there is a lot of traffic in this city,so look both ways bef

Both,本身有三个词性,adj,adv,pron,
在这儿是形容词!adj,因为后面跟的是可数名词复数,指二个办法.
权威解释和例句(来自牛津字典）：
adj,
(with pl ns; the n may be preceded by a def art,a demons det or a possess det 与复数名词连用,名词前可有定冠词﹑ 指示限定词或所有格限定词) the two; the one as well as the other 二者; 两者都:hold sth in both hands 双手拿着某物 * Both books/Both the books/Both these books are expensive.这两本书都很贵.
adv.
...and...not only...but also...不但...而且...; 既...又...; 又...又...:be both tired and hungry 又累又饿 * She spoke both French and English.她既会说法语也会说英语.* Both his brother and sister are married.他的哥哥和姐姐两人都已婚.
Pron
(of sb/sth) (referring back to a pl n or pron 复指前文之复数名词或代词) the two; not only the one but also the other 二者:He has two brothers:both live in London.他有两个兄弟,都住在伦敦.* His parents are both dead.他父母双亡.* We both want to go to the party.我们俩都想去参加聚会.* I like these shirts.I'll take both of them.我喜欢这种衬衫.我两件都要.
有问题还可以再问.