Replacing all NA values with a constant is actually rather simple. A dataframe can be indexed using another dataframe of logical values of the same dimension. The following will create a new dataframe that is a copy of roads3 and replace all of the NA values with 0:

roads.replace.na <- roads3
roads.replace.na[is.na(roads3)] <- 0

In this chapter, I won't use the dataframe with replaced NA values which was just created, so this is just for demonstration purposes. A more effective way to handle NA values, when possible, is to replace the missing value with an estimate based on existing data.