ITS at 5.9GHz band, how it do not interfer other radio services at 5.9GHz band

Hi every one, I refer one document about ITS (intelligent transport system) and it make me so confused.
Firstly, I just care about ITS's antenna so I just ask you about its antenna like others application.
For example, it works between 5855-5925MHz (70MHz bandwidth), and it does not interfer other app.
So, it has some conditions such as: < -55 dBm/MHz e.i.r.p. below 5850 MHz , what does this mean, please ?

In my opinion, I can design an antenna radiate at 5.9GHz but it's very hard to make it like a filter just radiate
at 5.9GHz (S11 at 5.9 is -40dB) and very small radiation at 5.850 (S11 is -3dB) to have e.i.r.p <-55dBm/MHz below 5850 MHz

Thank you !

Well....what they want is for you to insure that low frequencies out of your band are below -55 dBm/MHz. i.e. you set a spectrum analyzer to a resolution bandwidth of 1 MHz, and sweep from 5000 to 5850, and you should not see the trace go above -55 dBm at any frequency.

So the first thing you do is set up your system to transmit normally (you might have to gin up some random data to send at the correct data rate), and do the above test. If you meet it by a few dB...you are good to go. If you do not meet that spec, you have to improve the lower spectral mask you are transmitting. You can do that by modifying the modulation (modulation type, complexity, data rate, chip rate, etc etc), adding filtering (both microwave and DSP type filtering), and sometimes increase the IP3 point of your transmitter components.