How do I control the brightness of an LED street light
Not going to work.  LED emitters themselves are inherently constant-current devices.  

However, the street light is probably a packaged commercial product, designed to accept a constant voltage, and it includes a constant-current power supply.  These are usually switch mode, which tend to be inherently multi-voltage, because when they see a lower voltage, they simply hold the switch open for a bit longer, which draws more current. 

Aside from working on world-market AC power (i.e. 100V Japan through 277V USA 3-phase), this also gives them a natural ability to ride through brownouts at full brightness.  

It probably has some some capacitors too, to get across those AC zero-crossings without shimmering the LED (which annoys people in motion relative to a point light source, some car taillights do this.) 

So, as you dim it by messing with its voltage, it's just chopping harder and pulling more current to stay at full brightness.  

It will never work. You need to disassemble the light and replace the power supply with one that is friendly to dimming.