Cross bores are not an immediate safety hazard. The normal waste water passing through a sewer lateral will not harm a gas pipe. Depending on the location of the cross bore, it may cause a sewer lateral blockage over time. Cross bores can become dangerous if someone attempts to clear the blockage using mechanical cleaning equipment such as augers that can cause the gas pipe to become severed causing natural gas to leak from the pipe and can lead to a deadly explosion.
A sewer lateral is generally installed in a downward slope from a home to the street. Because natural gas is lighter than air, it will go up when released to the atmosphere. If a natural gas pipe is severed inside a sewer lateral, the gas will try to rise which means it travels towards the home rather than into the mainline sanitary sewer.