jermyn street unsuitable for long vehicles

which why is Jermyn Street unsuitable for long vehicles ?
is it because Mr Street has a lack of spacial awareness and a long vehicle would be too much of an intellectual challenge ?
or perhaps Jermyn is simply a very small person and isn’t strong enough to turn the wheel of a lorry or bus.
whatever it is that makes him unsuitable for long vehicles, i’m sure he’s not pleased to have it announced on this sign.