pulsed), i don't know if such a low rate of pulses reignition like in the following movie and also this one: http://www.youtube.com/watch?v=MQ5LFUPLV1s are common in Europe and/or North America.

Ignitors have to send pulse, when the ballast voltage is at it's maximum, to ensure the fat arc would build up, this mean their pulsing should be concentrated there. Some (mainly short range superimposed ones) send a burst of multiple pulses per each mains half cycle, but they are really concentrated only around the mains maximum. Other (mainly long range semiparallel ones) send only one pulse per half wave.
Only old ignitors (like Tesla TZ1 - designed in early 80's) send only few, but very high voltage pulses per second - even then the pulse is in the voltage maximum.

This flashing might be or from "subharmonic" lamp oscillation (the flash pushes momentarily the lamp pressure up, so next few pulses would not break down the lamp, till it cool down; often appear as random flashes). In such case the flashing frequency would increase just before successful reignition - this is hard to judge from the video due to camera stroboscopic effect.
Or from some (older style?) slow acting ignitor (like the TZ1).

These flashes in the video is the same flashes that i saw in reality. The only problem with the camera is that it didn't captures all the flashes. The lamp was flashed continuously at a fixed rate of 4-5 flashes per sec. I think the ignitor was made in Israel by Eltam in which they continues to manufacture "non independent" (Semiparallel) ignitors that can be operated at a distance up to 20m.
By the way, many of Kiriat Ata town HPS street lights (AEG and Philips, old and new) uses semiparallel ignitors with this rate of pulses (4-5 pulses per sec). Perhaps Eltam semiparallel ignitors.