your question depends on the speed limit of the roads in question, as well as the length of the road way being crossed. in addition the speed in which you would cross is varied, and as such cannot be accurately answered, as you didn't state your speed of crossing.
let's say it's a 30 mph roadway with a cross length of 10 feet.
an obect is traveling approximately 45 Feet per second at 30 mph.
so suppose the object closing in in you is 450 feet away, if you start to cross the 10 foot road, while said object is 450 feet away, you will have 10 seconds before said object reaches your path.
